Laser Spectroscopy of the Small Atmospherically Important ...

Laser Spectroscopy of the Small Atmospherically Important Molecules, °2 03 and SO2

Howard Allan Sheard

Doctor of Philosophy

University of Edinburgh

2003

Laser spectroscopy of the small atmospherically important molecules, 02, 03 and SO2

Declaration (University regulation 1.1.4)

I hereby declare that this thesis has been composed by myself and, except where due

acknowledgement is given, the work described in it is my own and was carried out at the

University of Edinburgh.

Laser spectroscopy of the small atmospherically important molecules, 02, 03 and SO2

Acknowledgements

I would firstly like to thank my supervisor Robert J. Donovan for his

continued support and his willingness to share both his vast general and spectroscopic

knowledge and his precious time. I would also like to thank my second supervisor

Kenneth P. Lawley who's daily visits are always accompanied with constructive

conversation pertaining to the theoretical aspects of spectroscopy. I also acknowledge

that the majority of the theoretical work performed by myself has been achieved with

Kenneth's help and encouragement. Thirdly, I must thank Trevor Ridley for his

support and advice in writing this thesis, and, his help in performing the many

experiments undertaken in the past three years.

Finally, I would also like to thank the many friends I have made (and lost) and

flatmates, who have let rue into their lives during my eight year stay in Edinburgh.

Laser spectroscopy of the small atmospherically important molecules, 02, 03 and SO2 U,

THE UNIVERSITY OF EDINBURGH

ABSTRACT OF THESIS-] Name of Candidate: Howard Alla--S/ward

Address: Room 274 School of Chemistry,

University ofEdlLnbITSA, West Mains Roa4

Postal Code: E119 3ff

(Regulation 3.9.14)

Degree:

Title of Thesis: Laser ~

Na of words in the main text of Thesis: —45,000 -

Laser optical-optical double resonance (OODR), and, resonant enhanced multi-photon ionisation

(REMIPI) spectroscopy, have been used to study a number of electronic states ofjet cooled 02 and SO2, and

their dissociation fragments SO, S and 0.

The nd.irI flSCg (n = 3 - 9) gerade Rydberg states of 02 excited using OODR spectroscopy via single

rotational levels of the b ('E) valence state have been studied. The use of OODR spectroscopy allowed a

detailed rotational study of the 3soi d (fi g ) Rydberg state to be undertaken and the rotational distribution

of the 02 b ( Z ' ), v = 0 state, produced in the photolysis of 03, at wavelengths corresponding to the

Huggings bands of 03, to also be identified. A comparison of the singlet/triplet ndirg and nscg states showed

evidence for the importance of spin-orbit coupling in the core. A transition from a (A,S) coupling scheme to

a (O,w) coupling scheme is observed as the principal quantum number, n, increases ton 2! 5. The nscrg

states also show evidence for an S-uncoupling interaction between the 'Hi - 31712 and the ii - states.

An experimentally similar OODRJREMPI study of the Rydberg states of SO2 within the 73,000-

83000 cm' t region identified three Rydberg states, i.e. the (A2 core) 4s, (A, core) 4p and (A, core) 'Ip. Two-

photon dissociation of SO2 within this energy region is energetically sufficient to allow the production of SO

X (3 E), a (a) and b ('E) states. The detection and wavelength dependence of primary SO and 0, and

secondary S (and O) ion-signals in REMPI experiments is also reported.

Vibrationally excited SO X (3 Z) molecules, formed via two-photon photolysis of SO2, can be

further excited via v = I and 2 of the SOB (3 E) state (found to be coupled with the Q (H) state) to a newly

observed ion-pair state of SO. The ion-pair dissociation channel is an extra source of S ions, which should

be considered in discussions pertaining to primary sulfur production from photolysis of SO2.

Laser spectroscopy of the small atmospherically important molecules, 02, 0 3 and SO2 iv

Table of Contents.

Declaration

Acknowledgements

Abstract

1. Introduction and background principles.

I.!. thiroduction................................................................................................

1.2. Atmospheric absorption of radiation...................................................................

1.3. The absorption of radiation by an atomic or molecular system....................................

1.3.1. Transition moment, JL .....................................................................

1.3.2. Detection and decay of excited states.....................................................

1.4. Electronic structures of relevant atomic/molecular systems........................................

IA.!. The SO2 molecule...........................................................................

1.4.2. The 03 molecule.............................................................................

1.4.3. The SO molecule.............................................................................

1.4.4. The 02 molecule.............................................................................

1 .4.5. The S and 0 atoms...........................................................................

1.5. Thesis structure............................................................................................

1 .6. References.................................................................................................

2. Experimental section.

2.1. Introduction................................................................................................

2.2 The laser system

2.2.1. The excimer laser............................................................................

2.2.2. The dye-lasers................................................................................

2.2.3. Frequency doubling (20(1 harmonic generation)..........................................

2.3. Polarisation methods.....................................................................................

2.4. Laser calibration methods...............................................................................

2.5. The molecular beam......................................................................................

2.6. Time-of-flight (TOF) mass spectrometer (MS).......................................................

2.7. References.................................................................................................

'I

ill

I

1

2

5

5

7

12

13

18

23

25

29

32

34

37

37

37

38

39

41

43

43

44

45

49

Laser spectroscopy of the small atmospherically important molecules, 02, 03 and SO2 v

3. The 3Scrg d( ri g ) Rydberg state of 02: Its OODR spectrum recorded via

v = 0 of the b(' E ) valence state and its possible use as a monitor of the

b(' ç) state produced by the photolysis of the Huggins band of ozone SO

3.1. Introduction................................................................................................ 50

3.2. Previous experiments detecting the b(' ) valence state.......................................... 51

3.2.1. Photo-fragment-excitation (PHOFEX) spectroscopy of ozone in the Huggins

bandregion................................................................................ 51

3.2.2. The Kinetic-Energy-of-Release (KER) spectra.......................................... 52

3.2.3. Previous detection of the 02 b('E)valence state via the 35Cg d('fl g )Rydberg

state using a [(1+1')+l'] REMPI excitation scheme................................. 55

3.2.4. Previous OODR study of v = 3 of the perturbed 02 3SCg d(' 11g) Rydberg state

excited via single rotational levels of the b(1 E) valence state.................... 56

3.3. An OODR study of v = 0-2 of the perturbed 02 35Cg d(l Hg ) Rydberg state excited via

single rotational levels of the b(' E) valence state............................................. 60

3.3.1. Previous studies of the 02 3soj Rydberg states.......................................... 60

3.3.2. Experimental................................................................................. 62

3.3.3. The b4—Xtransition in 02 ................................................................ 65

3.3.4. The Haul-London factors for the two-photon d 4-4—b transition in 02 ............. 67

3.3.5. An overview of the d(' Hg ) Rydberg state (v = 0 -2) region observed via

OODR spectroscopy..................................................................... 68

3.3.6. OODR spectroscopy of v = 0 of the d(' Hg) Rydberg state.......................... 70

3.3.7. OODR spectroscopy of v = I of the d('rlg ) Rydberg state.......................... 80

3.3.8. OODR spectroscopy of v = 2 of the d( 1 fl g ) Rydberg state.......................... 85

3.4. Analysis of the 02 b('L) state rotational distribution produced from the photolysis of

ozone in the Huggins band region (310-350 nm) probed using [(l+l')+l] REMPI via

the 02 d('Ilg ) states ................................................................................ 92

3.4.1. Introduction................................................................................... 92

3.4.2. The 02 b(1 L), v = 0 state rotational distribution produced from the photolysis

of ozone in the Huggins band region (at 337.2 and 344 nm) monitored via v =

I of the d('Hg ) state .................................................................... 94

3.4.3. The 02 b('), v = 0 state rotational distribution produced from the photolysis

of ozone in the Huggins band region (at 340 rim) monitored via v = I of the

d( I rig ) state .............................................................................. 97

3.4.4. The 02 b('E), v = 0 state rotational distribution produced from the photolysis

of ozone in the Huggins band region (at 340 rim) monitored via v = 2 of the

d( ' Hg ) state .............................................................................. 98

3.5. Conclusions................................................................................................ 104

3.6. References................................................................................................. 107

Laser spectroscopy of the small atmospherically important molecules, 02, 03 and SO2 vi

4. An optical-optical double resonance study of the ndg8 Ins org gerade Rydberg states of 02 excited via single rotational levels of the

b( E) valence state 109 4.1. Introduction ................................................................................................ /09

4.2. Experimental .............................................................................................. 111

4.3. General overview of the OODR/REMPI spectrum ................................................... III

4.4. The nd,r8 states, for n = 3-8 ........................................................................... i/S

4.5. The nsoi states, for n = 3 - 9 ............................................................................ 124

4.5.1. Vibronic data ................................................................................. 124

4.5.2. Rotational data ............................................................................... 131

4.6. Predissociation of the 02 Rydberg states .............................................................. 140

4.7. Conclusions ................................................................................................ 142

4.8. References ................................................................................................. 143

5. The wavelength dependence of the ionisation and dissociation products

Of SO2 145

5.1. Introduction ................................................................................................ 145

5.2. One-colour REMPI spectroscopy of S02 in the 212-230 rim wavelength region ............... 147

5.3. One-colour REMPI spectroscopy of 502 in the 410-455 tim wavelength region ............... 150

5.4. One-colour REMPI spectroscopy of SO 2 in the 245 -295 nm wavelength region ............... 154

5.5. One-colour REMPI spectroscopy of SO 2 in the 340-410 tim wavelength region ............... 157

5.6. A summary of the one-colour REMPI spectroscopy of SO 2 ........................................ 160

5.7. References ................................................................................................. 16/

6. The 1,11 and III Rydberg states of SO2 162 6.1. Introduction ................................................................................................ 162

6.1.1. Previous studies .............................................................................. 163

6.1.1 .1. The SO2 â(3B1) state and SO2 *'Al) State ................................. 163

6.1.1.2. Photoelectron study of the three lowest-lying electronic states of the

SO2 ion-core ................................................................... 164

6.1.1.3. VUV absorption experiments from the SO2 X('A1 ) state to the

singlet I, 1/and III Rydberg states ........................................... 166

6.1.1.4. REMPI studies of the 11 Rydberg state excited from the SO2

X('III) state .................................................................... 168

6.1.1.5. OODR studies of the 1! Rydberg state excited from the SO2 1('A 1 )

state via the (0,0,0) band of the 2i(3 B,) valence state ..................... 171

6.2. Experimental .............................................................................................. 17/

6.3. Results and discussion ................................................................................... 172

Laser spectroscopy of the small atmospherically important molecules, 02. 0 3 and 502 vii

6.3.1. An overview of the SO2, SOP, S(Oj) and 0 ion-channels observed via one-

colour REMPI spectroscopy in the 345-410 rim wavelength region............. 172

6.3.2. One-colour (3+1) REMPI study of the 'I, I.)jj, Ijjj Rydberg states................... 175

6.3.3. A two-colour OODR study of the Vi Rydberg state.................................... 178

6.3.4. Observation of the H Rydberg state using 000R spectroscopy..................... 179

6.3.5. Observation of the Vi Rydberg state in the SO, S, and O ion-channels........... 183

6.3.6. Determination of electronic assignments for the!, II and II! Rydberg states........ 185

6.4. Conclusions................................................................................................ 187

6.5. References................................................................................................. 188

7. Observation of the a (5f1) ion-pair state of SO following the OODR

excitation of the X( 3 E) ground state via the coupled B( 3 Z-) state 189

7.1. Introduction................................................................................................ 189

7.2. Experimental.............................................................................................. 192

7.3. Results and discussion................................................................................... 193

7.3.1. The SO a (I1) ion-pair state vibrational structure...................................... 193

7.3.2. The rotational structure for v= 0 of the a ('fl) ion-pair state......................... 196

7.3.3. The SO a (fl) ion-pair state rotational structure for v> 0 of the ion-pair state 207

7.3.4. Determination of the nature of the intermediate state.................................. 210

7.3.5. Determination of the nature of the ion-pair state........................................ 215

7.4. Rittner plot................................................................................................. 216

7.5. Conclusions................................................................................................ 218

7.6. References................................................................................................. 220

8. Analysis of the S ion-signal observed from the photodissociation of SO2 221

8.1. Introduction................................................................................................ 221

8.2. production via the a(511) ion-pair state of SO ...................................................... 221

8.3. Primary production of sulfur atoms from the dissociation of SO 2 ................................. 225

8.4. Secondary production of sulfur atoms from the dissociation of SO............................... 229

8.5. Conclusions................................................................................................ 231

8.6. References................................................................................................. 232

Appendix A: University regulations 233

A. I. Lecture courses attended................................................................................ 233

Conferences attended.................................................................................... 234

List of Publications....................................................................................... 236

Chapter 1. Introduction and background principles

Chapter 1.

Introduction and background principles.

1.1. Introduction.

The aim of this thesis is to advance the spectroscopic knowledge of small

atmospherically important molecules using resonant enhanced multi-photon ionisation

(REMPI) time-of-flight mass spectroscopy (TOF-MS). The three atmospherically

important molecules chosen in this study are sulfur dioxide, SO2, ozone, 03 and

oxygen, 02.

SO2 is isovalent with 03, both having 8 valence electrons. They also share

similar electronic ground states (X( 'A 1 )) and ground state equilibrium bond angles

(03, 116.8°; SO2, 119.4°) I,2• However, there are significant differences between SO2

and 03, especially in dissociation energies (03, 1.04 eV; SO2, 5.61 eV)'

The multi-photon absorption of UV radiation by SO2 and 03 primarily leads to

photolysis of the parent molecule to produce SO + 0 and 02 + 0 dissociation

fragments, respectively. This statement is especially true of ozone since it possesses a

low dissociation energy that can be exceeded using one UV photon and thus no

REMPI detection of 03 has been reported in the literature. Further multi-photon

excitation, ionisation and detection of the dissociation fragments can lead to a greater

understanding of the dynamics of dissociation events, the internal energy distribution


2

within the dissociation fragments and excited electronic state information within both

the parent and daughter molecules or atoms.

1.2. Atmospheric absorption of radiation.

Ozone is extremely important in the chemistry and dynamics of the

atmosphere . It can be found in trace amounts throughout the atmosphere, with the

largest concentration between about 15 - 40 km (Stratosphere). Firstly, ozone absorbs

virtually all solar ultraviolet radiation between 200 - 310 nm (Hartley band) as seen

in Fig. 1. 1, which would otherwise be allowed to penetrate to the Earth's surface.

Radiation of such wavelengths is lethal to simple unicellular organisms, and to the

surface cells of higher plant and animal life. Secondly, the upper atmosphere is

greatly influenced by the heating that follows absorption by ozone of UV, visible and

infrared radiation.

Chapter 1. Introduction and background principles 3

4. ------* N2,O

4.-----2---

O3 i i 150 liii 4.-------

-

IN

0 100 V

100 200 300_ Wavelength (nj

Fig. 1.1.: The atmospheric altitude of maximum absorption of ultraviolet radiation as a tünction of

wavelength. The atmospheric constituents principally responsible for absorption, in the indicated

regions, are shown. Taken from Watanabe, Ref. 6.

A discussion of the importance of atmospheric ozone is not complete without

mentioning the so-called "hole in the ozone layer", which has been widely publicised

in the popular media 4, 5 . Man's release of cholorfluorcarbons (CFC's) can lead to the

destruction of ozone via the following mechanism shown for CF2C12

CF2C12 + hv —* CF2C1 + Cl

Cl+03—* CIO +02

CIO +O—Cl+O2


net 0+03 —* 202

Oxygen, 02, molecules also attenuate solar ultraviolet radiation between - 100

- 200 nm as shown in Fig. 1.1. The structured absorption observed around 200 nm

in Fig. 1.1 relates to discreet bands of the B(3 Z ) +- X( 3 Z ) Schumann-Runge

system and the apparently structureless 130 - 170 nm region is the continuum of the

Schumann-Runge system. The observed structure attributed to 02 below 130 nm in

Fig. 1.1 relates to Rydberg states of 02; these states are discussed further in Chapters

3 and 4.

At wavelengths lower than - 100 nm the majority of the solar VUV radiation

is attenuated by N2 and by 0 atoms at wavelengths corresponding to its atomic

absorption lines '.

Within our own atmosphere SO2 is an atmospheric pollutant. The presence of

sulfur compounds in the troposphere leads to the production of "acid rain", a process

whereby H2SO4 builds up in clouds, lowering the pH of rainfall with disastrous effects

chiefly upon plant life. The conversion of SO 2 to H2SO4 in air via photochemical

processes is negligible; instead the reaction is thought to be initiated by an OH radical

via the following reaction:

OH+S02+M—HOS02 +M (1.2)

The HOS02 radical is further oxidised to H2SO4 via a complex mechanism.


1.3. The absorption of radiation by an atomic or molecular

system.

1.3.1. Transition moment, R,,.

When electromagnetic radiation interacts with an atom or molecule the energy

of the incident radiation can be absorbed and converted into internal energy

(electronic, vibrational or rotational) to form an excited state of the atomic or

molecular system. If an allowed electric dipole transition is considered, the absorption

of radiation is facilitated by the perturbation of the dipole moment of a molecule by

the electric field of the electromagnetic radiation. Herzberg 8 defmes the matrix

element of the electric-dipole moment, Ram, as being dependent upon the

eigenfunctions of the lower and upper states, V. and vi,, and the electric-dipole

moments of the system whose components are, M,,, M and M 7 shown by equation

(1.3).

irtm = 5 * MxWmdD + Jy' * Myi,,dr + * MzIIImdT (1.3)

where M =I ekxk , M, = ekyk and M = I ekzk

and Ck are the charges on the N particles of co-ordinates Xk, Yk and zk. If R differs

from zero, the two states combine with each other with a certain probability with


emission or absorption of radiation; if R" is zero the transition under consideration is

forbidden as an electric-dipole transition.

If, for a given transition, the electric-dipole transition moment is zero, the

corresponding transition may be observed if the matrix element of the magnetic or

quadrupole moment are different from zero. In these cases the corresponding quantity

- magnetic dipole moment or electric-quadrupole moment should be substituted for

M, M and M in equation (1.3). The transition probabilities for a magnetic dipole

transition and electric-quadrupole transition are typically 10-5 and 10 -8 times weaker

respectively than an electric-dipole allowed transition.


7

1.3.2. Detection and decay of excited states.

Several schematic excitation schemes for a molecule AB are shown in Fig.

1.2. The figure shows several different experimental techniques relating to the

spectroscopy of AB and AB*, which are discussed below.

'PAR

Ulu

Absorpti

y-' a,1

IPAB

WO.

Spontaneous Stimulated I + 1 2 + 1 0 + 1 1) + 1' Emission Emission REMPI REMPI REMPI

Fig. 1.2.: Some schematic excitation and emission schemes for a molecule AB.

When a molecule (AB) is subject to radiation whose energy corresponds to the

energy difference between two states (nhv = AE), absorption of the radiation can

occur. The absorption intensity varies with the population of the lower state, the

intensity of the radiation and the nature of the transition (allowed, spin forbidden, etc).

The process of absorption yields information about the position and nature of the


8

upper electronic state (AB*). Herzberg 8 defines the absorption intensity, jabsn,, for a

single-photon transition as:

'-'- 1abs nm _

- . nm o 'rn hvnm Ax

where Bmn = 87r3/3h2c IRh1 I 2

and N n is the number of molecules per cm in the initial (lower) state m, B,,, is the

Einstein transition probability of absorption and Ax is the thickness of the absorbing

layer. It is assumed that the incident radiation has a constant intensity throughout the

entire absorbing layer and that the wavenumber distribution of the radiation is

sufficient to cover the entire absorption linewidth.

The fate of the excited molecule (AB*) is dependent upon several factors. The

excited molecule may de-excite via emission of a photon. This process is known as

fluorescence or phosphorescence depending upon whether the transition is between

states of the same or different multiplicity, respectively. For spontaneous emission,

the emitted photon need not be the same energy as the absorbed photons, as states

other than the initial state (state m) may be reached. The intensity of spontaneous

emission is described by:

rUm _ AlL. A 'em JVnhiCVnmJT1nm (1.6)

where A nm = 647t4Vnm3/ 3/7 IR ""12 (1.7)

(1.4)

(1.5)


and A,,,, is the Einstein transition probability of spontaneous emission. Fluorescence

(or phosphorescence) yields information about the lower state (m).

A second type of emission known as induced, or stimulated, emission may

occur from an excited state. Stimulated emission requires the presence of radiation of

energy hvnm to induce the transition, such that;

AB* + hv nm —* AB +2 hvnm (1.8)

The process of stimulated emission is therefore similar to that of absorption since it is

dependent upon the intensity of the incident radiation, Ion' and the Einstein transition

probability of stimulated emission, Bnm , which is equal to Bmn . Unlike absorption,

stimulated emission is dependent upon the excited state, n, population, N.

r nm_ I nm se O Nn Bnm hvnm (1.9)

Resonantly enhanced multi-photon ionisation (REMPI) spectroscopy uses

absorption to access excited states, but before fluorescence can occur, further photons

are absorbed to produce ions. More than one photon can be absorbed in laser

experiments since the intensity of a focussed laser beam can be very high. Fig. 1.2

shows examples of (1 + 1), (2 + 1) and [(1 + 1') + 1'] REMPI processes. The numbers

denote the number of photons absorbed for resonance to be achieved with the AB*

state and the number of photons required to ionise the AB* state. In (2 + 1) REMPI

spectroscopy two photons are required to populate the excited state and these photons


are absorbed simultaneously (two-photon coherent absorption). In a [(1 + 1') + 1]

REMPI experiment two photons of different energies are used to excite the AB* state

(two-colour REMPI).

1

E C-)

>

ci)

W

c a) 4-, 0

11

12

We

El

1.0 1.2 1.4 1.6 1.6 2.0 2.2 2.4 2.6

Internuclear Distance / A

Fig. 1.3.: The [1+(2')+1'] optical-optical double resonances (OODR) excitation scheme used to detect v

= 3 of the 02 d('flg)Rydberg state via u= 0 of the 02 b('E)state.


II

The photon energies used in two-colour REMPI experiments may be chosen

such that an intermediate state is excited prior to excitation of the state of interest.

This technique is known as optical-optical double resonances (OODR) since two

excited states are resonantly populated (double resonance) both by laser radiation

(optical-optical). An example of an OODR excitation scheme is shown in Fig. 1.3,

which shows the [1+(2')+l'] OODR excitation scheme used to detect v = 3 of the 02 d

( ' lu g) Rydberg state via v = 0 of the 02 b(' ) state. This is a commonly used

technique, by fixing the first photon energy to a known transition (the red photon in

Fig. 1.3) and scanning the second photon energy (the blue photon in Fig. 1.3), detailed

vibrational and rotational structure of the state of interest state can be observed.

OODR spectroscopy will be discussed in greater detail in Chapter 3.

The observed intensities of the above spectroscopic techniques can be

diminished, or even cancelled, by several alternative processes. Such channels

include, dissociation where the AB* molecule dissociates to form A + B, A* + B, A

+B* or A* + B* before the excited AB* state can be detected in fluorescence or

REMPI experiments. Such excited AB* states are said to be predissociative.

Quenching of the excited AB* state may also occur, where the AB* molecule is de-

excited by collisions with other gaseous molecules (or the walls of the container in

static cell experiments). Thirdly, the AB* molecule may undergo internal conversions

or inter-system crossings which cause the population of the specific AB* state to be

reduced. Examples of such processes include 5, to S0 (internal conversion) and 5, to

T i (inter-system crossing) in polyatomic molecules.


12

1.4. Electronic structures of relevant atomic/molecular

systems.

To better understand the known electronic states of s0 2, 03, SO and 02,

potential energy diagrams for these molecules are included in this Section

accompanied by a brief description of their spectroscopy. The following text is not

intended to be a comprehensive review of the entire body of work completed on these

molecules.

The relative energies for the equilibrium geometries of the low-lying

electronic states of SO2 and 03 are shown schematically, see Fig. 1.4 and 1.6

respectively. It is difficult to generate potential energy curves for triatomic molecules

(ABC) since they possess three internuclear distances (A-B, B-C and A-C). 2D

potential energy surfaces for low lying states of both SO2 and 03 have been generated

using computational methods by Jones 9, displaying potential energy vs. O-S-0 and

0-0-0 bond angles, a s ,, and %0,, respectively.

Potential energy curves for the diatomic species, SO and 02, have been

generated by using a Morse Potential, see Equations (1.10 and 1.11), fitted to the

known spectroscopic constants for the specific states; dissociation energy, De , in cm',

equilibrium bond length, re , in meters, reduced mass, PA, in atomic mass units and

vibrational frequency, co,, in cm'. The resultant potential energy curve is displaced

vertically in energy by the term value, Te .

U(r)= D(1—e'')2 (1.10)


27r2 cp (0 =1.2177x10 where eh e (Oeijii

1.4.1. The SO2 molecule.

A schematic representation of the known electronic states of SO2 along with

the dissociation thresholds for SO +0 and 02 + S formation is shown in Fig. 1.4. The

SO2 states energies are taken from those of Herzberg 1, since at the scale shown exact

up-to-date energies are not required and the labelling system used by Herzberg is still

in use today. Fig. 1. 5, the electron energy-loss spectra (EELS) 10 of Vuskovic et al. 11

for the SO2 molecule, shows the entire ii - state range and is similar to that

observed in absorption experiments 12,13

The outer electron configuration of the singlet SO2 X(A 1 ) ground state is,

(6a1)2(2b 1 )2(7a1 )2(4b2)2(5b2)2( 1 a2)2(8a1 )2 (1.12)

and experimentally 9 it has been found that the ground state possesses an re of 2.712 A

and an a ) of 119.40 in its lowest energy equilibrium geometry.

Chapter 1. Introduction and background principles '4

100

80

IP SO2 .'( 2A)

Ti Band

Band m II

I F Band

If +

+ is

'A + is i f + 10

'A + ID +

3+ID >_1A+'D 3+ls 'E+ +'P,

+ 3PJ +

+ 3PJ /

3 3p Y- d1A+3PJ 3

, + 3p

so+o 02 +s

E () cv)

CD

>

a)

W

DO

40 DBand -

C('B2)

(3B,)

20 -

0 so

Fig. 1.4.: The relative energies for the equilibrium geometries of the known electronic states of SO 2

along with the dissociation threshold for SO + 0 and S +0 2 formation. The SO2 state energies and

assignments are taken from those of Herzberg'.


15

E0 MDeV lop

i A VV\j" e - ixP

3 5 7 9 11 ENERGY LOSS, eV

Fig. 1.5. The electron energy loss spectrum (EELS) of S02 of Vuskovic et al. The upper spectrum

was recorded using high-energy electrons (E 0 = 200 eV) detected at low scattering angle (0 = 10°), this

favours the excitation of electric-dipole allowed transitions. The lower spectrum wss recorded using

low energy electrons (E 0 =20 eV) detected at high scattering angle (0 = 1300), this change in

experimental conditions allows forbidden transitions to be observed, e.g. triplet - singlet transitions.

The lowest energy triplet state of SO2 is the a(3B 1 ) state, which correlates

with the SO X( 3 E ) + 0 (3Pj) dissociation limit. The one-photon SO2

( 3B 1 ) *-- !(A l ) transition is therefore spin-forbidden, triplet - singlet, which can

be optically pumped using high laser powers, high pressure conditions or large path

length apparatus. Studies 11 4-16 have shown that the high-energy vibrational bands,>

27500 cm- , possess irregular contours and are perturbed. Spectroscopic evidence

leads to the assumption that the ( 3B 1 ) state is vibrationally perturbed by a 3A1 state.

The 3A1 state is a "dark state", in that it can not be observed directly from the ground

state, only its effects upon the ( 3B 1 ) state can be observed.


16

The first one-photon, optically allowed electronic transition in SO2 involves

two transitions, the §('B1 ) - X(A1) and A('A 2 ) +- X(A 1 ) state transitions. An

example of the /, .-I absorption spectrum can be found in Fig. 1 of Shaw ci al.

17 An underlying continuum is observed due to the optically allowed B(B1 ) +-

X( 1A 1 ) state transition, whose continuum nature is due to Renner-Teller coupling of

the B('B 1 ) state with high lying, densely packed vibrational levels of the I('A 1 )

ground state. Against this continuum background A( 1A 2 ) +- I('A 1 ) transitions can be

observed, the so called Clements' bands, which are typically labelled A, B, C... etc

and should not be confused with the A, B, C, ...etc or A, B, C, ... etc states. The

A0 2 ) - I('A 1 ) transition is symmetry-forbidden by one-photon optical selection

1. rules and is observed via vibronic coupling to the B('B 1 ) state through the v3(b2)

antisymmetric stretching vibration. At energies lower than the B('B 1 ) state origin

ábsorption due to the 4('A 2 ) state is greatly reduced, due to its forbidden nature, but

the observed levels are still structured.

Both the ( 3B 1 ) and A(A 2 ) states are discussed in greater detail in Chapter 6,

where they are observed as (3 +1) REMPI signals from the ground state in the SO P, S

(and 0') ion-channels and where the ( 3B 1 ) state is used as an intermediate state in

OODR spectroscopy of the SO2 3 1 Rydberg state.

Of all previous publications pertaining to the spectroscopy of SO2, at least half

are dedicated to the SO2 C(B 2 ) valence state. This is due to several reasons; firstly,

the C(B2 ) state is observed strongly in absorption and fluorescence experiments,

secondly, it is easily accessibly by commonly used laser wavelengths, especially 193,

248 and 308 rim (ArF, KrF and XeCl fundamental excimer radiation), and thirdly, the


origin of the C('B 2 ) state lies 3000 cm' below the first dissociation threshold for

SO2 X( 1A 1 ) + hv —+ SO X( 3 ) + 0 (3Pj) products to which the higher vibrational

levels of the (B) state have been shown to dissociate 18

The C('B2 ) state does not correlate with the lowest SO X( 3 ) + 0 (3Pj)

asymptote, but with the SO a('A) +0 ('D) dissociation limit. The C( 1B2 ) state

structure is observed strongly in both one- and two-photon absorption due to the

allowed nature of the ('B2 )+-- ('A,) transition. The most thorough assignment for the

band structure of the C('B 2 ) state has been performed by Yamanouchi et al. 19 The

SO X( 3 ) + 0 (3P) dissociation limit occurs at energies exciting the (1,4,2) band

of the C(B2) state at 45725.3 cm. 18,19 Braatz and Tiemann 18 discuss the nature of

the repulsive states in the (1,4,2) band region above the dissociation threshold and

note that there is no barrier in the exit channel for SO X( 3 Z - ) + 0(3 Pj) dissociation.

REMPI spectroscopy of the C('B2 ) state is discussed in greater detail in Chapter 5,

where the SO2 C('B2) +_ X(3 1 - ) transition is pumped using one- and two-photon

REMPI excitation.

The Rydberg SO2 bands, E—H, have been observed in one-photon VUV

spectroscopy of the SO2 i('A, ) state and are therefore singlet in nature. The origins

of the I, II and III Rydberg states are also indicated in Fig. 1.4, which are discussed in

greater detail in Chapters 5 and 6.


1.4.2. The 03 molecule.

A schematic representation of the low-lying electronic states of 03, along with

the dissociation thresholds for 02 + 0 formation is shown in Fig. 1.6. The electronic

origins of the singlet and triplet 03 states in Fig. 1.6 were taken from theoretical work

of Palmer and Nelson 20 Fig. 1.7, the energy-loss spectra of Mason et al. 21 shows the

spectral range 0- 12 eV (i.e. from the ground state to the first ionisation potential of

03).

The outer electron configuration of the singlet 03 X('A 1 ) ground state is,

(5a 1 )2(3b2)2( 1 bi)2(6a1)2(4b2)2(l a2)2 (1.12)

and experimentally 9 i has been found that the ground state possesses an re of 2.403 A

and an a of 116.8° in its lowest energy equilibrium geometry.

Four different band systems are present in the 200 - 1000 rim absorption

spectrum of 03, similar to those observed in the electron energy loss spectrum of

Mason et al. 21 shown in Fig. 1.7. These bands are the Wulf, Chappuis, Huggins and

Hartley bands at 1000 - 662 nm, 739 - 451 rim, 370 - 300 rim and 310 - 200 rim,

respectively. All four of these bands lie above the first dissociation limit of 03

corresponding to 02 X(3 )+0(3 P2) formation at 1.02 eV (8226.8 cm)


100

'E 60 C)

c)

> 40

ci)

W

20

Ell

p 0 +(2)

1 3A 1

23A

23A 2_______ L\+S 21B1

23B1 21A2

i Y_ + + i 1 1 B 2

2 3 B 2 1A+ 1 D 2 1 A 1

1IA 2 i 3B1_______ 1B1

1 1 A 2

1 3B2 3E + 3p j

02 +0 1 1 A 1

triplet 03 singlet 03

Fig. 1.6.: The relative energy positions for the equilibrium geometries of the low-lying singlet and

triplet electronic states of 0 3 along with the dissociation thresholds for 02 + 0 formation. The 0 3 state 20 energies and assignments are taken from the theoretical values of Palmer and Nelson


a I I 0 I 1 I 1 t,I I I I -

x1I1000 - E=2OeV = = 9 U 1110

if AM

Ii I 0 2 4 6 8 10 12

Energy Lss,eV)

0.6

0.4

02

0.0

0 2 4 6 8 10 12 Energy Lo$(eV)

Fig. 1.7. The electron energy loss spectra of Mason et al. 21 for energies of 0 to 12 eV. Spectrum (a) is

recorded under conditions favouring singlet state detection and (b) is recorded under conditions

favouring singlet and triplet state detection. Note the Wulf, Chappuis, Huggins and Hartley bands

regions are labelled in (a).

Absorption to the three low-lying meta-stable triplet states of 03, the 1 3B,, I

3B2 and 1 3A, states, see Fig. 1. 6, between 1000 - 662 nm are traditionally grouped

together to form the Wulf bands. These bands were originally attributed to hot and

cold bands of the 1 'A2 state, but subsequently have been assigned to the three triplet

states 22 Evidence for the triplet nature of the Wulf bands can be observed in Fig. 1.7.


The Chappuis band system between 739 - 451 nm consists of an irregular

vibrational structure superimposed upon a continuum system. The vibrational

structure originates from the bound 1 'B, state, which correlates with the 02 a('A g ) +

0 ('D) dissociation limit. The underlying dissociation continuum originates from the

1 'A2 state, which is unbound and correlates with ground state products 02 X( 3 z ) +

0 (3P2). Experimentally, fluorescence cannot be observed from the bound 1 'B, state

indicating that it is short lived. This observation can be explained thus; if the 1 'B,

and 1 'A2 states are distorted from C2,, to CS symmetry both states are reduced in

symmetry to 'A" symmetry. An avoided crossing can therefore occur between the two

states in Cs symmetry, which can lead to dissociation to the ground electronic states

Of 02 X(3) +0(3 P2). This mechanism leads to the dissociation of the bound 1 1 B 1

state reducing its lifetime considerably. The vibrational structure of the Chappuis

bands has yet to be unambiguously assigned. Anderson and Mauersberger 22 note that

none of the five vibrational assignments for the Chappuis bands structure which have

been proposed satisfy all the experimental observations. It should also be noted that

the Wulf bandsprobably contribute to the irregular nature of the Chappuis bands.

The Hartley bands or continuum is probably the most experimentally studied

band of 03, due to its intensity and importance in atmospheric absorption of harmful

UV radiation 200 - 310 nm. It is generally accepted that the Hartley band of 03 is

due to the electronically allowed transition from the singlet 03 X(A 1 ) ground state to

the 1 'B2 state. The 1 'B2 state correlates with, and can dissociate to, the 02 a('L g ) +

0 ('D) dissociation limit. Irregularly spaced diffuse bands possessing energy spacings

ranging from 150 to 300 cm' cap the Harley continuum. Structure can also be


observed in the low energy tail of the Hartley continuum, see Fig. 1.7, at 310 - 370

nm, corresponding to absorption to the Huggins bands, with an upper electronic state

assignment of 2 'A 1 , as seen in Fig. 1.7. Both the Huggins and Hartley bands have

been discussed in detail by O'Keeffe et al. 23, where the UV absorption vibrational

spectrum of Huggins and Hartley bands 03 have been reassigned.

Several bands can also be observed at energies greater than 6 eV (- 48400 cm

1) in both VUV absorption and electron energy loss (Fig. 1.7) experiments. These

bands relate to Rydberg states of the 03 molecule, based upon low-lying ion-core

states of 03. Assignment of the observed Rydberg states' structures has been recently

been reviewed by Palmer and Nelson 20 Their theoretical spectra compare favourably

with experimental UV, VUV and EELS spectra.


1.4.3. The SO molecule.

A potential energy diagram for the known valence states of SO is shown in

Fig. 1.8. The potential energy surfaces have been generated using spectroscopic

constants for the specific states from various sources 24-26 fitted to the Morse equation

(1.10). The low-lying states of SO, indicated in Fig. 1. 8, have been reviewed in detail

by Bonn and Omellas 26

The lowest outer electron molecular orbital configuration of SO is,

(72(2)4(3*)2 (1.13)

This configuration gives rise to the three lowest-lying electronic states: the

X( 3 ) ground state and the meta-stable a(') and b(' Y-') states. The three valence

states correlate with the S (3Pj) + 0 (3P) dissociation limit and possess

experimentally determined 26 equilibriumbond lengths of 1.493, 1.502 and 1.514 A,

respectively. It should be noted that transitions to both the a('A) and b(' ) states

from the X( 3 ) ground state are spin-forbidden, but can be optically pumped using

high intensity laser radiation.

The bound c(' Y- - ), A'('A) and A' (3 ) states also correlate with the S ( 3P)

+0(3 Pj) dissociation limit. The states possess re values somewhat higher than the

ground state of 1.765, 1.770 and 1.719 A, respectively 26 This increase in r leads to

experimental difficulties in observing the origins of these states from the ground state

due to Frank-Condon constraints.


Of the 18 states that correlate with the S (3Pj) + 0 ('Pi) dissociation limit, six

have been discussed, the rest are either shallow-bound, e.g. the A( 3 [I) and Q( 5 U)

states or repulsive, e.g. the C ,(3 U) state. The shallow-bound and repulsive states may

perturb one another or higher energy states, which correlate with higher energy

asymptotes. An example of such a perturbation is the repulsive C (3 U) state, which

undergoes an avoided crossing with the C( 3 lIE) state 26

iLSIS]

\ /1P SO X(U)

E 60

2' 40 U)

W

20

C' (sri) S (10) + 0 (1 D)

C(U) S (0) +0 (3p)

( u ? ?

(3p)

\\

•\\

A' (3A)

c( 1 E-)

b ( ' z ')

( 'A)

I (3 ) . So

I

2

3

4

nA

Fig. 1.8.: Potential energy curves for the known valence states of SO. Singlet states are drawn using

solid lines and triplet states are drawn using broken lines.


25

The higher energy valence states which correlate with the S ('D) +0 (3Pj)

dissociation limit, B( 3 ), C( 3 [I) and d(' 11) states, and the S ( 1D) + 0 ('D)

dissociation limit, e(' IT) state, have been discussed in detail by Archer et al. 25 The

B( 3 ) state will be discussed in greater detail in Chapter 7, where it is used as an

intermediate state in OODR experiments excited from the X( 3 ) ground state to an

SO ion-pair state.

1.4.4. The 02 molecule.

A potential energy diagram for the electronic states of 02 is shown in Fig. 1.9.

The potential energy surfaces have been generated using spectroscopic constants for

the specific states 27 fitted to the Morse equation (1.10). Morill 27 has reviewed the

electronic states of 02, indicated in Fig. 1.9, in detail.

The potential energy surfaces for the electronic states of 02 are somewhat

similar to those of SO, showing the isovalant nature of SO and 02. Due to the

homonuclear nature of 02, the electronic states of 02 possess gerade, g, or ungerade,

u, symmetry indicating that the molecular orbitals are either symmetric or

antisymmetric, respectively, to an inversion point through the centre of the 02

molecule. Only ungerade states can be observed or accessed as allowed transitions

from the gerade 02 X(' Y- - ) ground state via single photon absorption. Transitions to

states of gerade symmetry from the ground state become optically allowed if the

transition involves the coherent absorption of two photons. A second consequence of

the homonuclear nature of 02 is that due to the zero nuclear spin of 160 only totally


symmetric levels (with respect to interchange of nuclei) of a state can exists. This

results in the absence of even rotational levels for negative states, e.g. the

X( 3 ;) state, andthe absence of odd rotational levels for positive states, e.g. the

b(' Y,' ) state. The rotational levels of the X( 3 ) and b(' Z) states are shown

schematically below in Fig. 1. 10, the one-photon optically forbidden transitions

between the two states are also shown, which are discussed in greater detail in

Chapter 3.

°2 x (2n) " f4sa

3d,t

5sc d( 1 r19)

•-\_ 'jj / C (3

nd

lf (,) 0(1D)+0(1D

C.)

TC) 60

/1(3 ng)

B •. 0(

3 P + 0 (3P

C 40 -

C ) .. I CHg)

20

b (1+) . A' (3A9)

a (Ag)

0 2 X(39)

I • I

1 2 3

nA

Fig. 1.9.: Potential energy curves for some known electronic states of 02. Singlet states are drawn using

solid lines and triplet states are drawn using broken lines.

120

100

6

b('Ya+ ) 4

0

1

N(=K)

5

X(3) 3

J

6 5 4

4 3 2

2

0


J (=K=N)

Fig. 1.10.: The rotational structure of the 02 X(3 E-) ground and b(' ) valence states. The missing

levels are due to the zero nuclear spin of homonuclear 02. The spectroscopically observed branches of

the X( 3 Z) - b( ) transition are labelled P, PQ, Q and R as shown, see text Section 3.3.3 and

Fig. 3.7.

The Rydberg state potentials observed in Fig 1.9 relate to some of the gerade

Rydberg states of 02. Rydberg states possess a potential energy surface similar to that

of the ion-core state on which they are based and all of the Rydberg states shown in

Fig. 1.9 converge upon the lowest ionisation potential of 02, the X( 2 H g ) ion-core

state, which has the configuration

(3c g)2 (17r)4 (lltg)' (1.14)


The Rydberg states, observed in Fig. 1.9, can therefore be formed by the addition of a

Rydberg electron [usually represented by the notation n12] to the molecular ion-core

(1.14), as shown below

(3a,)2 (17t)4 (17tg) 1 n12 (1.15)

The lowest energy Rydberg states of 02 are the 3scr C(3 r'0,1,2) and d(' fl)

Rydberg states, which are based upon the lowest ionisation potential of 02. Both the

C(3 oj2)and d(' fl y ) Rydberg states have been studied extensively, primarily from

the 02 X(3 E )

state using two-photon spectroscopy. The C( 3 H0,1,2) state is found to

be predissociated, possessing large bandwidths, as a result of perturbation by the II

(3171g) state. The high vibrational levels, v ~!: 2, of the d( H) state are perturbed as a

result of an interaction with the II( 1rig) state. The C( 3 ri o,1,2), and especially the

d( u) Rydberg states shall be discussed in more detail in Chapter 3.

The higher-energy nso and ndrg Rydberg states also converge upon the

lowest ionisation potential of the ion. Although only ns, n = 4 - 5 and ndir, n =3 -

4, Rydberg states are indicated in Fig 1.9 the Rydberg series extend to much higher n.

In fact, (in Chapter 4, where the nscrg and ndir Rydberg states are experimentally

observed and discussed in greater detail) the nscrg and fld2tg Rydberg states have been

observed up to, and including, n = 9 and 8, respectively, using OODR spectroscopy.


1.4.5. The S and 0 atoms.

The electronic states of atoms are quantised, in that, excitation occurs at

discrete energies due to the lack of vibrational and rotational structure. The electronic

configurations and term energies for the electronic states of S and 0 atoms are well

known '. Both atomic species have 3Pj ground states, where the total angular

momentum, J, takes the value 0, 1 or 2. The three Jcomponents (or multiplets) of the

term are split apart by spin-orbit coupling, which is dependent, to a first

approximation, on the spin-coupling constant, A, and J,

AE=E-E 1 =AJ

(1.16)

which is known as the Landé interval rule. In general, the rule holds true for atoms

with low Z, i.e. light atoms. A can be either positive or negative, if A is positive the

components align themselves in order of increasing J from low to high energy and are

know as normal multiplets. If A is negative, as in the case of S and 0, the components

align themselves in order of increasing J from high to low energy and are know as

inverted multiplets. There is no general rule regarding the normal or inverted nature of

the multiplets arising from excited terms. For ground terms, i.e. the ground state,

when an orbital is partially filled with equivalent electrons and the orbital is less that

half full, the multiplet that arises is normal. If the orbital is more that half full the

reverse is true, and the multiplet that arises is inverted. The ground term for both S

and 0 are inverted giving three states 3P2 , 3P 1 and 3P0 in order of increasing energy.


The term differences for these three states 7 are (3P2 - 3P1) 369.055 and (3Pi - 3P0)

177.585 cm-1 for S atoms and (3P2 - 3Pi) 158.256 and (3P i - 3P0) 68.712 cm' for 0

atoms. From equation (1.16) it is possible to calculate an approximate spin-coupling

constant for the 3Pj ground state of A = -181 ± 3.5 cm' for the 0 atom and A = -73.9 ±

6.2cm 1 for the S atom.

Higher-energy states of S and 0 include the 1 D and 1 S states at 923 8.609 and

22 179.954 cm_ i for S atoms and 15867.862 and 33792.582 cm for 0 atoms. These

states, due to their singlet nature, do not show any multiplet behaviour. States of still

higher energy are Rydberg in nature and are based upon different states of the S and

0 ion-cores.

Resonant detection of atomic species can be achieved in REMPI experiments.

The two most common experimental techniques that use atomic resonant detection are

photo-fragment excitation (PHOFEX) and kinetic energy of release (KER)

experiments, see Chapter 3. The most common resonant atomic detection scheme

used is fixed wavelength (2+1) or (3+1) REMPI excitation. To better illustrate this

statement detection of the three states arising from the 3Pj term of 0 atoms will be

discussed.

The three 0 (3P2), 0 (31? 1 ) and 0 (3P0) states can be detected via (2+1) REMPI

by monitoring the 0 ion-channel between 225.5 and 226.6 nm, as shown in Fig 1.11.

The three peaks correspond to two-photon excitation of the three states arising from

the 0 (3Pj) term to states arising from the 0 (3p 3Pj) term, i.e. 0 (3p 3P) +-*-- 0 (3P)

transitions, a further single photon absorption from the 0 (3p 3Pj) term results in

ionisation of the 0 atom to O, which is detected in REMPI experiments. The two-

225.0 225.5 226.0 226.5 227.0

Co C 0)

(I) C 0

+ 0


photon transition occurs at discrete wavelengths, due to the quantised nature of atoms,

of 225.656 nm, 226.059 and 226.233 nm for the 0 02), 0 (3P 1 ) and 0 (3P0) states,

respectively. These excitation wavelengths also show experimental evidence for the

inverted multiplet nature of ground states 0 atoms.

Wavelength / nm

Fig. 1.11.: The (2+1) REMPI spectrum of the 0 (3P) states.

The excited states of 0 and S atoms can also be detected using a similar

REMPI excitation scheme, although the resonant wavelengths and the intermediate

state terms will differ from those described above.


1.5. Thesis structure.

The present work completed upon the 02 molecule follows on from work

initiated by Patrick O'Keeffe who graduated from Edinburgh University in 199928.

O'Keeffe primarily studied one-photon excitation and photolysis of ozone at

wavelengths exciting the Huggins bands of 03, see Section 1.4.2. It is energetically

possible to produce X(3 y-)

, a(1A g ) and b(' ) oxygen photofragments, together

with an 0 (3P1) atom state, from the photolysis of the Huggins bands. Kinetic-energy-

of-release (KER) experiments performed by O'Keeffe show that the one-photon

photolysis of ozone in the Huggins band region (310 - 350 nm) produces both

X( 3 ) and a( g ) state oxygen molecules together with 0 (3Pj) state atom. At

certain wavelengths, corresponding to bound levels within the Huggins band region, a

third dissociation channel is accessible resulting in a b(' E) state oxygen molecule

together with an 0 (3Pj) state atom.

O'Keeffe attempted to detect the b( 1 ), v =0 state, produced from the

photolysis of 03, using a [(1+1')+l'] REMPI detection scheme via the 3so

d( H g ) Rydberg. The 3so d(' fl g) Rydberg state is perturbed and the resulting

photolysis spectra were found to be complicated by overlapping and unassignable

rotational structure. It is therefore necessary to study the 3so d(' Hg) Rydberg state

to better understand its rotational perturbation and allow the ozone photolysis spectra

to be assigned and interpreted correctly. The experiments described in Chapter 3

investigate the 3so d('fl g ) Rydberg state using OODR spectroscopy and the

possibility of using other, less perturbed vibrational levels of the d(' ri g ) state.


Chapter 3 also attempts to use the OODR spectra to assign O'Keeffe's photolysis

spectra and estimate rotational distributions of the v= 0 state produced at

three photolysis wavelengths exciting bound regions of the Huggins band.

In Chapter 4 these experiments are extended to include the possibility of using

higher-energy nd Rydberg states as intermediate states for REMPI detection of the

b( 1 ) state. The nd, and underlying ns, Rydberg states are also of spectroscopic

interest and both Rydberg series are also thoroughly discussed in chapter 4.

In the present work the ionisation and dissociation products of SO2 is also

discussed, chosen due to its isovalence with 03. In Chapter 5, an overview of the

wavelength regions used to investigate the spectroscopy SO2 and its dissociation

products (SO, S/(02) and 0) is presented. Four specific wavelength regions in the

one-colour spectrum of SO2 are of spectroscopic interest, these being; 212 - 230 nm,

245 -295 run, 340 - 410 nm and 410 - 455 nm. Each region corresponds to a different

aspect of SO2 spectroscopy. The 212 - 230 nm and 410 - 455 nm regions correspond

to the one- and two-photon excitation of the SO2 C(B2 ) - X('A) transition,

respectively. The SO2 C(B2 ) state is discussed, in detail, in Chapter 5.

One important spectroscopic observation within the 340 -410 nm wavelength

region is that the parent SO2 ion is observed. The parent S02 ion is rarely observed

in the spectroscopy of SO2, presumably due to dissociation of SO2 prior to ionisation.

For an S02 ion-signal to be observed a resonance is required at the three-photon level

with a SO2 Rydberg state. REMPI and OODR experiments described in Chapter 6

investigate the 1,11 and III SO2 Rydberg states observed in the S02+, SO+ , S+ and 0+

ion-channels.


The fourth specific wavelength region, 245 - 295 nm, shows resonances in the

SO ion-channel that are attributed to the one-photon SO d(1 171) +- a(s) and SO

e( 1 fl) *— a( 1L)transitions. Resonances in the S ion-channel result from atomic sulfur

transitions and resonances, in SO, to a newly observed SO ion-pair state. OODR

experiments described in Chapter 7 investigate the SO ion-pair state and the coupled

SO B( 3 E) state, which is used as an intermediate state in the OODR excitation

scheme. The source of S ions within this wavelength region, 245 - 295 nm, is

discussed in Chapter 8.

1.6. References.

G. Herzberg, "Molecular spectra and molecular structure, II Electronic spectra and

electronic structure of polyatomic molecules", D. Van Nostrand company, Inc

Princeton, New Jersey, New York (1966).

W.H. Kirchhoff. J.Mol. Spectrosc. 41 (1972) 333.

R.P. Wayne, "Chemistry of the Atmosphere" second edition, Oxford University

press, Oxford (1991).

M.J. Molina and F.S. Rowland, Nature 249 (1974) 103.

J.C. Farman, B.G. Gardiner and J.D. Shanklin, Nature 315 (1985) 207.

K. Watanabe. J. Chem. Phys. 21 (1953) 1651.

C.E. Moore, "Atomic energy levels", Circular of the National Bureau of Standards

467. vol. 1(1949).


35

G. Herzberg, "Molecular spectra and molecular structure, I Spectra of diatomic

molecules", Krieger publishing company, Malabar, Florida, 2" Ed. (1989).

R.O. Jones, J. Chem. Phys. 82 (1985) 325.

S. Trajmar, J.K. Rice and A. Kuppermannn, Adv. Chem. Phys. 18 (1970) 15.

L Vuskovic and S. Trajmar, J. Chem. Phys. 77 (1982) 5436.

D. Colomb, K. Watanabe and F.F. Marmo, J. Chem. Phys. 36 (1972) 958.

J.M. Ajello, G.K. James, I. Kanik and B.O. Franklin, J. Geophys. Res. 97 (1992)

10473 and J.M. Ajello, G.K. James and I. Kanik, J. Geophys. Res. 97 (1992)

10501.

J.C.D. Brand, V.T. Jones and C. Di Lauro. J. Mo!. Spectrosc. 45, (1973) 404.

J.C.D. Brand and R. Names. J. Mo!. Spectrosc. 46, (1973) 194.

A.J. Merer. Discuss. Faraday Soc. 35, (1963) 127.

R.J. Shaw, J.E. Kent and M.F. O'Dwyer, Chem. Phys. 8, (1976) 155.

C. Braatz and E. Tiemann, Chem. Phys. 229, (1998) 93.

K. Yamanouchi, M. Okunishi, Y. Endo, S. Tsuchiya, J. Mo!. Struct. 352, (1995)

541.

M.H. Palmer and A.D. Nelson, Mol. Phys. 100, (2002) 3601.

N.J. Mason and S.K. Pathak, Contemporary Phys. 38, (1997) 289.

S.M. Anderson and K. Mauersberger, J. Geophys. Res. 100 (1994) 3033.

P. O'Keeffe, T. Ridley, K.P. Lawley and R.J. Donovan, J. Chem. Phys. 115,

(2001) 9311.

R.S. Speth, C. Braatz and E. Tiemann, J. Mol. Spectrosc. 192 (1998) 69.

C.P. Archer, J.M.F. Elks and C.M. Western. J. Chem. Phys. 112 (2000) 6293.


A.C. Bonn and F.R. Omellas, Chem. Phys. 247 (1999) 351.

J.S. Morrill. PhD. Thesis, University of Maryland (1999).

P. O'Keeffe. PhD. Thesis, Edinburgh University (1999).

Chapter 2. Experimental section. 37

Chapter 2.

Experimental section.

2.1. Introduction.

The apparatus used for the laser spectroscopy studies described in this thesis is

shown in Fig. 2.1. The individual components of the apparatus will be described in

some detail below.

2.2 The laser system.

A XeC1 excimer laser (Lambda Physik EMG 201 MSC) was used to pump

two tuneable dye-lasers (Lambda Physik FL2002 and FL3002). The XeC1 excimer

laser produced intense ( 400 mJ/pulse), ultraviolet (308 nm), short pulses ('-' 25 ns)

of monochromatic radiation. The typical operating repetition rate in our experiments

was --'5 Hz. For two-colour experiments, the excimer laser beam was split into two,

using a beam splitter, so as to pump two dye-lasers simultaneously. These two-dye-

lasers possess a bandwidth of— 0.2 cm'.

38 Chapter 2. Experimental section.

Extraction Lens mounted Lens mounted Plates lonisation on a xyz stage Soled

on a xyz stage (-21 OOV) Chamber (fflOmm) Babinet

(f'=tiOmm) Polarise

HI IPulsedi Molecul Pulse

ar Nozzle

generator TOF Beam

lye Laser Einzel Lens

/

JTube (-800V)

L2002 V on Beam Microchannol Plate Detector

10 I Computer I To Turbo Puns

Digital Oscilloscope H Boxcar

I Chan Timing - 10

_______________

Power Meter or reourder I Noun Optalnic call

XeCI Excimer Dye Laser

Laser FL3002

EMG2OIMSC 308 nm

12 Cell .. Beam

1_a Stop QLens

Photo- multiplier tube

Monoebromalor

Beam Stop

Bean( Diffraction Dye Dye Doubling Beam Turning Splitter Grating Oscillator Amplifier Crystal Separator Prism

Fig. 2.1.: Schematic diagram of the experimental apparatus used to record a REMPI-TOF-MS

spectrum.

2.2.1. The excimer laser.

An excimer is a molecule, which is bound in its excited electronic state but is

unbound in the ground state. XeC1 is such a molecule and a laser system can therefore

be easily based upon it, see Fig. 2.2(a). Since the ground state is very weakly bound,

the XeC1 dissociates rapidly, (typically in a few picoseconds) and therefore its

population is extremely low. A population inversion is created between the strongly

bound excited state and the weakly bound ground state The excited state is pumped

(produced) in an electrical discharge (typically 45 kV) through Xe and HC1 in a Ne

carrier gas. The efficiency of such a system is high, about 20 percent. Emission from

the excited state to the ground state produces the XeC1 Excimer beam (308 nm). The

output of the excimer laser is pulsed with a repetition rate of--' 5 Hz. Examples of

Chapter 2. Experimental section.

other excimer systems include ArF (193 mu), KrF (248 nn), XeF (3 51 nm), KrCI

(222 nm) and XeBr (282 nn).

2.2.2. The dye-lasers.

Dye-lasers use a series of highly fluorescent organic dyes, which absorb

strongly at the output wavelength of the pump laser. The dyes are circulated to avoid

degradation by the intense excimer source. A population inversion is provided by the

slightly different equilibrium geometries of the So and S1 states of the dye molecules,

see Fig. 2.2(b).

(a) (b)

XeCl* S, state

relaxation (f (fast)

308 nm 308 n

emission S0 state

r relaxation XeCI Xe + Cl (fast)

r

> 0) I-

w

Fig. 2.2.: Schematic representation of the (a) Excimer emission process and (b) the dye-laser pump and

probe scheme.


Molecules in the S1 state, produced by the optical pumping using the excimer

beam, relax rapidly to their equilibrium geometries before the emission of a photon

takes place. Molecules in the So state, produced by photon emission from the Si state,

must also relax to their equilibrium geometry before re-excitation can occur. The rate

of relaxation for both states is faster that the rate of emission, such that, a population

inversion is created between the S1 and So states of the dye molecule. Although only a

single fluorescence wavelength is shown in Fig. 2.2(b), in the real working system,

fluorescence is observed over a wide wavelength range due to the vibrational and

rotational sub-levels being broadened to such an extent, by collisions in the liquid,

that the levels form a continuum.

The advantage of using laser dyes is that they emit radiation over a wide range

of wavelengths. A single wavelength can be selectively produced if the laser cavity is

modified to include a diffraction grating. The wavelength of the emitted light is

scanned by changing the angle of the diffraction grating. Different dyes can be used to

produce wavelengths in the 332 - 985 nm range, as shown in Table 2.1.


Dye Range / nm Dye Range I nm

p-Terphenyl 332-350 Rhodamine 6G 569-608

DMQ 346-377 Rhodamine B 588-644

QUI 368-402 Sulforhodamine B 594-642

BiBuQ 367-405 Rhodamine 101 614-672

PBBSO 386-420 DCM 632-690

DPS 399-415 Rhodamine 700 701-768

Stilbene 1 405-428 Pyridine 1 670-760

Stitbene 3 412-443 Oxazine 750 735-796

Coumarin 120 423-462 Pyridine 2 695-790

Coumann 2 432-475 Rhodamine 800 776-823

Coumarin 47 440-484 Styryl 9 810-875

Coumarin 102 460-510 HITC 837-905

Coumann 307 479-553 IR 144 + IR 125 842-965

Coumarin 334 506-537 IR 125 890-960

Coumarin 153 522-600 IR 140 882-985

Table 2.1.: Laser dyes and their wavelengths used for a XeCI excimer pumped dye-laser.

2.2.3. Frequency doubling (2 w' harmonic generation).

It may be noted from Table 2.1 that wavelengths below 332 nm cannot be

generated using the fundamental output of any of the dyes. To increase the available

range of wavelengths, it is necessary to use non-linear optics. The non-linear optics

used in this study include beta-barium borate (BBO type I and II) and potassium

dihydrogen phosphate (KDP) crystals. These crystals can frequency double radiation

which is incident upon them between 440 - 630 nm, 410 - 444 rim and 530 - 670 nm,

respectively. Frequency doubled radiation therefore extends the range of available

wavelengths by 205 - 335 tim. The crystals also have the effect of rotating the plane

of polarisation of the incident laser radiation by 90 degrees, which must be taken into

account during polarisation experiments. Frequency doubled radiation is separated


from the fundamental by the use of a coloured filter (UGS) (for wavelengths> 500

nm) or a Pellin Broca prismatic beam separator.

The non-linear effect of the frequency doubling crystals can be explained as

follows: the oscillating electric field, E, of the intense monochromatic dye-laser

radiation induces an electric dipole, p, dependent upon the polarizability, a, of the

crystal (equation (2.1)). The polarizability being a measure of the degree to which the

electrons in the crystal can be displaced relative to the nuclei.

1u=aE+ 1/2 P E2 +

(2.1)

where f is known as the hyperpolarizability. The effects due to the second (or higher)

terms in the series are referred to as non-linear effects, since they arise from terms

which are non-linear with respects to E. These effects are normally small, but at high

laser powers they become important.

since EA sin cot

then E2 = 1/2 A2 (1 - cos 2ot)

(2.2)

where A is the amplitude. From equation (2.2) it can be seen that the non-linear I

term is dependent upon twice the linear laser frequency, 2a.


2.3. Polarisation methods.

The Fundamental laser radiation originating from our dye-lasers is vertically

polarised. Thus, the final output radiation is either horizontally or vertically polarised

depending upon whether or not frequency doubling is used. The plane of polarisation

of the radiation, relative to the TOF axis, is altered in this study using a Soleil Babinet

compensator. The laser radiation, from either dye-laser, can therefore be set to be

perpendicular or parallel to the TOF axis. The Soleil Babinet compensator can also be

used to produce either left or right handed circularly polarised light.

2.4. Laser calibration methods.

Calibration of the laser wavelength was achieved by simultaneously recording

one of two types of spectrum; the neon optogalvanic spectrum and the 12 fluorescence

excitation spectrum. In both techniques, the fundamental, not the frequency doubled,

wavelength is calibrated. The beam splitter used, in both cases, only sends - 10% of

the fundamental beam into the optogalvanic or 12 cell.

The use of the neon optogalvanic spectrum is a well-established calibration

technique. It is achieved by measuring the voltage and current changes induced in the

optogalvanic cell by laser radiation. In a plasma, whenever the wavelength of a laser

coincides with absorption by an atomic or molecular species, the rate of ionisation of

the species momentarily increases or decreases due to the increase in the excited state

population. Such changes in the ionisation are monitored as a variation in the plasma


current. For the experiments reported here a Pb/Ne hollow-cathode lamp was used. A

spectrum of neon absorption lines was therefore recorded by monitoring the

optogalvanic response and the wavelengths at which peaks occurred were calibrated

with the known atomic resonances of neon 1 . Calibration using the neon optogalvanic

response can be achieved from wavelengths between 335 - 670 nm, although several

wavelength regions within this range show little optogalvanic activity.

Calibration via 12 fluorescence excitation is achieved in a similar way. An

iodine cell is irradiated with laser radiation, which is absorbed if the wavelength of

the radiation correspond to an absorption band in iodine. The total fluorescence from

the relaxation of excited iodine molecules is recorded using a photomultiplier. The

recorded fluorescence excitation spectrum is then calibrated by comparing it to the

known iodine absorption spectrum 2 Calibration by 12 fluorescence can be achieved

for wavelengths 500 - 675 rim.

Wavelength calibration of some of the SO 2 / SO spectra has also been

achieved using (2+1) or (3+1) REMPI sulfur atomic resonances

2.5. The molecular beam.

A molecular beam consists of a sample gas expanded from a high to low

pressure through a small pinhole (250 gm). A high-pressure gradient is achieved by

evacuating the chamber in which the molecular beam is to expand to a background

pressure of 1 x 10 -6 Ton and using a relatively large backing pressure of the sample

gas ( 760 Ton). Under such conditions numerous collisions occurring in, and


immediately beyond, the nozzle converts the random motions of the sample gas into

mass flow in the direction of the beam. The temperature of the beam is therefore

greatly reduced, since relative to one another the atoms and molecules have an

extremely low velocity.

The molecular beam was generated from pre-prepared bulbs of SO2 seeded in

He (He 95%, SO2 5%) or 100% 02 at a pressure of 1 atmosphere (760 Ton). The

gaseous samples pass through a pulsed nozzle (General Valve, 250 .tm diameter

orifice), into the ionisation region of a linear time-of-flight mass spectrometer. The

typical repetition rate of the pulsed nozzle was —5Hz with a pulse duration of —180-

300 p.s, driven by a Farnell (PG 102) pulse generator.

2.6. Time-of-flight (TOF) mass spectrometer (MS).

A schematic diagram of the time-of-flight mass-spectrometer (TOF-MS) is

shown in Fig. 2.3. The spectrometer consists of two main regions, the ionisation

chamber and TOF tube. Within the ionisation chamber ions are generated via the

interaction between the laser beam(s) and the molecular beam. Acceleration of the

ions into the TOF tube is achieved by an electric field applied to the repeller plates.

The ions then move through the TOF tube under field free conditions and finally

encounter a large potential drop where they are detected by a microchannel plate

(MCP) detector.

The ionisation chamber consists of a small hollow metal cube of dimensions

70 x 70 x 70 mm. Quartz windows are mounted on two opposing faces of the


chamber, through which the counter-propagating laser beams pass. The remaining

four faces are shown schematically in Fig. 2.3 and it can be seen that the laser beams

travel orthogonal to both the molecular beam and the TOF axis. This allows a specific

region within the molecular beam to be probed and permits ionised particles to be

selectively accelerated in to the TOF domain. Any unionised gases are simply

pumped away by an oil diffusion pump (with a liquid nitrogen trap to reduce any oil

contamination) backed by a Maruyama Type 150 rotary pump. The major advantage

of using such a small ionisation chamber is that short focal length lenses (60 mm) can

be positioned outside the ionisatibn chamber to produce high power densities in the

focal regions of the laser beams. The position of the lenses can be seen in Fig. 2.1.

The high laser power density allows weak or multi-photon transitions to be observed.

Also, since the lenses are mounted externally, they can be easily manipulated using a

XYZ stages to allow the best possible control over the laser beams positioning and

overlap.


Repeller (-21OOV)

1' Molecular I I

Beam

Field compensati Pulsed

ring electrode -i J Nozzle

To Diffusion o Pump

Extraction Cone Laser

(

Beam OV)

Einzel Lens

IF (--800V)

Quarter Turn Valve

Field Free

TDrift Region

Turbo Pump

Plate Detector

Fig. 2.3.: A schematic diagram of the TOF-MS . spectrometer.


The iónisation chamber also encompasses the ion extraction optics which

consist of two conical electrodes mounted to the top and bottom faces of the chamber,

see Fig. 2.3. The two electrodes are separated by 16 mm such that the molecular beam

and the laser beams intersect perpendicularly, midway between the two electrodes. In

REMPI experiments the lower electrode (extraction cone) is held at earth (0 kV) and

the upper, repeller, electrode is typically held at +2.1 kV. Field compensation ring

electrodes, shown in Fig. 3.2, are added to the design of the extraction optics to

providing a uniform electric field, which would otherwise be deformed by the conical

shape of the electrodes. Since a positive voltage is typically applied to the repeller,

only positive ions are extracted and accelerated towards the TOF region. Negative

ions and electrons will be accelerated toward the positive repeller plate. The lower

grounded extraction cone has a small aperture of 2 mm diameter, which allows the

positive ions to travel from the ionisation chamber into the TOF chamber.

Immediately following the extraction optics the positive ions encounter an Einzel

lens, which collimates the ion beam allowing increased ion detection efficiency.

Typically, +700 to +900 V is applied to the Einzel lens.

The positive ions then move under field-free drift condition through the time-

of-flight tube (561 mm), Figs 2.2 and 2.3. The flight tube is independently pumped by

a turbo pump (Turbotronik NT150/360) backed by a Maruyama 150 rotary pump.

After travelling down the flight tube, the ions hit the front of the microchannel plate

detector and produce a cascade of electrons, which are collected by an anode. This

signal is monitored using a digital oscilloscope (LeCroy 9344C) and processed using

a Stanford Research Systems SRS250 boxcar integrator. The resulting spectra were

recorded on a PC and a chart recorder.


The pressures in both chambers are monitored using Pirani and Penning

gauges (Edwards). Under operating conditions the pressure in the ionisation chamber

could reach 8 x 10 -6 Torr whereas the pressure in the time-of-flight chamber was

typically :!~ 2 x 10 Ton.

2.7. References.

G.R. Harrison, "MIT Wavelength Tables", John Wiley & sons, I nc, New York,

(1939).

S. Gerstenkorn and L. Luc, "Atlas du spectra d'absorption de la molecule de I'ode

entre 14800 - 20000 cm 1 ", presses du CNRS, Paris, (1978).

C.E. Moore, "Atomic energy levels", Circular of the National Bureau of Standards

467. vol. 1(1949)181.

Chapter 3. The 3s d(' fi g ) Rydberg state of 02 recorded via v = 0 of the b(' E') valence state 50

Chapter 3

The 3so d('rlg ) Rydberg state of 02: Its OODR

spectrum recorded via v = 0 of the b(' ) valence state

and its possible use as a monitor of the b(' ) state

produced by the photolysis of the Huggins band of

ozone.

3.1. Introduction.

The experimental studies discussed in this Chapter follow on from work

initiated by Patrick O'Keeffe, who graduated from Edinburgh University in 1999 1 . In

this work, 02 b(' Z' ) state molecules were observed, for the first time, from the one-

photon photolysis of 03. Specifically, the 03 V(A l ) -> 02 b(' ) + 0 (3P)

dissociation channel was observed when bound levels of the Huggins band were

excited 2-4 The 02 b(' Z' ) state was detected directly by [(1+1 ')+ 1'] REMPI via both

v = 1 and 2 of the 3so d(' Hg ) Rydberg state and indirectly by kinetic-energy-of-

release (KER) experiments monitoring the 0 (3Pj) co-fragment. These results are

summarised in Section 3.2.

Both v= 1 and 2 of the 3so d('fl g ) Rydberg state are perturbed and one

major impetus for the present work was to find alternative probe schemes for the

detection of 02 b(' )state molecules. The experiments described in this Chapter

investigate the possibility of using other, less perturbed vibrational levels of the 3so

Chapter 3. The 3s d(' Hg) Rydberg state of 02 recorded via v = 0 of the b(' ) valence state 51

d(' ri g ) Rydberg state. In Chapter 4 these experiments are extended to include the

possible use of higher-energy nd Rydberg states. In both sets of experiments the

b(' Y-' ) state is optically pumped from v 0 of the 02 X( 3 ) ground state.

3.2. Previous experiments detecting the b(' E )valence state.

3.2.1. Photo-fragment-excitation (PHOFEX) spectroscopy of ozone in

the Huggins band region.

A two-colour REMPI experiment, where the pump laser (312 - 345 nm) is

responsible for dissociating 03 and the probe laser (226.233 nm) resonantly detects

the 0(3 Po) atomic dissociation fragment via (2+1) REMPI, as previously discussed in

Section 1.4.5, is considered first. Experiments such as these produce photo-fragment-

excitation (PHOFEX) spectra, where the resonantly detected fragment's population is

probed as a function of photolysis wavelengths (or energies). The PHOFEX spectrum

recorded by O'Keeffe' for dissociation wavelengths between 312 - 345 nm, detecting

the 0 (3P0) atomic fragment, is reproduced in Fig. 3.1. The vibrational assignments

used in Fig. 3.1 refer to the (v i ',v2 1,v3 ') levels of the Huggins band, where v 1 ' =

symmetric stretch, v2' = bending mode and v3' = asymmetric stretch.

4u 0

L , 0 -

I

0

Chapter 3. The 3scrg d(' Hg) Rydberg state of 02 recorded via v = 0 of the b(' ) valence state 52

8

6

• I • I • I • I • I • I •

-I

-1

•

(a) (c)

Kr

3000

2000

04

, 1000

310 315 320 325 330 335 340 345

Photolysis Wavelength/ rim

Fig. 3.1.: The PHOFEX spectrum of 0 (3P0) fragments produced via photolysis of ozone in the 312 to

345 nm region recorded by detecting the 0 (3P0) state via the (2+1) 3p (3P) +--- 2p (3P0) REMPI

transition at 226.233 rum. Sections (a), (b) and (c) were recorded with frequency doubled RiO! and

DCM and the fundamental of PTP respectively. The 226K absorption spectrum of Molina and Molina 5

(dotted spectrum) and the vibrational assignment of the features investigated by O'Keeffe are also

shown. Adapted from Fig. 1(a) Ref. 4.

3.2.2. The Kinetic-Energy-of-Release (KER) spectra

Kinetic-energy-of-release (KER) experiments are time-resolved versions of

fixed photolysis wavelength PHOFEX experiments. The fixed wavelength photolysis

laser dissociates 03 molecules at the laser focus, the probe laser then resonantly

ionises the 0 (3Pj) co-fragments whilst they are still at the laser focus. The 0 ions

(and the 02 co-fragments) are then allowed to drift apart under field free conditions at

velocities determined by the dissociation event. After a fixed time delay an extraction

Chapter 3. The 3scr d(' hg) Rydberg state of 02 recorded via v = 0 of the b(' E') valence state 53

pulse is applied to force the 0 ions into the time-of-flight region of the apparatus

where the ions again move under field free conditions. The time-of-arrival at the MCP

detector is determined by the several factors including the fixed time delay, the

extraction voltage and the velocity of the 0 (3Pj) co-fragments (both speed and

direction) imparted during dissociation. Only ions that move with a velocity towards

or away from the time-of-flight chamber are detected due to the core extraction

condition, which are typically used in KER experiments. The time-of-arrival of the O

ions can be indirectly related to the internal energies of the 02 co-fragments produced

during dissociation.

KER experiments were reported following photolysis at various wavelengths

in the Huggins bands. An example is shown in Fig. 3.2. The three raw TOF profiles

were recorded by averaging over 3000 shots with a 200 ns delay between the laser

pulse and the high voltage extraction pulse. The profiles were recorded using

radiation polarised perpendicular to the TOF axis. Only partial core-extraction was

used to allow an ion-signal to be observed for off-resonance wavelengths, profile (b).

Profile (a) shows the one-colour probe-only spectrum where the 0 ions are formed

from the dissociation of ozone at 226.333 nm. This profile should be subtracted from

further experimental data, although this has not been attempted in Fig. 3.2. Profile (c)

was recorded whilst exciting the (63 1) level of the Huggins band at 317.64 nm.

Chapter 3. The 3sag d( l Fig ) Rydberg state of 02 recorded via v = 0 of the b(' E') valence state 54

(c)

0

O(P)

0, (h L

E4)

O , (a t)

O,(X 3E)

45 5.0 5.5 6.0

Time- of-Flight/ jis

Fig. 3.2.: The partially core-extracted TOF profiles of the 0 (3P0) fragment produced from the

photolysis of 03 at various wavelengths in the Huggins bands. Profile (a) is the probe signal only which

has not been subtracted from the other profiles while (b) and (c) are the TOF profiles produced

following photolysis of ozone at 316.64 nm (off-resonance) and 317.64 nm (on-resonance, the (63 1)

Huggins band see Fig. 3. 1), respectively. Taken from Figure 5 Ref. 4.

Chapter 3. The 3s d( I Fi g ) Rydberg state of 02 recorded via v = 0 of the b(1 ) valence state 55

It was possible to observe, in Fig. 3.2(c), the on-resonance profile, a trimodal

distribution of internal energies relating to 02 X( 3 ), a( g ) and b(' )

production. No significant 02 b(' Z) state peaks can be observed in the off-

resonance profile shown in Fig. 3.2 (b). Spectra such as these showed, for the first

time, experimental evidence that the b(' ) state is produced directly from the

dissociation of the bound levels within the Huggins band region of ozone.

3.2.3. Previous detection of the 02 b(' )valence state via the

3so d('H g )Rydberg state using a [(1+1')+1'1 REMPI

excitation scheme.

The b(' ) state oxygen molecule can also be resonantly detected using a

[(l+l')+l'J REMPI excitation scheme via the 02 3so d(' fi g ) Rydberg state. The

d(' hg) state, the lowest energy singlet Rydberg state of 02, was chosen to monitor

the b(' Y- ) state population since it had been previously studied by many groups,

primarily from the X(3 Z ) ground state 713, although the a('A g ) state (generated

using a microwave discharge) had been used in later studies 13-15

Experiments where the b(' E) state population, produced from one-photon

fixed wavelength photolysis of ozone, was resonantly detected using a [(I + I ')+ 1']

REMPI detect scheme via v =2 of the d( l H g ) Rydberg state has been reported 3 . The

resulting spectra, which will be discussed in Section 3.4, were found to be

complicated by overlapping and unassignable rotational structure resulting from

perturbations in the d(' H g ) Rydberg state rotational structure. While the perturbed

Chapter 3. The 3s d(' H g ) Rydberg state 0102 recorded via v = 0 of the b(' ) valence state 56

rotational contours of the v =2 level provided a distinctive fingerprint for its

identification, it made the determination of a rotational distribution of the 02

) state almost impossible.

A further experimental difficulty resulted from the inability to ionise the

d( H g ) state, v :5 2 levels efficiently due to the energy requirements of the probe

laser. For this reason a [(1+1')+l'] REMPI excitation scheme was chosen to be more

efficient that a (2+1) scheme for the observation of the v :!~ 2 levels.

3.2.4. Previous OODR study of v = 3 of the perturbed 02

3so d(' H g ) Rydberg state excited via single rotational

levels of the b(' E') valence state.

It was therefore necessary to study the d(' H g ) Rydberg state more closely to

better understand its rotational perturbation and allow the ozone dissociation spectra

to be assigned and interpreted correctly. O'Keeffe etal. 16 studied v = 3 of the

d(' 11lg) Rydberg state, as ionisation of this level could be efficiently achieved using a

(2+1) excitation scheme. The experimental detail will be discussed in Section 3.3.2.

Briefly, single rotational lines of the known (0,0) band of the b - X transition in 02

were pumped. The b(' Y- ' ) state single J levels excited were then detected using (2+1)

REMPI excitation via v =3 of the d(' fl g ) Rydberg state. The resulting OODR

spectra confirm that the v =3 level also undergoes strong rotational perturbation. The

OODR spectra, recorded by O'Keeffe et al. 16 are shown in Figs. 3.3 and 3.4. The

Chapter 3. The 3sc d(' hg) Rydberg state of 02 recorded via v = 0 of the b(' E') valence state 57

spectra and their analysis will be discussed in greater detail, in Sections 3.3.6 and

3.3.8.

In the present work the OODR studies of v = 3 of the d(' Ji g ) Rydberg state

has been extended to include the v =0 - 2 levels with the purpose of finding a suitable

probe scheme for the detection of the b(' E' ) state rotational distribution produced

from the photodissociation of ozone. These studies are described in Section 3.3.

Chapter 3. The 3s d(' [I g) Rydberg state of 02 recorded via v = 0 of the b(' E') valence state 58

,,S

O(b,v=OJ=O) Q

O'P' 4

0(bv=0,J=2)

R

L__

02 (bv=0,J=4) P R

0(bv=0,J=6)

_j

0 R S

Q A o (b,v=OJ=8)

2 o P

58750 58775 58800 58825 58850 58875

Two-Photon Energy / cm- '

Fig. 3.3.: (2+1) REMPI spectra of v =3 of the d(' Hg ) Rydberg state, excited from various rotational

levels, 4, of v = 0 of the b(' E) state, in which essentially unperturbed rotational levels of the

Rydberg state are accessed. Adapted from Fig. 3 Ref. 16.

0

C

Chapter 3. The 3scrg d(' Hg ) Rydberg state of 02 recorded via v = 0 of the b(' ) valence state 59

P O(bv=O)

jj LJ"= 12,

JJ'JIJ

J'=14 R'

R OF

s o

J 16

:

.0 OF ULL 4 OF

i I iiii

58700 58750 58800 58850 58900 58950 59000

Two-Photon Energy / cm7 1

Fig. 3.4.:(2+1) REMPI spectra of v =3 of the d(' Hg ) Rydberg state, excited from various rotational

levels, .4, of v =0 of the b(' E' ) state, in which perturbed rotational levels of the Rydberg state (J')

are accessed. The starred band is due to a (3+1) signal from the ground states 02. Adapted from Fig. 4

Ref. 16.

0

0

Chapter 3. The 3s d(' fig ) Rydberg state of 02 recorded via v = 0 of the b(' E') valence state 60

3.3. An OODR study of v = 0 -2 of the perturbed 02 3so

d(' H g ) Rydberg state excited via single rotational levels of

the b(' )valence state.

3.3.1. Previous studies of the 02 3sclg Rydberg states.

The one-photon optically forbidden gerade Rydberg states of 02 are also of

great spectroscopic interest. The lowest-energy gerade Rydberg series converging

upon the ground X( 2 11 g ) ion-core state are the 3scr states. The 3scrg C( 3 H g )and

3scrg d( fi g ) states have previously been investigated using (2+1) REMPI. 7-15

The Q' 1_1 g ) state, which lies at lower energy than the d( H g ) state (-j 800

cm'), is a typical triplet state, in that it comprises three Q components, 0 = 0, 1 and 2

arranged in order of increasing energy. Although all three triplet states were observed

via (2+1) REMPI from the triplet X( 3 E) ground state only the 92 = 1 component was

observed from the singlet a('A g ) state due to a spin-orbit interaction (- 98% 3flig, 2%

1 111ig)'3 with the two-photon allowed d(' fi g ) state. The observed spectra show rich

rotational structure, which can only be partially resolved due to the relatively large

linewidths produced by predissociation.

Previous studies pertaining to the d(' H g ) Rydberg state revealed that, v =

0 of the d( H g ) Rydberg state is unperturbed, indicated by a regular rotational

distribution which possesses a B value similar to that of the 02 ground ion-core state.

However, V = I shows signs of a slight perturbation for 1= 10-14, indicated by a

broader bandwidth. For v = 2 and 3 of the d(' H g ) Rydberg states are significantly

Chapter 3. The 3s d(' fIg) Rydberg state of 02 recorded via u = 0 of the b(' E' ) valence state 61

perturbed, reflected in irregular band contours and varying rotational linewidths,

caused by interactions with a spectroscopically "dark" II ( ' Hg) valence state. The

crossing between the d(' H g ) and the II ( 'flg) states occurs between the u = 2 and v =

3 levels of the d Rydberg state, which were observed to be perturbed most strongly.

Typical two-photon spectra for both the 3so C(3 fl g ) and 3so d(' Ji g ) states

from both the X( 3 >.) and a('A g ) states are reported in a review article by Morrill et

al. " Linewidth information for both the 3so C(3 11 g ) and 3so d('fl g ) states from

both low and high temperature REMPI and KER spectra and theoretical CSE

(coupled-channel-Schrodinger-equation) calculations are reported 14 Due to the

variability and uncertainty of the Jdependence upon linewidths, CSE calculations

were modelled for both J = 1 and J = 15.

Linewidths for the 3sa C( 3 fi g ) state exhibited an oscillatory vibrational

pattern expected for an outer crossing between the C(3 11 g ) Rydberg and JJ(3 fig)

valence states, with maxima for v = 1 ( - 320 cm') and 3 (- 90 cm-1 ), and minima for

v =0 ('-j

50cm 1 ), 2 (- 6 cm-) and 5 (- 2.6 cm'), in agreement with experimental

observations.

Linewidths for the 3so d(' Jig ) state v ~: 2 are complicated by bound-bound

interactions with the 11( fI g ) valence state for energies below the 0 ('D) + 0 ('D)

dissociation limit. Linewidths for the lower vibrational levels were found to be

dominated by contributions from the triplet predissociation channels via spin-orbit

interactions between the d( fl g) and the C( 3 H g ) states. The low J linewidths showed

maxima for v2 (- 5.5 cm) and 5 ('-S 100 cm - 1 ), and minima for v = 0 (= 0.7 cm'),

3 (- 0.6 cm) and 7 (- 5.5 cm'), intermediate values for v = I (— 2.8 cm4), v = 4 (-S

Chapter 3. The 3sc d(' IT 5 ) Rydberg state of 02 recorded via v = 0 of the b(' ) valence state 62

7.8 cm) and 6 ('-j 7 cm'). Higher J linewidths showed a significant increase for v

2 (from 5.5 cm' to - 20cm 1 ) due to a near degeneracy with v= 8 of the II('fl g )

state, which is heavily predissociated through 1( fl g) - 11(1 U 5 ) interactions.

3.3.2. Experimental.

A schematic potential energy diagram showing the "pump and probe" scheme

used to record the OODR spectra is shown in Fig 3.5. Two different excitation

schemes were used, a [1+(2')+l'] OODR scheme, Fig. 3.5(a), used to detect v = 2 of

the d( 11 g ) Rydberg state and a [1 +(I + I ')+ 1'] OODR scheme, Fig. 3.5(b), used to

detect v =0 and 1. Different schemes were used due to the inability to ionise both v =

0 and 1 using a single probe photon in a [1+(2')+1'] OODR scheme. The

nomenclature used to describe the OODR scheme is discussed in Fig. 3.6.

1:

ii

120

III"

E C)

C.,

>..

ci)

W

Cu

ci) 0 0.

U

80

2) 60 I)

W Cu

40 ci) 0 0

20

0 0

Internuclear distance I A

Chapter 3. The 3scrg d(' Hg ) Rydberg state of 02 recorded via v = 0 of the b( 1 Z ) valence state 63

(b)

\

0'* X('11.)

A-•- C3n9

•d1119

\ H1A9

."ll'fl 3 - ...

... 11119

1,3

b

a

• X 3z9•

I • I • I • I • I • I • I •

).5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Internuclear distance I A

Fig. 3.5.: Potential energy diagram for 02 showing the relevant electronic states and the pump and

probe scheme for the detection of (a) the d(' Hg ), v = 2 Rydberg state detected via a f +(2')+l']

excitation scheme and (b) the d(' Fig ), v = 0 Rydberg state detected via a [1 +(1 +1 ')+ 1'] excitation

scheme. Note: the d(' Hg ), v = 1 Rydberg state must also be detected via a [1 +(1+1 ')+l'] excitation

scheme.

Experiments performed upon the d(' fl g) Rydberg states required a two-laser

set-up. A 30/70 beam splitter was incorporated into the laser set-up. 30 % of the

excimer power was used to optically pump the fundamental output of R700 dye

generating 760 rim radiation ( 5 mJ per pulse). This radiation was used to pump the

(0,0) band of the b — X transition. 70 % of the excimer power was used to optically

pump the dyes DMQ and QUI ( 15 mJ per pulse) used to probe the d —+-- b

transitions in a (2')+ 1' REMPI scheme. For the (1 + I ')+ 1' REMPI excitation scheme,

Chapter 3. The 3sog d(' fi g ) Rydberg state of 0 2 recorded via v = 0 of the b(' E) valence state 64

frequency doubled, C307 (-j 3 mJ per pulse) was used to generate the required probe

photons.

The number before the round brackets describes the number of photons used to pump the first resonant transition. In these experiments the pump photon relates to the b - X transition. I.e. the red photon in Fig. 3.5. Note: The pump photon is denoted by an unprimed number i.e. 1 not 1'.

[1+(2')+l']

[1 +( 1 + 1 ')+ 1']

The number(s) inside the round bracket describes the number and origin of the photon(s) used to probe the second resonant transition. In these experiments this transition relates to the d -*-- b transition. Note: The pump photon is denoted by an unprimed number and a probe photon is denoted by a primed number i.e. 1 (pump) and 1' (probe).

The final number after the round bracket denotes the minimum number and type of photon(s) used to ionise the upper state. In these experiments a single probe photon is generally used to ionise the d state. An exception occurs when v = 0 or 1 of the d state is probed using 2' photons, the overall scheme would then be [1+(2')-i-l+l'] or [I+(2')+2'1.

Fig. 3.6.: The nomenclature used to describe the OODR excitation scheme.

The pump laser was calibrated from known b - X transition energies 17 The

probe laser was calibrated by simultaneously recording the neon optogalvanic

response and its one-photon fundamental energy was estimated to be accurate to ± 0.2

cm

Chapter 3. The 3sc d( 1 H g ) Rydberg state of 02 recorded via ii = 0 of the b( E) valence state 65

3.3.3. The b - X transition in 02.

Most electronic transitions are electric-dipole transitions, which possess the

following selection rules for a single photon transition.

AA = 0, ±1 (E.g. - , U - allowed, A - , 1 forbidden) (3.1)

AS = 0 (E.g. ' - 'Z allowed, ' - forbidden) (3.2)

A=0,±1 (3.3)

+ 4-> + and - - for an allowed transition. (3.4)

g 4-> u and u *-* g for an allowed transition. (3.5)

The b(' E ) X(3 ) transition is therefore an electric-dipole forbidden one-

photon transition, since the transition is between a 1 1 4- 3y(AS = 1), +4— - and g +-*

g. If the magnetic-dipole, not the electric-dipole, is used to pump the b - X transition

then the transition becomes allowed and can be optically pumped. Magnetic-dipole

transitions tend to be 10 5 times weaker than electric-dipole transitions.

Some of the energies at which the b +- X transition can be pumped are shown

in Fig. 3.7. The two spectra, Figs. 3.7(a) and (b), probe Jb =0 - 6 at 339.934 rim and

Jb = 12 -24 at 351.021 nm, respectively. The probe wavelength in (a) is able to probe

the(d('1T1 5 ), v=3)4-4—(b('), v0) transition at-58835cm'(2x339.934

rim in cm'), for the following rotational transitions (2,0), (3,2), (5,4) and (7,6) which

corresponds to the S-branch observed via 02 (b, v =0, J =0) and the R-branches

observed via 02 ( b , v = 0, J = 2, 4 and 6) respectively, as observed in Fig. 3.3. The

Chapter 3. The 3s d(' fi g ) Rydberg state of 02 recorded via v = 0 of the b(1 E) valence state 66

probe wavelength in (b) is able to probe the (d(' U ), u =2) - (b(' ), v =0)g

transition at 56977 cm-1 (2 x 351.021 nm in cm'), for the following rotational

transitions (11,12), (13,14), (15,16), (17,18), (19,20), (21,22) and (23,24) which

corresponds to the P-branches observed via 02 (b, v = 0, J = 12, 14, 16, 18, 20, 22

and 24) respectively. It is important to note that due to the zero nuclear spin of the

homonuclear 160 only even rotational levels of the b(' E ) valence state exist, see

Section 1.4.4. The labelling scheme used in Fig. 3.7 has been taken from that of S-L

Cheah etal. 17 and their transition energies match closely with those observed in Fig.

3.7.

Pp

Jb 6 4 2 0

PQ

--t-1 J, =6 4 2

(a)

[ R

Jb2 4 6 8 10 14 I I 1822

rQ

b2 4 6 810 14 I I Ii 1822

(b)JJUJU

RZ 10

Co C 0)

U)

0 +

c..l

0

13090 13100 13110 13120 13130 13140 13150 13160 13170

b ( 1 E9

) < X (3ç) transition energy / cm -1

Fig. 3.7.: The rotational structure of the (0,0) band of the b <— X transition recorded probing (a) Jb = 0

- 6, probed at 339.934 nm and (b) .4 = 12 -24 (in the r and Q region) probed at 351.021 nm. Note: only even rotational levels of the b(' Z) state exist. The rotational branches have been labelled

according to S-L Cheah et al. 17 The pump wavelengths used in the OODR experiments correspond to

the Pp branch for 1b = 0 and the r branch for .4 = 2 - 24.

Chapter 3. The 3sog d(' hg) Rydberg state of 02 recorded via v = 0 of the b(' E' ) valence state 67

3.3.4. The HönI-London factors for the two-photon d --- b transition

in 02.

Hönl-London factor calculations have been used to simulate the rotational

band intensities for a U +-4-- Z, two-photon transition (d —+— b); see Equations

(3.6) - (3.10).

0 branch (AJ = -2) (J±-1)(J±)(J±-2)(J±) (3.6)

15J(J- 1 )(2J-1 )(2J+1)

P branch (M = -1) (J±-1)(J±)(J±+ 2 (3.7)

30J(J+1)(J-1) (2J+1)

Q branch (AJ = 0) (J±+1)(2)±1)2(2J+1)(J±Q) (3.8)

1 OJ(J+ 1)(2J- 1)(2J+3) )(2J+ 1)

R branch (AJ = + 1) (J± ~+1 )(J±c~+2)(J±2 ~)2 (3.9)

30J(J+1XJ+2)(21+1)

S branch (J = +2) (J±+1 )(J±}4-2)(J±c+3)(J±c2+1) (3.10)

I 5J(J+1)(J+2)(2J+3)(2J+1)

Equations (3.6) - (3.10) were taken from the work of Bray and Hochstrasser 18; note

Equation (3.7) has been corrected from its incorrectly cited form 19 and a degeneracy

factor of (2J+1) has also been removed from the Equations (3.6) - (3.10). The

resulting 1 16n].-Lôndon factors were then plotted against the rotational level of the

b('Y- ' ) valence state, Jb, shown in Fig. 3.8. It can clearly be seen from Fig. 3.8 that

the intensities of the 0-, P-, R- and S-branches converge (0.0 1667 at Jb = 5000), as Jb

increases to infinity. The Q-branch, although present at low values of Jb, is very weak

and by Jb ~: 6 the Q-branch is essentially absent. When Jb = 0 is populated only one

branch, the S-branch, is observed and when Jb = 2 is populated the P-, R- and 5-

,M w e

S.

- 0.05 I- 0

0.04 CU

0.03

o 0.02 —J

:0 0.01

I 0.00

Chapter 3. The 3sog d(' fi g ) Rydberg state of 02 recorded via v = 0 of the b(1 ) valence state 68

branches are observed. The overall general trend for the 0-, P-, R- and S-branches

observed at higher values of Jb is weak, strong, weak, strong respectively.

sIij J 0-branch, &J=-2

P-branch, J=-1

Q-branch, tJ=O R-branch, EJ+1

IJ S-branch, AJ=+2 -

0 2 4 6 8 10 12 14 16 18 20 22 24

b level.

Fig. 3.8.: HOnI-London factors for a [I +-+-- , two-photon transition using linearly polarised light, for

b (J", the rotational level of the lower state) = 0 - 24. The distributions are calculated from Equations

(3.6)- (3.10).

3.3.5. An overview of the d('Hg )Rydberg state (v = 0-2) region

observed via OODR spectroscopy.

An overview of the OODR spectrum recorded for the d( fI g ), v =0 - 2

Rydberg states region via the b( Z' ), v = 0, J = 0 using a [1+(2')+1'] excitation

scheme, is shown in Fig. 3.9. It is possible to observe, in Fig. 3.9, the relative

intensities for the three d state levels assigned. It is perhaps unsurprising that, the d,

348 350 352 354 356 358 360 362 364 366 368 370 372 374 376 378

C D)

CI) C 0

+ 04

0

Chapter 3. The 3s d(' Hg) Rydberg state 0102 recorded via v = 0 of the b(' E ) valence state 69

Probe wavelength / nm

Fig. 3.9.: Spectroscopic overview of the wavelength region required to excite (n, 0) of the

d4—'--b transition, where n0 -2, via the b('), v= 0, J= The b( 1 Y, +), v0,J=0 state

is produced by pumping at 762.309 nm (13118.04 cm '), as shown in Fig. 3.7(a). The ladder indicates

the position of d(' Hg ), v = 0, 1 and 2, J =2 states. The arrows represent the three-photon ionisation

thresholds for the 02 X( 2 H(112,3/2)g ) ion-core states from the b(1 ), u = 0, J =0 state. The structure

between 350 - 351 nm and 369 - 375 nm is due to one-colour probe only excitation of ground state 02.

The weaker unassigned features are due the three-photon probe only excitation of the b(' ), u =0, J

=0 state 20 Note: both v =0 and 1 of the d state cannot be ionised by the absorption of one further

photon in this experiment.

v = 2 level possesses the greatest intensity due to the fact that it is possible to ionise v

=2, albeit only to v =0 of the ion, with one further probe photon, whereas both the v

=0 and 1 levels cannot be ionised directly. The three-photon (probe) ionisation

thresholds for the O2 X(2 11(, / 2,3/2)g) ion-core states from the b( 1 ), v = 0, J = 0

state are indicated in Fig. 3.9. The d, v =0 band possesses an intensity significantly

Chapter 3. The 3sci d(' H g ) Rydberg state of 02 recorded via v = 0 of the b(' E ) valence state 70

greater than that of the d, v = 1 band due to accidental resonant enhancement of the

d, v =0 state signal, at the three-photon level above the b(' E) state which increases

the ionisation efficiency of the v =0 level with respect to the v = 1 level where no

three-photon resonances are observed. With the exception of the structure between

350 - 351 rum and 369 - 375 run, which is due to one-colour probe-only excitation of

ground state 02, the unassigned weaker structure is due to further three-photon

resonances from the b('), v= 0, J= 0 state 20

3.3.6. OODR spectroscopy of v =0 of the d(' H g ) Rydberg state.

The 02 d(' ITI v =0 state was first studied using a [1 +(2')+2']/

[1 ±(2')+ 1+1'] OODR excitation scheme, which involved first exciting the one-photon

b - X transition (at 760 run), followed by two photon absorption, using a second

laser wavelength (374-376 nm), to excite the d +-E--b transition. The absorption of

multiple photons from the d(' fl g ) state resulted in ionisation of the 02 Rydberg state.

The 02 ion-yield was measured as a function of the total d(' fi g ) state energy

relative to the X( 3 Z), v= 0, J= 0 ground state as defined by Slanger and Cosb y21

The resulting spectra for the d(' fi g ) v =0 state recorded via the b(' E), v =0, J=

0 - 6 states using a [1+(2')+2']/ [1+(2 1)+l+1 1] OODR excitation scheme, are shown in

Fig. 3.10.

Chapter 3. The 3scrg d( 1 H g ) Rydberg state of 02 recorded via v = 0 of the b(' E') valence state 71

Jd =1 46

Jb O

LU 5f[1 :: 5f[3/2]

Co D) (I) C 0

+

0

Jb =2

MM

66300 66350 66400 66450 66500 66550 66600 66650

Energy I cm-1

Fig. 3.10.: The d( 1 H g ), v= 0 state spectra recorded via the b('), v= 0,J= 0-6 state using a [1±(2')+2']I [1 +(2')+l + 1'] OODR excitation scheme. Energy scale relative to X( 3 E ), v = 0, J =0 ground state as defined by Slanger and Cosby 21 The dotted ladder relates to the position of the

rotational levels of the d(' Hg ), u = 0 state. The solid ladder relates to observed rotational structure

excited by a [1+(3')+l'] OODR excitation scheme recorded via 1b =0. The ungerade states resonantly enhance the d(' flg ), v = 0 state signal for all observed levels.

Chapter 3. The 3s d(' fi g ) Rydberg state of 02 recorded via v = 0 of the b(' E) valence state 72

As previously stated, in experiments where two-probe photons (2) were used

to excite the d —i—b transition, the d([Ig ) v =0 and 1 state cannot be ionised by

the absorption of a single pump or probe photon. The observed spectra in Fig. 3.10 for

the d(' LI g) v =0 state possess extra structure to that predicted by the Hönl-London

factor calculations, shown in Fig. 3.8. The extra structure arises from accidental

resonant enhancement of the 02F ion-signal by (3'+1') REMPI transitions from the

v =0, J = Jb states to several rotational levels of the 5f v = 0 ungerade

Rydberg cluster, which converge upon 02 X( 2 11 (l/23/2)g ) ion-core states 20

It has been previously noted that similar experiments recorded for the

dC fi g ) v = 1 state show no resonant enhancement and the observed 02 ion-signal

is weak due to the two photon ionisation step. This was clearly observed in Fig 3.9,

where the signal intensity for the d( flg) v = 0 state is greater than that of the

d('fl g ) v= 1 state, due to the accidental resonant enhancement of the d('fI g ) v=

0 state. A second excitation scheme was therefore used, [l+(1+1')+l'] OODR, which

involved exciting the same one-photon b +— X transition (' 760 nm), followed by two

colour two-photon coherent absorption, (one pump, 760 nm and one probe, 248.5-

250 run), as illustrated in Fig 3.5(b). The probe photon, due to the d '---b energy

requirements, now possessed enough energy to ionise the d(' fl g ) v = 0 state

directly. No extra peaks due to accidental resonance enhancement were observed in

the [1+(1+1')+l'] OODR spectra recorded for the d('fl 5 ), v= 0 state via Jb = 0- 8

and 10 - 16, respectively, shown in Figs. 3.11 and 3.12.

Chapter 3. The 3scr5 d(' ri g ) Rydberg state of 02 recorded via v = 0 of the b(' ) valence state 73

1234 5 6 7 8 9 10

JdII I I I I I I I I

I J=0 b *

MON

C', C C)

C,) C 0

+ ('1

0

2

Jb 1

JL LLJb=8

66250 66300 66350 66400 66450 66500 66550 66600

Energy I cm -1

Fig. 3.11.: The [1+(l+1')+l'] OODR spectra recorded for the d('115 ), v 0 state via Jb = 0- 8 levels

of the b(' ), v = 0 state. Energy scale relative to X( 3 E), v =0, J = 0 ground state as defmed by

Slanger and Cosby 2 ' The * and the + peaks refer to v = 0 of the 5dit5 () state and v = 2 of the 5sag

(3 H 1 ) state respectively observed via a [1+(2')+1'] OODR excitation scheme via the assigned

rotational levels of the b(' ), v = 0 state.

Chapter 3. The 3scr d(' hg) Rydberg state of 02 recorded via v = 0 of the b(' Y,') valence state 74

8 9 10 11 12 13 14 15 16 17 18 JdI I I I I I I I I 1 1

Jb 10

*

Co C

C,)

0 +

('1

0

JLiUJLJ\Jb=1 2

b 16

66400 66500 66600 66700 66800 66900 67000

Energy I cm

Fig. 3.12.: The [1+(l±1)+1'] OODR spectra recorded for the d( 1 11 g ), v= 0 state via Jb = 10- 16

levels of the b(' E), v =0 state. Energy scale relative to X( 3 ), u = 0, J= 0 ground state as defined

by Slanger and Cosby 21 The * peaks refer to v= 0 of the 5dltg ( 1 Y, + ) state observed via a [1+(2')+1']

OODR excitation scheme via the assigned rotational levels of the b(' E), u =0 state.

Chapter 3. The 3scrg d(' Hg) Rydberg state of 02 recorded via v = 0 of the b(' ) valence state 75

Although no accidental resonance enhancement was observed, in Figs. 3.11

and 3.12, extra peaks were observed due to different higher n gerade Rydberg states

resulting from a [1+(2')+1'] OODR excitation scheme, labelled * and +. The * peaks

arise from a Rydberg state with 0 =0, as only a single Q-branch was observed, and

peaks were assigned to v = 0 of the 5d7tg ('F1 ) state. The + peaks arise from a

Rydberg state with Q = 1, as a similar intensity distribution was observed to that of

the d('IIEg ) v=O state, and peaks were assigned to v= 1 of the 5sc g ( 3 fl 1 )state.

Both the 5thtg (' Y- + ) state and the 5STg (3 [T ) state will be discussed in Chapter 4.

The peak distributions and intensities for the d( [11 v = 0 state, Figs. 3.11

and 3.12, can be assigned using the two-photon selection rules for a Q (14—(-0)

transition and the HOnl-London factor calculations (3.6) - (3.10), Fig 3.8. The

spectrum recorded via Jb = 0 consisted of a broad peak due to an S-branch, whilst that

recorded via .J, =2, showed four peaks arising from P-, Q-, R- and S-branches. Those

spectra recorded via Jb =4 - 8 had clearly resolved 0-, P. R- and S-branches and

showed the characteristic weak, strong, weak, strong intensity distributions. Those

spectra recorded via Jb = 10 - 16 also possessed clearly resolved 0-, P, R- and 5-

branches, although the signal-to-noise ratio was much lower.

Since the energy scale is relative to X(3 y ), v =0, J =0 ground state as

defined by Slanger and Cosby, 21 common rotational levels will occur at the same

energies, i.e., the S-branch of the spectrum recorded via Jb = 2 (Jd = 4) overlaps in

terms of total energy with the 0-branch of the spectrum recorded via Jb = 6 (Jd 4)

since both branches have a common Jd level. This allows the rotational structure to be

Chapter 3. The 3sc.rg d(' Hg ) Rydberg state of 02 recorded via v = 0 of the b(' E') valence state 76

easily identified. The rotational term energies can also be easily measured as the

peaks are distinct and each Jd level is observed in two separate spectra.

The rotational term values for the d(' Ilg) v =0, J= 1 - 18 state are shown in

Table 3.1. The values, in Table 3. 1, were calculated using the known molecular

constants for the b(' Z) state 21 and transition energies between the X( 3 ) ground

state and the b(' ) state 17 . Most rotational levels were observed via two different

branches and the values, in Table 3. 1, are an average of the term energies for these

levels, which typically were consistent to ±1 cm -1 . The increase in error, relative to

that of the calibration process alone, is due to uncertainties in locating the line

maxima.

The rotational term values shown in Table 3.1 and have been rotationally

analysed using Equation (3.11) to produce the spectroscopic constants T,0 / cm' and

B / cm for each of the vibrational levels and these are shown in Table 3.2.

T,j=To+BJ(J+ 1)

(3.11)

The spectroscopic constants are compared to those reported by Sur et al. 1 1 ,

van der Zande et aL 22 Morrill et al. 14 and Lewis et al. 23 The B values for the lower

vibrational levels of the 02 (2flg) molecular ion 14 are also included, in Table 3.2.

Chapter 3. The 3scr5 d(' Hg) Rydberg state of 02 recorded via v = 0 of the b(' E') valence state 77

U'

J' 0 1 2 3

0 - - - - - -

1 66361.4 68233.1 70019.6 - 71952.4 -

2 66367.7 68240.7 70023.3 - 71956.7 -

3 66377.9 68250.7 70034.7 - 71963.6 -

4 66391.4 68262.1 70044.4 - 71973.3 -

5 66408.2 68280.5 70057.8 - 71985.4 -

6 66428.5 68299.5 70072.9 - 71998.5 -

7 66451.7 68322.1 70089.7 - 72014.9 -

8 66478.9 68348.6 70108.6 - 72034.3 -

9 66509.2 68378.9 70125.6 - 72054.0 -

10 66543.7 68410.9 70146.7 70284.3 72076.2 -

11 66580.1 68440.7 70170.6 70313.5 72102.0 72259.1 12 66621.3 - 70196.9 70346.2 72127.3 72278.0 13 66663.6 - - 70384.3 72154.2 72303.0 14 66711.5 68583.3 - 70425.4 72183.4 72328.8 15 66762.4 68633.6 - 70468.6 72211.8 72358.7 16 66816.0 68685.7 - 70520.8. 72237.5 72387.3 17 66874.9 68741.0 - 70571.9 - 72428.0 18 66932.3 68797.5 - 70624.2 - 72466.6 19 - - - 70680.5 - -

20 - - - 70740.8 - -

21 - - - 70802.7 - -

22 - - - 70865.4 - -

23 - - - 70927.5 - -

24 - - - 70986.9 - -

Table 3.1.: Term values, in cm, for the rotational levels of V = 0, 1, 2 and 3 of the 02 d(' Hg ) state.

Energies relative to X( 3 E), u = 0, J= 0 ground state as defined by Slanger and Cosby '. Results for

v = 3 taken from O'Keeffe et al. 16

T 0 /cm Present work Ref. 11 Ref. 22 Ref. 23 CSE Ref. 14

0 66357.8 66380 66300 66358 66356 1 68230.4 68237 68200 68226 68226 2 70017.5 - 70108 70015 70011 3 71950.2 70950 71920 71950 71950

B,,/cm' Present work Ref. 11 Ref. 23 CSE Ref, 14 Ion-core state

Ref. 14

0 1.684 [1-18] 1.68 [25] 1.68 [30] 1.68 1.68 1 1.641 [1-10] 1.58 [12] 1.63 [10] 1.64 1.66 2 1.31 [1-7] - 1.29 [6] 1.29 1.64 3 1.15 [1-10] 1.15 [8] 1.21 [15] 1.14 1.62

Table 3.2.: Spectroscopic constants for W = 0, 1, 2 and 3 of the d(' Hg ) Rydberg state. Square

brackets indicate the number of J levels used to determine the constants. Term values relative to

u = 0, J= 0 ground state as defined by Slanger and Cosby 2 ' Results for v = 3 taken from

O'Keeffe etal. 16

Chapter 3. The 3sag d(' ri g ) Rydberg state of 02 recorded via v = 0 of the b(' E' ) valence state 78

The spectroscopic constants for v = 0, T0 = 66357.8 cm- ' and B0 = 1.684 cm,

accurately reproduce (± 0.5 cm') the presented experimental values. Lewis et al. 23

also note that the B0 value is similar to B0 =1.68 cm- 1 for v =0 of the 0i1 X( 2 fl g )

ion-core state, which is to be expected for an unperturbed Rydberg state.

Rotational linewidths for the d(' fl g ) v = 0, J = 1 - 18 levels show significant

signs of power broadening especially for those spectra, shown in Fig 3.12, recorded

via Jb = 10 - 16 where the signal to noise ratio decreases. The S-branch from Jb 'O

also possess an anomalous profile, which only appears when the pump power is

increased. The same final state, Jd = 2, accessed via the 0-branch from Jb = 4 has a

normal profile. The pump and probe wavelengths required for the two routes are

different and an extra pump photon in the former route apparently excites a further

resonant transition via an ungerade state, thereby enhancing the ionisation cross

section. The line position in the d(Jd)—+-b (Jb)+-X (J") transition, [l+(1+1')+l']

OODR spectra, can also be shifted by increasing the pump power. Shifts of up to 2

cm-

I to the blue were induced in the J= 24—+--0 line.

Linewidths of low Jlevels of v = 0 - 2 of the 3d7t (')Rydberg state have

been reported 24 be :!~ 0.05 cm- 1 . Accessing these levels using a [1+(2')+1'] OODR

excitation scheme via the b state provided a minimum linewidth of 0.4 cm - 1 ,

consistent with the expected laser fundamental linewidth for a two-photon process.

The same line with a more powerful probe beam can be broadened to 2 cm and

further broadened by increasing the power of the pump beam. The minimum

observable linewidth for the d (Jd)*-4-- b (Jb)4— X (J") transition for Jd =2 pumped

via Jb =0 measured with various pump and probe powers was found to be 1.5 cm-

1

Chapter 3. The 3sa d(' Hg ) Rydberg state of 02 recorded via u = 0 of the b(' E') valence state 79

which is an upper limit to the true linewidth. The observed linewidths of the other

rotational levels of the d(' H g ) ' v =0 state increase with J, probably the result of

increasing the pump and/or probe laser powers to maintain a similar signal to noise

ratio, but the minimum linewidth observed remained an order of magnitude less that

the value of- 30 cm -1 for v = 0 of the C(3flg)state 14 The latter is homogeneously

broadened by a repulsive 3flg state that passes through the C( 3 r1 5 ) state near its

potential minima. Since the minima of the d and C states are only 800 cm - 1 apart

and their re values are very similar, the same repulsive triplet state will pass close to

the minima of the d state. The very much reduced width of the d state resonances

compared with the corresponding C states resonances thus underlines the nearly pure

singlet nature of the d state.

Overall v = 0 of the 02 3so d( 1 H g ) state is shown to be unperturbed, since

the observed rotational term values can be accurately reproduced using B0 = 1.684

cm-1 which is similar to that of the O2 X( 2 H g ) ion-core state, B0 =1.68 cif', which

the d('fl g ) state is based upon. The v= 0 level of the 02 3sa d('Hg ) state is,

therefore, suitable as an intermediate state to monitor the production of v = 0 of the

b(' ; ) states produced from the photolysis of ozone in terms of its unperturbed

nature, but is unsuitable in terms of the inability to ionise it directly using a

[l+(2')+l'] OODR excitation scheme.

Chapter 3. The 3scrg d(' H g ) Rydberg state of 02 recorded via u = 0 of the b(' ) valence state 80

3.3.7. OODR spectroscopy of V = 1 of the d( fi g ) Rydberg state.

The 02 d('fl g ) v= 1 state was studied using a [1+(1+1')+l'] OODR pump

and probe scheme for similar reasons to those of the d(' fl g ) v =0 state. This

scheme involved exciting the one-photon b - X transition (- 760 urn), followed by

two colour two-photon coherent absorption, (one pump, 760 urn and one probe,

237.5 - 238.5 nm) exciting the d (—'--b transition. Further probe photon absorption

from the d( H g ) state resulted in ionisation of the d state to produce 02. The ion

yield was measured as a function of the total d(' H g ) state energy relative to the

X( 3 ), v= 0,J= 0 ground state. The resulting spectra are shown in Figs. 3.13 and

3.14 for OODR excitation via Jb = 0 - 6 and 8 - 16, respectively.

Although no accidental resonance enhancement was observed for the spectra

in Figs. 3.13 and 3.14, extra peaks were observed due to different higher n gerade

Rydberg states resulting from a [l+(2')+l'] excitation scheme, labelled *, • and +.

The , • and + peaks, although visible from Jb = 0, quickly lost their intensities as Jb

was increased and the peaks were assigned to the 5dir(3 E), v 2, 7dit(' ), v = 1

and 7dit (3 ), V= I states, respectively.

The peak distribution and intensities for the d( 1 H g ), v = 1 state did not

follow the intensity distribution predicted by the Hönl-London factor calculations.

The spectrum recorded via Jb =0 consisted of a single peak due to an S-branch, whilst

that recorded via Jj, =2, showed four peaks arising from F-, Q-, R- and S-branches.

Those spectra recorded via Jb = 4-8 had clearly resolved 0-, F-, R- and S-branches but

did not show the characteristic weak, strong, weak, strong intensity distribution

Chapter 3. The 3Sag d( 1 Hg) Rydberg state of 02 recorded via v = 0 of the b(' E') valence state 81

predicted by the Hönl-London factor calculations. For those spectra recorded via Jb =

10-16, Fig. 3.13, the 0-, P-, R- and S-branches were again clearly observable and did

not show an intensity distribution as predicted by the Hönl-London factor calculations

and, in addition, branches corresponding to Jd = 12 and 13 rotational levels were

clearly absent.

The clearly observable absence of .Jd = 12 and 13 in Fig. 3.13 is consistent

with the findings of Lewis et al. 12, who proposed that d( Jig ), v = 1 state is

perturbed by the more strongly dissociative II ( 'Hg) valence state, see Fig. 3.5. The

perturbation which occurs near Jd = 12, results in a diminished OODR!REMPI signal

due to the increased dissociation of the unobservable rotational levels. Lewis et al. 12

concluded that the d(' fl g ), v = 1 state is perturbed atJd 12, 30, 39.5 and 46 by the

rotational structure of v = 5, 6, 7 and 8 of the II ('Hg) valence states, respectively.

Chapter 3. The 3sog d(1 Hg) Rydberg state of 02 recorded via v = 0 of the b(' Y,') valence state 82

1234 5 6 7 8

- 'JdIIII I I I I

Jb 0 M Mw

IMMM

CU C 0) (I) C 0

+ ('1

0

68150 68200 68250 68300 68350 68400

Energy I cm -1

Fig. 3.13.: The [1+(l±1')+V] OODR spectra recorded for the d(1 fl g ) , v— I state via Jb = 0- 6 levels

of the b(1 ), v 0 state. Energy scale relative to X( 3 ), v= 0, J 0 ground state as defined by

Slanger and Cosby 21 . The , • and + peaks refer to v2of the 5dit g (3 ), v1ofthe

7dltg ( ) and v 1 of the 7ditg (3E- ) states, respectively, observed via a [1+(2')+]'] OODR excitation

scheme via the assigned rotational levels of the b(' ), v = 0 state.

b 8

Chapter 3. The 3sog d( 1 H g ) Rydberg state of 02 recorded via u = 0 of the b(' ) valence state 83

6 7 8 9 10 11 14 15 16 17 18

I I

JOVOW" ib =12

\f\J1J&/Jb_ 14

b16 44 Energy/cm -1

Fig. 3.14.: The [1+(1+U)+1'] OODR spectra recorded for the d(1 fl g ) v= 1 state via Jb 8 - 16 levels

of the b(1 ), u= 0 state. Energy scale relative to X( 3 ), u= 0, J 0 ground state as defined by

Slanger and Cosb y21 Note: branches corresponding to Jd = 12 and 13 are absent.

CU

0) (I)

0 + c'J 0

Chapter 3. The 3scrg d( I H g ) Rydberg state of 02 recorded via v = 0 of the b(" E + ) valence state 84

The term values for the 02 d( lug)' v = 1, J= 1-10 rotational levels, Table

3. 1, can be accurately reproduced (± 0.5 cm) by the spectroscopic constants T1 =

68230.4 cm and B1 = 1.641 cm. The present values are somewhat higher (+ 5.0 ±

0.5 cm-') than those generated from T1 = 66238 cni' and B 1 = 1.682 cm-1 obtained

from a room temperature (1,0) d(' H g ) a('Ag ) spectrum by Lewis et al. 23

If the room temperature spectrum and the corresponding spectrum calculated

using a CSE model of Lewis et al. 23 are compared it is apparent that structure relating

to the lower rotational levels form several overlapping band heads, which are difficult

to analyse. The rotational analysis has therefore probably been based upon higher

rotational levels above the perturbation, which has been extended to lower J.

Therefore the term values, hence T1 and B1 values, for the lower rotational levels are

not based upon experimental observation of these levels.

If the experimental and computational spectra of Lewis et al. 23 are once again

compared, it is apparent that the experimental spectrum possesses significantly less

ion-signal for the lower J bands than that predicted by theory. These observations lead

to the conclusions that Lewis et al. 23 had difficulty in observing, assigning and

determining the term values of low rotational levels of the d( 1 Fi g ), v = 1 state. This

observation highlights the advantage of the OODR spectra, where rotational, structure

is easily observed and identified.

The rotational linewidths (6.5 ± 0.5 cm) for d( 11 g) v = 1 show no

systematic variation for Jd 1-10 and are twice that for v = 0 (low Jb) and V = 3. One

reason for such an observation would be that a dissociative repulsive state is mixing

Chapter 3. The 3s d(' fi g ) Rydberg state of 02 recorded via v = 0 of the b(' E') valence state 85

with the d(' fl v = I state, causing the rotational levels to broaden. This shows

further evidence supporting the hypothesis that d( 1 Hg)' v = 1 is perturbed.

Overall, v = 1 of the 02 3so d(' Fig ) state shows evidence of being perturbed,

both from its missing rotational lines and large linewidths, rendering it unsuitable as

an intermediate state to monitor the production of v =0 of the b(' F,' ) states produced

from the photolysis of ozone. In addition, it is also unsuitable in terms of the inability

to ionise v = 1 of the 02 3so d('H g ) state directly using a [1+(2')+l'] OODR

excitation scheme. However, v = 1 of the 02 3so d(' Hg) state has recently been

used to monitor the production of v =0 of the b(' ) states produced from the

photolysis of ozone using a [(1+l')+l'] REMPI detection scheme 25 This analysis will

be discussed in section 3.4.

3.3.8. OODR spectroscopy of v =2 of the d(' H g ) Rydberg state.

The 02 d(' H g ), v = 2 state could not be studied using a [l+(1+1')+l'] OODR

pump and probe scheme, as a large one-colour (3+1) REMPI signal was observed at

the probe wavelengths required. Therefore a [1+(2')+l'] OODR pump and probe

scheme which involved first exciting the one-photon b - X transition ( 760 nm),

followed by two-photon absorption, using a second laser (349.5 - 352 rim), to excite

the d ---- b transition. Absorption of a single further probe photon can energetically

result in ionisation of the 02 molecule to produce 02 (2flg). The ion yield was

measured as a function of the total d(' Hg) state energy relative to the X(3 E ), V =

Chapter 3. The 3sa d(' Hg ) Rydberg state of 02 recorded via v = 0 of the b(' ) valence state 86

0, J= 0 ground state. The resulting spectra are shown in Figs. 3.15 and 3.16 for

OODR excitation via Jb = 0 - 10 and 12 - 22, respectively.

The spectrum recorded via Jb =0 should consist of a single peak due to an S-

branch, whilst that recorded via Jb =2 should show four peaks arising from P-, Q-, R-

and S-branches. Extra peaks were observed in both spectra at 70000 cm and

70035 cm' and were attributed to three-photon resonances from the b(' E), v = 0

state.

Although, it is energetically possible to ionise the d(' Fi g ), v = 2 state

directly using one probe photon, the ionisation efficiency is low, due to the fact that

the ionisation threshold for v =0 of the 02 X( 2 Fi g ) ion-core state is only just

exceeded, which possesses a poor Franck-Condon overlap with the d( fl g ), v =2

state. lonisation of the d( Fi g ), v = 2 state is therefore greatly enhanced, at low

values of Jb, by three-photon resonant enhancement from either autoionising states or

high n Rydberg states converging on higher vibrational levels of the ion. The

consequence of such an excitation scheme is that additional peaks were observed in

the low Jb spectrum, of d( 1 Fi g ), v = 2 state, (Fig. 3.15). This effect is observable, to

a lesser extent, throughout the entire .J,, series.

Chapter 3. The 3scrg d(' Hg ) Rydberg state of 02 recorded via v = 0 of the b(1 ) valence state 87

246 8

j'III 111111 89101112 -

.. I I I —F-1 i,

- -

-

I • I i I • I I

69900 70000 70100 70200 70300 70400 70500

Energy I cm.-1

Fig. 3.15.: The [1+(2)+1'] OODR spectra recorded for the d( 1 H g ) v= 2 state via Jb = 0 - 10 levels of

the b('), v= 0 state. Energy scale relative to X(3), V= 0, J= 0 ground state as defined by

Slanger and Cosby 21

P-4, ca D)

C')

0

+ c.,1

0

MM PDII

Co

D) Co

0 ~ c%J 0

Chapter 3. The 3sc d(' fl g) Rydberg state of 02 recorded via v = 0 of the b(' 4 ) valence state 88

8 10 12 14 16 18 20 22 24

16

- .Jl1IIIIl 111111 I I I I I I I

-

Jb=1 8

JLJb_ 18/20

I I I • I I

70200 70400 70600 7.0800 71000

Energy / cm-1

Fig. 3.16.: The [1+(2')±1'] OODR spectra recorded for the d(' fi g ), v = 2 state via Jb = 12 - 22 levels

of the b(' ), v =0 state. Energy scale relative to X( 3 ), v = 0, J= 0 ground state as defined by

Slanger and Cosb y 21 Note 'b = 20 can not be independently pumped, see Fig 3.7.

Chapter 3. The 3scrg d(' Hg) Rydberg state of 02 recorded via v 0 of the b(' E ) valence state 89

If the resonantly enhanced structure is ignored, observed in the spectra

recorded via .J, = 0 -6, then the number of observed rotational levels at each value of

.Jb is similar to that predicted from Hönl-London factor calculations. The Hönl-

London factors do not predict the observed rotational intensities, indicating that the

d(' H g ), v =2 state is perturbed or that the ionisation efficiency of the observed

rotational levels is Jdependent. As Jb increases to 8 and 10 a second higher energy

system becomes observable. The higher energy system is caused by the interaction

between the d( Hg) Rydberg state and a valence state, the II ('Hg) state, whose

rotational structure is becoming degenerate with the d( 1 Hg) states. As Jb increases

still further only the upper of the two systems is observed. Again the Hönl-London

factors predict the number, but not the intensities, of the rotational branches. A

secondary interaction can also be observed in the splitting of the d(' H g ), v =2, J

16 rotational level by the I (Ag), which crosses the II ('Hg), to which it is

heterogeneously coupled.

The perturbation of the d( I fl g ), v =2 state observed in the energy region

studied can be explained thus; the 'dark' valence II ('Hg) state (re = 1.49 A) cannot be

observed directly due to a poor Franck-Condon overlap with the 02 b(' ) (re = 1.22

A) state and its almost zero ionisation efficiency (r e = 1.14 A,02+ (2flg)). The 'dark' II

('flg) valence state can be observed indirectly by its perturbational effects upon the

d(' H g ) state. It has been previously proposed that v = 8 and 9 levels of the II ('Hg)

valence state perturb v =2 of the d(' fi g ) state and the two states rotationally couple

together 12 At low .J,, values the lower of the coupled rovibronic levels are

predominantly Rydberg in character and the upper of the coupled rovibronic levels are

Chapter 3. The 3sc d( 1 Hg) Rydberg state 0102 recorded via v = 0 of the b(1 ) valence state 90

predominantly valence in character. Since only the Rydberg state can be detected due

to its greater ionisation efficiency, only the lower Rydberg-like coupled levels will be

observed, as observed forJb = 0 - 6 in Fig. 3.15. As Jb increases, since the rotational

spacing for the II ('Hg) valence state is smaller than that of the d(' Hg) state, the

rovibronic levels of the two states become increasingly isoenergetic (degenerate).

When the two isoenergetic states couple the resulting coupled states share almost

equal amounts of valence and Rydberg character. Therefore, both the upper and lower

coupled states can be observed, since they both possess some Rydberg state character

which can be ionised and detected, as observed for Jb = 8 - 10 in Fig. 3.15. As Jb

increases still further the rovibronic levels of the Rydberg state become higher in

energy than those of the valence state. Therefore, once the two states mix only the

upper of the coupled states can be observed as it alone now possess the Rydberg

character, which can easily be ionised. Therefore the observed spectra seems to shift

to higher energy, as observed forJb = 12 - 22 in Fig 3.16.

The term values for the 02 d('IIl g ) v= 2,J= 1-7 state, Table 3. 1, can be

accurately reproduced (± 0.5 cm') by the spectroscopic constants T 2 = 70017.7 cm-1

and B2 = 1.31 cm-1 . The present values are somewhat higher (+5.0 ±0.5 cm-1 ) than

those generated using the T2 = 70015 cm-1 and B2 = 1.29 cm-1 constants reported by

Lewis et al. 23 for similar reasons to those proposed for the v = 1 state.

Overall the dClTl g ), v= 2 state spectra, Figs 3.15 and 3.16, show regions of

significant perturbation, reflected in irregular band contours and varying rotational

linewidths, caused by interactions with a spectroscopically dark valence state. The

primary interaction is with the 11 ( ' Hg) valence state, which is bound in the energy

Chapter 3. The 3s d(' fl g) Rydberg state of 02 recorded via v = 0 of the b(1 ) valence state 91

region of the crossing with the d(' H g ) state, but which has a much larger equilibrium

bond length. A strong rovibronic coupling between v = 2, J4 = 8 - 12 of the

d('171 g ) Rydberg state and the near degenerate rotational structure of v = 8 of the II

( ' Hg) states is predicted by Lewis et al. 12 This type of behaviour is also observed in

the spectroscopy of the 02 d('lTl g ), v= 3 state, Figs. 3.3 and 3.4. Thus, both v .= 2

and v =3 are unsuitable as an intermediate state to monitor the production of v = 0 of

the b(' ) states produced from the photolysis of ozone although they can both be

ionised using a (2+1) REMPI process. However, an attempt to use v =2 of the 02

3sa d( Hg ) state as a monitor of the rotational distribution of v =0 of the

b(' Y-' ) state produced from the photolysis of ozone is described Section 3.4.

Chapter 3. The 3scr d(' Fig ) Rydberg state of 02 recorded via v = 0 of the b(' E' ) valence state 92

3.4. Analysis of the 02 b(' ) state rotational distribution

produced from the photolysis of ozone in the Huggins band

region (310 - 350 nm) probed using 1(1+1')+lJ REMPI via

the 02 d('II g )states.

3.4.1. Introduction.

It has been demonstrated, 145 photolysis of ozone at wavelengths

corresponding to absorption to bound levels within the Huggins bands produces

02 X( 3 E), a('A g ) and b(' ) state oxygen molecules together with 0 (3P2, 1 ,o) state

atoms. It was shown that the 02 b(' Y-') state photofragments can be detected using a

[(1+1')+l'] REMPI excitation scheme via v = 1 and 2 of the d(' Hg) Rydberg state.

Spectra were recorded to monitor the 02 b(' ), v = 0 state rotational distributions

produced from the photolysis of ozone in the Huggins band region at 337.2, 340 and

344 nm, which correspond to bound regions within the Huggins band possessing

(4,0,1), (3,1,1) and (3,0, 1) vibrational quanta, respectively. The relative position of the

three Huggins bands can be observed in Fig 3.1.

Thus far it has not been possible to rotationally analyse the d('Hg ) 44

b(' Z') spectra, produced via ozone photolysis, due to the presence of complicated

overlapping rotational structure and perturbation within the d(' Hg ) rotational

structure. An example of the complicated rotational structure can be seen in Fig. 3.17,

where the 02 b(' E), v =0 state rotational distributions produced via ozone

photolysis at 344 and 337.2 nm detected via v = 1 of the d(' Hg ) state are shown.

Chapter 3. The 3s d(' 11g ) Rydberg state of 02 recorded via v = 0 of the b(' E') valence state 93

The use of OODR spectra, such as those discussed in Section 3.3, to determine

the rotational energies, intensities and perturbations of the d( H g ) state observed via

the state allow the rotational structure of the d( 1 fl g ) ----b() transition to

be assigned. It should now be possible to estimate the low J (Jb =0 22) rotational

distribution of v = 0 of the 02 b(' ) state produced from the photodissociation of

ozone.

One important experimental difference between the photolysis and the OODR

spectra is that the pump and probe wavelengths used to detect the d( H g ) state are

different. As previously discussed, v = 1 and v = 2 of the d( fi g ) states were detected

using a [(1+1')+l'] and [(2')+l'} OODRIREMPI scheme via v = 0 of the b('ç) state,

respectively, where the pump energy is dependent upon the b +— X transition

energies. In spectra, recorded by O'Keeffe et al. 1,25 monitoring the photolysis yield

of the b('), v 0 state via the v— 1 and v= 2 of the d( 1 H g )states, a [(1+1')+l']

REMPI detection scheme was used. Here, the pump photons are no longer determined

by the b +— X transition energies, but relate to the photodissociation wavelengths

used; 337.2, 340 and 344 nm. Since the probe wavelength in each of the above

detection schemes is dependent upon the pump energy, and a single probe photon is

assumed to be used to ionise the d('H g ) states, the ionisation efficiency may differ in

each experiment. This means that the intensities of the rotational structure observed in

the photolysis spectra and the OODR spectra may differ. Hopefully the ionisation

efficiency will be similar for all Jand, therefore, not distorted the rotational line

intensities and hence rotational distributions calculated.

Chapter 3. The 3sa d(' 11g ) Rydberg state of 02 recorded via v = 0 of the b(' ) valence state 94

3.4.2. The 02 b('), v= 0 state rotational distribution produced

from the photolysis of ozone in the Huggins band region (at 337.2 and

344 nm) monitored via v= 1 of the d('H g )state.

Collaboration with B.R. Lewis has resulted in a publication 25 concerning the

02 b(' V), v =0 state rotational distributions produced from the photolysis of ozone

in the Huggins band region (at 337.2 and 344 nm) monitored via v = 1 of the

d('fl g ) state using a [(1+1')+l'] REMPI scheme. The analysis was based upon

spectra recorded by excitation into the (4,0, 1) and (3,0, 1) levels of the Huggins bands.

The spectra were analysed using v = 1 of the d(' H g ) state OODR data, Figs 3.13 and

3.14, for Jb < 16. Higher Jb levels were derived from the term values of Lewis et al. 22

and the photolysis spectra themselves. The 337.2 and 344 nm ozone photolysis

spectra are reproduced in Fig. 3.17, adapted from Fig. I of Ref. 25.

Three local perturbations of v = 1 of the d(' H g ) state are evident, in Fig. 3.17,

at Jb 30, 39.5 and 46. Previously 23" 1 a perturbation at Jb 12 has been

experimentally observed in the rotational structure of v = 1 of the d('Hg ) state, as

observed in the OODR data, Fig. 3.14. This perturbation at Jb - 12 results from a

homogenous interaction with v = 5 of the II ( H g ) valence state 23 Therefore, the

higher Jperturbations should result from interactions with v =6 to S of the same

valence state and the new experimental data should be useful in refining the CC

model of Lewis et al. 23

95 Chapter 3. The 3scr d(' Hg ) Rydberg state of 02 recorded via v = 0 of the b(' ) valence state

iii' m I

30i9-ri842 46 48 ___

11r111 I ii P(Jb)24 30 32 40 46 48 50

I 11 r-71 11R(J )22 2830 3840 44 46 48

b I I I F_71

I z 28 28 36 38 42 4 4S(J)

(301) 344.0 nm

* (401) 337.2 nm

54800 55000 55200 55400 55600 55800 56000 56200

Transition Energy I cm

Fig. 3.17.: [(1+1')+l'] REMPI spectra for the d('Hg )+f_ b(')(1,O) band of O3 obtained following

the photodissociation of 03 in the Huggins band at 337.2 and 344.0 nm, together with rotational

assignments. The peak marked with an asterisk results from a [(1+2')+l'J REMPI process from the 02

X(3 E) state and is not associated with the d(' Hg ) state. Adapted from Fig. 1 of Ref. 25.

The rotational distribution for the 02 b( 1 ), v =0 state formed from the

photolysis of 03 was deduced from the spectra of Fig. 3.17. The apparent strength of

the S-branch lines were corrected for the effects of probe-laser intensity variations, the

two-photon line strengths, and, using the CC calculations, the anomalous effects on

the electric part of the strengths caused by the Rydberg-valence interactions. In this

procedure the ionisation step in the REMPI process was assumed to be saturated and

the relative ion-signals deduced from the CC absorption intensities were scaled

0) C,) C 0

+ (1

0

Chapter 3. The 3scrg d(1 11 g ) Rydberg state of 02 recorded via v = 0 of the b(' Y, + ) valence state 96

according to the inverse of the CC predissociation linewidths. The distribution was

emphasised to only be roughly determined, with an estimated uncertainty of-- 30 %.

The calculated rotational distribution for the 02 b( 1 Z), v = 0 state formed

from photolysis wavelengths of 337.2 and 340 nm, corresponding to the (4,0,1) and

(3,0,1) Huggins bands respectively peaked near Jb = 34 and 28, respectively, with

FWHMs Mb - 15. A pictorial diagram of the distributions can be found in Fig. 3.18.

344 nm 3372 nm

-o

0

m 75 0

CD

0 0

> •1-• 'U U,

CC

20 30 40 50 J

Fig. 3.18.: The 02 b(' ), v = 0 state rotational distribution produced from the one-photon photolysis

of ozone at 337.2 and 344 rum corresponding to the (4,0, 1) and (3,0, 1) Huggins bands of 0 3 . Taken

from Figure 4 of Ref. 25.

Chapter 3. The 3scr d(' Hg) Rydberg state of 02 recorded via v = 0 of the b(' E') valence state 97

3.4.3. The 02 b('), v= 0 state rotational distribution produced

from the photolysis of ozone in the Huggins band region (at 340 nm)

monitored via v= 1 of the d( 1 rT g )state.

A similar rotational analysis to that shown in Fig. 3.17 has been performed for

excitation of the (3,1,1) Huggins band at 340 nm and is shown in Fig. 3.19. The

Figure includes the rotational assignment of Lewis 25

16

rm n-i-i 42FT-1 1 46 48

124 26 3232 38

4650

r-i R(Jb)

P(J ) 2-1216 30 44 30

2830 38 40 44 46 48

II fl fl 40

2 10 1416 ______ ___________ ______

III 11 FT-1

b inn ___ III I

d8 42

8 1216 2 28 28 36 0 I LS(J,)

(311) 340.0 nm

344.0 nm

54800 55000 55200 55400 55600 55800 56000 56200

Transition Energy I cm'

Fig. 3.19: [(l+l')+l'] REMPI spectra for the d( 1 Hg )*—*—b( l )(1,0)band of 02, obtained following

the photodissociation of 0 3 in the Huggins band at 340 nm, together with rotational assignments.

Photodissociation via 344 nm is included for comparison.

Although no rotational distribution calculations have been attempted for

dissociation via the (3,1,1) Huggins band, observed in Fig 3.19, the observed

Co C 0)

Cl) C 0

+ N

0

Chapter 3. The 3scrg d(' F15 ) Rydberg state of 02 recorded via v = 0 of the b(' ) valence state 98

rotational structure can be assigned and compared to spectra recorded via other

Huggins bands. If the (3,1,1) and (3,0,1) Huggins band spectra are compared, as

shown in Fig. 3.19. The spectrum recorded via the (3,1,1) band possesses a rotational

distribution which extends to lower energy and probably populates lower Jb levels

(extra peak(s) observable between 55050 - 55125 cm') than that observed via the

(3,0, 1) Huggins band.

3.4.4. The 02 b('E), v= 0 state rotational distribution produced

from the photolysis of ozone in the Huggins band region (at 340 nm)

monitored via v= 2 of the d('H 5 )state.

Further experimental evidence for the population of low 4, rotational levels of

02 b('E), v = 0 state formed from the photolysis of 03 at 340 nm, (3,1,1) Huggins

band, is shown in Fig. 3.20. The spectra, in Fig. 3.20, were recorded via v =2 of the

d('fl g ) state for ozone photolysis wavelengths of 340, 337.2 and 344 nm

corresponding'to the (3,1,1), (4,0, 1) and (3,0, 1) Huggins bands. The rotational

assignments, in Fig. 3.20, have been taken from OODR spectra of the d(' fl v =2

state, Figs. 3.15 and 3.16, for .Ji, = 0 -22 (indicated in black) and the rotational

energies of Morrill et a! 26, for 4, = 18 -44 (indicated in blue). It should be noted that

the spectrum recorded via (3,0,1) of the Huggins band is recorded at lower resolution

than those spectra recorded via the (3,1,1) and (4,0, 1) bands.

Chapter 3. The 3scrg d(' 11 g ) Rydberg state 0102 recorded via u = 0 of the b(' ) valence state 99

S(Jb)m FFFTQ122 6 2 2630 Ti

3€38 40

R(Jb)I UF12 1Ti I I fl 2.8 10 1 38-32 34-36 38 40 42

P(i I rlT3Z Ii I fl b 12-24 30 34 3638 42 42 44

°( b) fl, ~11 1 fl1 H2234 \36 40 40 42 44

18-20 36-38

JU 3,1,1) 340 nm

Co 0)

(I)

0 (4,0,1) 337.2 nm

+ c'1

0

n m

56800 57000 57200 57400 57600 57800

Transition Energy I cm - '

Fig. 3.20: [(1 + I ')+l 'J REM spectra of the d(' H g ) ~--- b( ) (2,0) band of 02, obtained following

the photodissociation of 0 3 in the Huggins band at 340, 337.2 and 344 nm, together with rotational

assignments for .1,, :5 22 from OODR experimental data (in black) and Jb = 18 -44 from Morrill 26 (m

blue). The peaks marked with an asterisk in the 340 nm spectrum represent rotational levels, Jb > 22.

Note, the spectrum recorded via (3,0,1) of the Huggins band, is recorded at lower resolution than those

spectra recorded via the (3,1,1) and (4,0,1) bands.

If the spectrum recorded via the (3,1,1) Huggins band is first considered it is

apparent that the majority of the observed rotational structure, excluding peaks

marked with an asterisk, can be assigned to rotational structure resulting from the

detection of Jb :5 22. Spectra recorded via the (4,0, 1) and (3,0, 1) Huggins bands show

rotational distributions that extend to high Jb values, shown by an increased intensity

for the peaks marked with an asterisk and the presence of rotational structure

observed outside the Jb = 0 - 22 region (at energies> 57200 cm'). The spectra in Fig.

Chapter 3. The 3scrg Al 11 g ) Rydberg state 0102 recorded via v = 0 of the b(' ) valence stale 101

28% Jb=14 50% -J =16 68%- jb18 / I ki I 100% Jb 20 \ 83%— J b=22 80% J b=24 *1 !, I

Observed I * I

—Simulation I (b)

J) II I J :i\ .IL (c)

56700 56800 56900 57000 57100 57200 57300

Wavenumber (th-4--b) / cm-1

Fig. 3.21.: The observed (a) and simulated (b)(1+1')+I' REMPI spectra of the (2,0) d( 1 H g )4_*

b(' Z + ) band of 02 following the photo-dissociation of ozone at 340 run, (3,1,1) Huggins band. The

component OODR spectra used in the simulation are shown in (c). Note the peaks marked with an

asterisk represent rotational levels, 1b > 22.

The assignment of the observed rotational structure for photolysis via the

(3,1,1) Huggins band detected via the d('fl g ), v = 2 state (Figs. 3.20 and 3.21)

should be compared with that detected via the d(' H g ), v = 1 state (Fig. 3.19). One

would expect to observe similar rotational levels in both spectra, since the ozone is

being dissociated using the same photolysis wavelength in each case. In Fig. 3.19, it is

apparent that a significant proportion of low it, is being detected, indicated by the

appearance of a large, broad 02 ion-signal around 55100 cm. This broad peak has

Chapter 3. The 3s d(' 11g ) Rydberg state of 02 recorded via v = 0 of the b(' Y.') valence state 102

been assigned to the 0-branch relating to Jb = 4 - 12, possessing a peak maximum at

Jb =6. If this assignment were correct one would also expect to observe a similar

peak(s) relating to the same rotational levels in Figs. 3.20 and 3.21. The 0-branch, Jb

= 6 and 8 positions are indicated in Fig. 3.20 but no peaks corresponding to these

lines are observed. Similarly, no resonances are observed relating to Jb =4 - 8, in the

P-branch. These experimental observations lead to the conclusion that Jj, =0 8-12

cannot be readily detected in a (1+1')+l' REMPI excitation scheme via the d('lTI g) , v

- =2 state but can be observed via the d( H g ), v = 1 state. The low Jievels of the

d('Hg ), v= 2 state can be observed using [1+(2')+l] OODR since they are

resonantly enhanced by ungerade Rydberg states (see text Section 3.3.8) at the three

photon level (it is unknown at this time whether these resonances are due to (2'+1') or

(2'+l) excitation) above the b(') state. But the low Jievels of the d('Hg ), v= 2

state are absent in Figs. 3.20 and 3.21 since these levels are observed via a [(l+1')+l']

REMPI excitation scheme where no resonant enhancement is expected. The

requirement for the low J levels of the d(1 H g ), v =2 state to be resonantly enhanced

for a signal to be observed can be explained thus; the rotational linewidths for the low

J levels of v =2 (Fig. 3.15) appear to be larger that those observed at higher J (Fig.

3.16). This observation indicates that the lower rotational levels are broadened to a

greater extent by predissociation and resonant enhancement, making their detection

difficult in a [(1+ 1 ')+ 1'] REMPI excitation scheme. REMIPI studies, such as that of

Lewis et al. 23, show a significant decrease in intensity for the lower J levels of the

d(' H g ), v =2 state than that predicted by theory.

Chapter 3. The 3sc d(' 11g ) Rydberg state of 02 recorded via v = 0 of the b(' E') valence state 103

The overall conclusion is that the lower rotational levels, Jb = 0 - 8-12, of the

b( 'Y-' ), v =0 state are observed to a greater extent in spectra recorded via d(' [Ig ) v

= 1, that those observed via d('JIlg ), v = 2 in (1+1')+l' REMPI. For low Jlevels to

be observed via d('fl g ), v= 2 resonances at the three-photon level are required as

observed in the OODR experiments (Fig.3.15).

In the spectrum shown in the top trace of Fig 3.20, the strong peaks can be

predominantly assigned to the four rotational branches relating to Jb = 12 - 30. This

assignment agrees well with the distribution observed in Fig. 3.2 1, of Jb = 12 - 22,

peaking for .J, = 20. Rotational levels >22 were not factored into the distribution,

although these rotational levels are expected to be produced during dissociation to

allow a smooth distribution. The presence of the peaks marked with asterisks, in Fig.

3.20, show that higher rotational levels are produced, but with a lower population due

to their peak intensities. Overall the distribution produced from the dissociation of

ozone at 340 run is assigned to Jb = 12 - 30, peaking at Jj, = 20, which is consistent

with data relating to detection via the d(' fl g) v = 1 level.

Photo-dissociation of ozone at 340 nm appears to produce a significantly

lower rotational distribution of 02 b(' ), v =0 state molecules. The distinct

difference, other than the photolysis wavelengths used, between 340 nm (3,1,0), 337.2

rim (4,0, 1) and 344 nm (3,0, 1) photolysis is that at 340 nm a single quantum of

bending motion is excited prior to dissociation, whereas at 337.2 / 344 nm no bending

quanta are excited. The change in the rotational distribution of the 02 b(' ), v =0

state from high to low Jb values probably originates from an overall change in the 03

geometry during the bending motion.

Chapter 3. The 3s d(' 11g ) Rydberg state 0102 recorded via ii = 0 of the b(1 ) valence state 104

3.5. Conclusions.

The 02 b(' ) state photo-fragments produced from photolysis of 03 can be

detected using a [(1+1 ')+I'] REMPI excitation scheme via the d(' Ji g ) Rydberg state.

Such spectra were found to be complicated by overlapping rotational structure and the

presence of perturbation within the d( 1 Ji g ) rotational structure. The rotational

distributions and perturbations of the d(' Ji g ). state have therefore been investigated.

In the present work, u= 0, 1 and 2 levels of the 3sa d('fl g )Rydberg state

were probed using rotationally selective OODR experiments in which single

rotational levels of the b(' Y-' ) state were first populated by direct optical pumping

from the X(3 y ) ground state. The d( 1 Ji g ) state was then excited from these

rotational levels by coherent two-photon absorption, using [1+(1+1')+l'] and

[1+(2')±1'] overall pump and probe schemes.

The d(' Ji g ), v =0 Rydberg level is found to be unperturbed, as shown by it

having narrow rotational linewidths and rotational linestrengths that are predicted by

theory. This lack of perturbation of v =0 is also shown by it having a B0 value that is

similar to that of v =0 of the molecular ion. The d( 1 Ji g ), v = 1 level is perturbed as

shown by it having larger rotational linewidths and rotational linestrengths that are not

predicted by theory. This perturbation of v= 1 occurs atJd= 12-13. The d('fl g ), v=

2 level also shows a region of significant perturbation between Jd = 8-12. A secondary

smaller perturbation at Jd = 16 was also observed and attributed to I ( 1 Ag) state, which

is weakly coupled to the II ( 1 11g) that is responsible for the large perturbations.

Chapter 3. The 3scr d(' 11g ) Rydberg state of 02 recorded via u = 0 of the b(' 4 ) valence state 105

The 02 b(' ), v =0 state rotational distribution produced from the

photolysis of ozone at wavelengths corresponding to bound regions within the

Huggins band, namely the (4,0,1), (3,1,1) and (3,0, 1) levels, has been investigated.

The b(' ), v = 0 state rotational levels have been detected using a [(1 + F)+ 1']

REMPI scheme via both v = 1 and v =2 of the d( 1 H g ) states.

The ozone-photolysis spectra, recorded by O'Keeffe 1 , have been rotationally

analysed and insights into the rotational distribution of the 02 b(' ), v =0 level

produced by photolysis of 03 at various wavelengths have been gained. Photolysis via

the (3,0, 1) and (4,0, 1) Huggins bands produces a distribution which peaks for Jb =28

and 34, respectively. The distributions for these levels were calculated using OODR

spectroscopic data obtained via excitation of v = 1 of the d( Hg) state. Spectra

recorded via excitation of v = 1 of the d(' H g ) state following photolysis of the

(3,1,1) Huggins band were observed to possess structure relating to low Jb levels,

which were absent from (3,0,1) and (4,0,1) ozone-photolysis spectra.

Ozone-photolysis spectra detected via excitation of v =2 of the d( H g ) state

are also reported. Photolysis via the (3,1,1) Huggins band was investigated and found

to produce a rotational distribution Jb = 12 - 30 which peaks forJb = 20. Spectra

recorded via the (3,0,1) and (4,0,1) Huggins bands were also observed and noted to

possess a higher Jb distribution.

The spectra and subsequent analysis of the 02 b(' ), v = 0 state rotational

distributions produced from the photolysis of ozone detected via the d('H g ) states is

complicated. Experimental difficulties include the inability to ionise both v = 0 and 1

of the d(H5 ) state directly, the presence of accidental resonances and the perturbed

Chapter 3. The 3sag d(' 11 g ) Rydberg state of 02 recorded via v = 0 of the b(' I') valence state 106

nature of the d( Jig) states due to coupling with the II ( fI g ) valence state. It is

therefore proposed that the d(' Hg) state of 02 should not be used to study the 02

v =0 state rotational distributions produced from the photolysis of ozone.

The search for a more suitable intermediate state, which can be used to detect the 02

b(' E) state, was undertaken and is discussed in Chapter 4.

Chapter 3. The 35øg d(' 11g ) Rydberg state of 02 recorded via v = 0 of the b(' ) valence state 107

3.6. References.

P. O'Keeffe. PhD. Thesis, Edinburgh University (1999).

P. O'Keeffe, T. Ridley, S. Wang, K.P. Lawley and R.J. Donovan, "Resonance

lonisation Spectroscopy", Edited by J.C. Vickerman, I. Lyon, N.P. Lockyear and

J.E. Parks, American Institute of Physics Conference Proceedings (1998) 103.

P. O'Keeffe, T. Ridley, S. Wang, K.P. Lawley and R.J. Donovan, Chem. Phys.

Lett. 298 (1998) 368.

P. O'Keeffe, T. Ridley, R.R.J. Maier, K.P. Lawley and R.J. Donovan, J. Chem.

Phys. 110 (1999) 10803.

L.T. Molina and M.J. Molina, J. Geophys. Res. 91 (1986) 14501.

P. O'Keeffe, T. Ridley, K.P. Lawley and R.J. Donovan, Chem. Phys. Phys. 115

(2001) 9311.

A. Sur, C.V. Ramana and S.D. Colson, J. Chem. Phys. 83(2) (1985) 904.

A. Sur, C.V. Ramana, W.A. Chupka and S.D. Colson, J. Chem. Phys. 84(1)

(1986)69. -

W.J. van der Zande, W. Koot, J.R. Peterson and J. Los, Chem. Phys. Lett. 140

(1987) 175.

J.A. Stephens, M Braunstein and V. McKoy, J. Chem. Phys. 89 (1988) 3923.

A. Sur, R.S. Friedman and P.J. Miller, J. Chem. Phys. 94 (1991) 1705.

B.R. Lewis, S.T. Gibson. S.S. Baneijee and H. Lefebvre-Brion, J. Chem. Phys.

113 (2000) 2214.

Chapter 3. The 3scrg d(' 11g ) Rydberg state 0102 recorded via v = 0 of the b(' E ) valence state 108

R. Ogorzalak Loo, W.J. Marinelli, S. Arepalli and R.W. Field, J. Chem. Phys. 91

(1989) 5185.

J.S. Morrill, M.L. Ginter, B.R. Lewis and S.T. Gibson, J. Chem. Phys. 111 (1999)

173.

R.D. Johnson III, G.R. Long and J.W. Hudgens, J. Chem. Phys. 87 (1987) 1977.

P. O'Keeffe, T. Ridley, S. Wang, K.P. Lawley, R.J. Donovan, H.H. Telle, D.C.S.

Beddows and A.G. Urena, J. Chem. Phys. Phys. 113 (200) 1.

S.-L. Cheah, Y.-P. Lee and J.F. Ogilive, J. Quant. Spectro. Rad. Trans. 64 (2000)

467.

R.G. Bray and R.M. Hochstrasser, Mo!. Phys. 31 (1976) 1199.

A.C. Kunimel, G.O Sitz and R.N. Zare, J. Chem. Phys. 85 (1986) 6874.

A.M. Sjödin, T. Ridley, K.P. Lawley and R.J. Donovan, in preparation.

T.G. Slanger and P.C. Cosby, J. Phys. Chem. 92 (1988) 267.

W.J. van der Zande, W. Koot, J. Los, and J.R. Peterson, Chem. Phys. Left. 140

(1987) 175.

B.R. Lewis, S.T. Gibson, J.S. Morrill and M.L. Ginter, J. Chem. Phys. 111 (1999)

186.

H. Park, L. Li and W.A. Chupka, Chem. Phys. Left. 162 (1989) 317.

P. O'Keeffe, T. Ridley, H.A. Sheard, K.P. Lawley, R.J. Donovan and B.R. Lewis,

J. Chem. Phys. 117 (2002) 8705.

J.S. Moth!!. PhD. Thesis, University of Maryland (1999).

Chapter 4. OODR study of the nd / nso Rydberg state of 02 excited via the b ('E) state 109

Chapter 4.

An optical-optical double resonance study of the nd

Insag gerade Rydberg states of 02 excited via single

rotational levels of the b(' ) valence state.

4.1. Introduction.

The OODRJREMPI excitation scheme previously used to study the

d( Hg ) states was extended to higher energy to investigate the higher-energy nd2rg

Ins clg gerade Rydberg states of 02 excited via single rotational levels of the

b(' Z' ) valence state. The purpose of such an investigation is twofold. Firstly, to

investigate the spectroscopy of the ndir /nsci Rydberg states themselves and,

secondly, to find a probe scheme to monitor the b( 1 E) valence state rotational

distribution produced from the photolysis of ozone which possesses none of the

disadvantages of the d( Hg ) states.

As previously stated, the lowest energy gerade Rydberg series converging

upon the ground X( 2 1T1 (1/23/2)g )ion-core states are the nsc.rg states. The 3so

C(3 rig ) and 3scr d('flg) states have been investigated using (2+1) REMPI primarily

from the X( 3 E)ground state 1-12 although the a('Ag)state 10,12-14 (generated using a

microwave discharge) and the b( ) state 12,15-17 have been used in later studies. The

C( 3 Hg ) state, which lies at lower energy than the d(' Hg ) state, is a typical triplet

state, in that it comprises three 0 components the 3Hog, 3ITIig and 31112g in order of

Chapter 4. OODR study of the nd,r5 / nso Rydberg state of 02 excited via the b (4) state 110

increasing energy. Although all three triplet states were observed via (2+1) REMPI

from the triplet X( 3 ) ground state only the 3Hlg state was observed from the singlet

a('L g ) state due to a spin-orbit interaction in the core (- 98% 3111g, 2% 'lug) 12 with

the two-photon allowed d(1 Hg) state. The observed spectra show rich rotational

structure, which can only be partially resolved due to relatively large linewidths

produced by predissociation. Typical spectra for both the 3so C(3 Hg )and 3so

d(' Hg ) states from the X( 3 ) and a('A g ) states are reported in a review article by

Morrill et al. 10

The lower vibrational levels of n = 4 and 5 of the nscrg Rydberg series have

previously been observed using (2+1) REMPI from the ground state 12,18,19• However

analysis of these spectra also proved difficult partly because of the spectral congestion

which arises from the number of J levels that are populated in the ground state, even

in jet cooled molecular beam experiments, and partly because of the large number of

strong electronic transitions that are observed from the ground state. All of the

previous experimental studies on the nso Rydberg states have been reviewed in detail

by Moth!l et al. 10

The lower vibrational levels of n =3 - 9 of the ndr states have also been

investigated using (2+1) REMPI, primarily from the X( 3 ) ground state 121822

although the a('A g ) state 12,21-25 and the b(' Y- + ) state 12 have been used in later studies.

This body of work, shows that the ndirg series converge on the ground state of the 02

ion and that nd spectra are complicated by spectral congestion leading to difficulty

in assignment and analysis. It Will be shown in the following discussion that OODR

spectroscopy significantly simplifies nd'r spectra and allows greater accuracy in

Chapter 4. OODR study of the ndirg / nso Rydberg state of 02 excited via the b (g4) state 111

spectral assignment and Rydberg state energy determination. It has been shown by

Pratt etal. 20 thatthe spectrum of the ndrg series for n ~! 5 becomes very simple. Only

the higher members of the 3 g /' g coupled pair are observed and the levels clearly

correlate with X( 2 lTl 1i2g )and X( 2 '3/ 2g) ion-core states, respectively.

4.2. Experimental.

The fundamental output of R700 dye was used to pump the (0,0) band of the

b(' Y-+ ) - X( 3 ;) transition of 02, around 760 rim. The frequency-doubled outputs

of C 102, C307 and C153 dyes were used to generate the 235-285 rim probe photons.

The wavelength of the probe laser was independently calibrated by simultaneously

recording the neon optogalvanic spectra to give an estimated accuracy in the two-

photon energy of± 0.5 cm_I .

4.3. General overview of the OODRIREMPI spectrum.

One of the major advantages of the OODR technique is that, due to the

simplified nature of the resulting spectra, underlying states or structure that would

normally be obscured by dense rotational structure becomes observable. A good

illustration of this effect is shown in Fig. 4.1. Fig. 4.1(a) shows the (2+1) REMPI

spectrum in the region of v = 1 of the 3dffg states excited from the X( 3 ) ground

state and is similar to the spectrum of v = 0 of the 3d7z states recorded by Glab et al.

22 Fig. 4.1(b) shows the same energy region excited by [1 +(2')+ 1'] OODR/REMPI, via

Chapter 4. OODR study of the nd / nso Rydberg state of 02 excited via the b (g4) state 112

.Jb = 0, v = 0 of the b(') valence state. At most, two rotational lines in each of the

electronic states are observed in the OODR spectrum. This contrasts with the many

rotational lines seen in each of the eight 3dlrg states observed in Fig. 4.1(a). Thus, it

was possible to observe weaker structure, identified as v =2 of the 4so ( t [Ig) and

4so (3171g) states, which is obscured by the overlapping dense rotational structure of

the 3dirg states in the (2+1) REMPI spectrum (Fig. 4.1(a)).

The spectra, shown in Fig. 4. 1, give a very good illustration of the different

oscillator strengths observed for transitions from X( 3 ) and b(' ) states to the ndrg

Rydberg states. A similar comparison of the oscillator strengths for transitions

observed from X( 3 E)and a('A 5 )states to the ndrg Rydberg states is shown in Fig. 3

of Ogorzalek-Loo et al. 12 The b(' ) and a('A 5 ) states are both strongly singlet in

nature and transition from these states will occur predominantly in the singlet

manifold, i.e. to other singlet states. Of the 24 nthrg Rydberg states, only six;

1 g 1 flg (x2), 1 Ag, and '(Dg are singlet states. Thus if spin, parity and the two-photon

selection rules are observed only transitions from the b(1+ g) to the 1 >.g+ 1

, fl 1

g and Ag

Rydberg states are allowed.

Chapter 4. OODR study of the nd / nso Rydberg state of 02 excited via the b ('g) state 113

(a)

1+

C 0)

(1)

(b) 3

C 0

+ ('1

0

3 II

Iq

86800 87000 87200 87400 87600 87800

Energy I cm-1

Fig. 4.1.: (a) The (2+1) REMPI spectrum of v = 1 of the 3d complex of 02 excited from

the X(3 Z,) ground state. The extent of the rotational envelope of the states is indicated by the dark

horizontal lines. (b) The [1 +(2')+ V] OODRJREMPI spectrum of u = I of the 3d complex of 02

recorded via J =0 of the b(1 ) valence state. Energy scale relative to X( 3 ), u = 0, j= 0 ground

state as defined by Slanger and Cosby 26

Chapter 4. OODR study of the nd / nso Rydberg state of 02 excited via the b ('g4) state 114

Transitions to four states are seen in the spectrum in Fig. 4.1(b); only those to

the 1g and 1 Ag states are expected (the 1Hg states are predissociated, see below). The

two other weaker transitions to the :3; and 3 A states are observed due to spin-orbit

coupling 3d[94%',6%3] 12 and 3d[97% 'A2, 3% 3A2] 12 It is important to note

that it is the AS = 0 component of the coupled states that carries all of the oscillator

strength from either the b(' Y- +) or the a(1A g ) states.

The two 1 f1g states are never seen strongly, mainly because of predissociation,

which will be discussed later in connection with the nscrg Rydberg states. It is likely,

however, that the weak peak at 87313 cm - 1 in Fig. 4.1(b) is due to one of the 1 r1

states. Transitions to the'(Dg state, forbidden in the b(' Z+) state spectrum, are seen

strongly in the a('A g ) state spectrum 12,21,25

An overview of the 85000 - 99000 cm -1 energy region recorded by

OODRJREMPI via .4=0, v = 0 of the b(' Z+) state is shown in Fig. 4.2. Within this

region it was possible to observe the fldff g states for n = 3-8 , A2 j and

3 A2 states were observed as previously discussed). The convergence of the nd-series to

the X(2 [1 112g )(97348 cm t , v= 0) and X( 2 fl 312g )(97548 cm4 , v= 0) ion-core states

can clearly be observed. Several one-colour probe only (2+1) REMPI signals from the

ground state of 02 were also observed and assigned to the 3SCTg and 3dltg states. The

flSCTg states with n = 4,5,6,7 and 9 are also observed, although only those where n = 5

and 6 (marked using asterisks) are indicated in Fig. 4.2. The other nsag states were

too weak to be seen with the intensity scale used for Fig. 4.2 and will be shown in

expanded form later. States for n =8 could not be observed due to overlapping ndlrg

structure.

Chapter 4. OODR study of the nd.ir / nscrg Rydberg state of 02 excited via the b ( 'Eg ) state 115

D) C,) C

+ (.4

0

u'=O u'=l u'2 o'=3 u'=4 u'=5 u6 - 3J11 II liii liii liii liii liii I

u' =O u'=l u'=2 u'=3 • .4.dII II liii liii I

u'=O u'l u'=2 - 5d11 III II

01= o'1 6d11 H

- ,e u-

_ u 0

7d1 ' u'O u' l

- 8d 11 11 u'O )'=l )'2

• 6sF 1 1i II - -

2 IC)

II 'Q )'l u'2

kUjL

84000 86000 88000 90000 92000 94000 96000 98000 Q=112 312

Energy I cm 1 0+(2fl)ttuI = 0

4.2.: An overview of the 85000 - 99000 cm 1 energy region recorded by OODR/REMPI via J,, =0, v =

0 of the b(' E) state. The two-photon ionisation thresholds from .J,, =0, v =0 of the b(1 E) state to v

= 0 of the 02 X( 2 111/2g ) and X( 2 113/2g ) ion-core states are labelled. The 5sorg and 6sag states are

marked with asterisks. Several one-colour, probe laser only, peaks are labelled.

Both ndlz g and flSag states show a transition from (AS) coupling to (Q,w)

coupling as the principal quantum number, n, increases. The transition between the

two coupling schemes has been discussed in detail by Glab et al. 22, Morrill et al. 10

and Ogorzalek-Loo et al. 12 for the 3d,z and 3so states, and is summarised below.

In (AA, Hund's case (a), coupling, A, Y, S, Q and fare all good quantum

numbers. If the spin-orbit interaction is neglected, the energy difference between the

I 3 - and the 'j1 - f1 states are determined primarily by the Coulomb,

J and exchange, K, integrals.

where J= !cA(1) çOB(2) W q(l) coB(2) dt j d't2 (4.1)

and K = !9A(1) q ij(2) W q,(2) ço,(1) d'r1 d'r2 (4.2)

For states of low n, the (AS) coupling description is appropriate since the

energy splitting between the various states are much larger than the spin-orbit

coupling constant, A,, responsible for the splittings between the 02 X( 2 fl 112g )and

X(2 II3i2g ) states of the ion-core. As n increases the splittings decrease, since VA(l)

q(2) (and similarly coA (2) 91j(1)) is dependent upon the overlap of the two

eigenfunctions, and it can be shown that the overlap varies as n-312 for Rydberg states,

the energy splitting due to the exchange (and Coulomb) interactions decrease as n 3 (n-

3/2

x n 3 = [n 3 ]2 = n 3). Thus, in the limit of high n, the spin-orbit interaction

completely mixes the Hund's case (a) basis states, i.e. A >> K. This limit corresponds

to Hund's case (c) in which L and S are decoupled from the internuclear axis and

where A and are no longer defined. Good quantum numbers are n, J, 0, 1 and w,

where co is the projection of the angular momentum of the excited electron on the

internuclear axis. The total angular momentum about the internuclear axis, 0, is

therefore obtained by adding the core angular momentum about the internuclear axis,

c, with w, Q = + a). The coupling between the core and the Rydberg electron is

therefore known as (92,) coupling.

Chapter 4. OODR study of the nd / nso Rydberg state of 02 excited via the b (g1) state 117

One consequence of the (Q,w) coupling, at high n, is that the singlet/triplet

description of the Rydberg states breaks down since the outer electron (the Rydberg

electron) is so weakly coupled with the rest of the molecule and Z is no loner defined.

The Rydberg states should therefore be labelled according to the (1,co) coupling

scheme of [L]nl; where [U] is the ion-core to which the Rydberg states converges

([1/2] =X( 2 [Ti , 2g )or [3/2] =X(2 [1312g )) , n is the principal quantum number of the

Rydberg state, / is the Rydberg electronic orbital angular momentum (1 = 0, 1, 2,

corresponding to s, p, d, ... orbitals respectively) and 0 is the orbital angular

momentum of the Rydberg states, i.e. the 3d,r(1 A2) state corresponds to [3/2]3d;2

state.

Experimentally, for low n, since for (AS) coupling A <<K, one would expect

the two coupled states to possess a splitting > 200 cm 1 (the X( 2 11 112g )X( 2 JIl312g )

splitting). The intensities of the two-coupled states also differ since they are not

completely mixed. Whereas for higher n values, corresponding to (0, w) coupling,

one would expect the two coupled states to possess a splitting of 200 cm -1 and

possess similar intensities, since they are completely mixed as A >> K.

Chapter 4. OODR study of the ndir / nso Rydberg state of 02 excited via the b ('g) state 118

4.4. The ndirg states, for n = 3 -8.

The ndlrg states, observed in Fig. 4.2, were assigned on the basis of their

effective principal quantum numbers, n, and quantum defects, 6 = n - as

calculated from the Rydberg formulae (4.3).

= IE 1 - R0 /[n - (4.3)

where T is the term value for the v th vibrational level of the electronic state

observed, R0 is the mass-corrected Rydberg constant for 02 (109735.5 cm 1 ) 20 and

IE is the ionisation energy for the particular vibrational level of the spin-orbit

component of the ion to which the Rydberg state converges. The ionisation energies

were determined from the X( 2 11112g ), v = 0 ionisation energy, the

X(2 1-1 g )vibrational constants, and the X( 2 fl 112g )-X( 2 111 312g )spin-orbit splittings as

described by Pratt et al. 20 ionisation energies used are shown in Table. 4.1. For The

the ndrg states 6 was found to be -0.045 ± 0.05.

V X(2 11i12g) Term energy / cm X(2 11312g ) Term energy / cm'

0 97348.0 97548.0 1 99220.2 99420.2 2 101059.9 101059.9 3 102867.1 103067.1 4 104641.8 104841.8 5 108384.0 106584.0 6 108093.7 108293.7

Table 4.1.: lonisation energies, in cm -1 , , for v = 0-6 of the 02+ X(2 H + 1125 )and X( 2 113/2g) ion-core

states.

Chapter 4. OODR study of the ndrg / nso Rydberg state of 02 excited via the b ( 'Eg ) state 119

The experimental and literature energies 19,2021 for the nd2tg states, n = 3 - 8,

are shown in Table 4.2, along with the effective quantum number, n, and quantum

defect, 6. The ndirg states, n :!~ 5, should strictly be labelled according to the (Q,w)

coupling scheme. The (AS) coupling labels, appropriate for the 3d states, are retained

in Fig. 4.2 and Table 4.2 for convenience. The experimental energies for ndlrg states

quoted in Table 4.2 refer to the lowest Jievel of the Rydberg states observed via .Jj, =

0, v = 0 of the b(' Y-')

states. i.e. the ndrg (g) and ndlrg (3g) states refer to J' = 0

and the fld7l g (3Ag) and ndrg ( 1 L g) states refer to J' "2. The literature values quoted in

Table 4.2 are reported to refer to the rotationless level of the states, J' = 0, for the nd,r

g (' g ) and ndirg (3w) states ' 9 and to J = n for the ndlrg (3Ag) and ndrg ( 1 Ag) states

21 i.e. J' = 2. The energies of Pratt et al. 20 refer to transition energies of the most

intense feature of the vibronic bands.

Chapter 4. OODR study of the ndir / nso Rydberg state 0102 excited via the b ('g) state 120

State n* Calibrated Literature cm CM7 1

3d7tg 3A 2,V=0 3.000 0.000 85154.2 85153a

3d,z 3A2,V1 2.999 0.001 87020.0 87021a

3d7c? 3A 2, v2 2.999 0.001 88856.6 88853 a

3dir 3A 2 , v=3 2.998 0.002 90659.1 90655 a 2.997 3d 3A 2, v = 4 0.003 92428.0 -

3dc 3A 2, v=5 2.997 0.003 94167.3 -

3dr, 3E,v=0 3.011 -0.011 85245.7 852438b

3diz 3 ,v=1 3.010 -0.010 87106.6 87109.6" 3d7ç, v=2 3.009 -0.009 88943.4 88942.2" 3d,r v = 3 3.008 -0.008 90742.5 90742.0 b

3dç o, v=4 3.008 -0.008 92509.7 92508.8 b

3d,r 3 , v5 3.006 -0.006 94243.7 - 3d7rg 3 o , v=6 3.005 -0.005 95945.4 -

3d 'A 2, v= 0 3.021 -0.021 85524.9 85527 a 3dç 'A 2, ü= 1 3.020 -0.020 87390.5 87390 8

3dn 'A 2, v =2 3.020 -0.020 89228.9 89226 a 3d7rR 'A 2,v=3 3.019 -0.019 91029.9 - 3d,r.'A2,v=4 3.018 -0.018 92797.9 - 3diç,'A 2,v=5 3.018 -0.018 94533.0 -

3d2rR v = 0 3.042 0.042 85691.5 85690.5 b

3d7tg 'Y ', v= 1 Eo 3.042 -0.042 87559.6 87558.6 b

3d ', v= 2 3.041 -0.041 89395.2 89394.3 b

3dz 0 ,v3 3.041 -0.041 91197.3 91196.8" 3d,z 'E, v = 4 3.040 -0.040 92967.4 - Mg,' o

U = 5 3.039 0.039 94699.0 - 3 dg, ' 0 , v=6 3.038 -0.038 96405.0 -

4d,r 3A 2, v=0 4.019 -0.019 90552.7 - 4dr 3A 2,v=1 4.018 -0.018 92422.6 - 4dn 3A 2, v = 2 "4.017 -0.017 94260.0 - 4d,r 3 A 2,v=3 4.015 -0.015 96061.0 -

4dn o, U 0 4.034 0.034 90605.8 90605.5" 4d2rR 3 o , v = 1 4.034 -0.034 92476.1 92476.1" 4dn 3 , v2 4.033 -0.033 94314.3 - 4d,r o, v = 3 - 4.032 4_ -0.032 96117.4 -

4dç 'A 2, u = 0 4.034 -0.034 90804.3 - 4d,z 'A 2, u= 1 4.034 -0.034 92675.6 - 4d,z 'A 2, u = 2 4.033 -0.033 94512.0 - 4d2rg 'A 2, v3 4.032 -0.032 96316.4 -

4d'r1 'Y o', v = 0 4.054 -0.054 90870.8 90870.4 5--0870.4 b

4dr 'E, v= 1 4.053 -0.053 92741.2 92741.0" 4dç, v= 2 4.052 -0.052 94577.7 - 4d v= 3 4.052 -0.052 96383.0 -

Chapter 4. OODR study of the ndrg / nso Rydberg state of 02 excited via the b ('g) state 121

State n' Calibrated Literature cm cm

5dir v = 0 5.048 -0.048 93042.3 93042.8

5d2tg30, V= 1 5.049 -0.049 94914.8 94914.0

5d'r , v =2 5.049 -0.049 96754.6 96757.2 '

5d v= 0 5.060 -0.060 93262.5 93261.4 C

5d' 0 ,u=1 5.060 -0.060 95134.5 95133.7C

5d'r, ', v= 2 5.058 -0.058 96969.9 96971.7 C

6d,r, v = 0 6.060 -0.060 94360.0 94349.1 C

6d,z 3 0 , v= 1 6.061 -0.061 96233.0 96232.5 C

6d,r, , v = 2 - - - 98072.3 C

6d,r, ', v = o 6.064 -0.066 94564.0 94554.0 C

6dit v= 1 6.065 -0.065 96435.0 9375 C

6dir,' 0 ,v=2 - - - 98277.1 C

7d,r 3 , v=0 7.075 -0.075 95155.5 95154.8 C

7d,z o, v = 1 7.072 0.072 97026.3 97026.4 C

7d ', v= 0 7.076 -0.076 95356.2 95355.7 C

7dr, v= 1 7.075 -0.075 97228.1 -

8d,z 3E0 , v =0 8.085 -0.085 95669.2 95669.7 C

8d,ç v = 1 8.092 -0.092 97544.4 97543•5 C

8dn o t , v = 0 8.087 -0.087 95869.9 95869.3 C

8diç u = 1 8.088 -0.088 97742.6 97742.1 C

Table 4.2.: The experimental and literature term values, in cm', effective quantum numbers, n, and

quantum defects, 8, for the nd gerade Rydberg states of 02 calculated using ionisation energies of 02X(21-1 112g )afld 02X( 2 H3/2g ) ion-core states found in Table 4.1. Energies relative to X( 3 ), u

=0, J = 0 ground state as defined by Slanger and Cosb Y26 The literature values are taken from

experimental data of (a)Yokelson et aL, Ref. 21. (b)Yokelson et al., Ref. 19. (c) Pratt et al., Ref. 20.

Chapter 4. OODR study of the nd'r / nso Rydberg state of 02 excited via the b (1 4) state 122

The majority of the calibrated fld7tg state energies agree favourably, ±2 cm i ,

with literature values 19-21 A discrepancy between literature values for the 3dirg ('A 2)

and (3A2) states should be noted. Three separate publications 19,21,24 by the Yale

University research group give different values for the 3 dlrg ( 1 '3Ag) state energies. The

first publication 24 using the 3dirg ( 1 Ag) +-a('A 5 )transition, quotes band origins (J=

0) for v = 0 -4 of the 3dirg ( 1 Ag) state of 85506.2, 87378.5, 89217.9, 91022.3 and

92789.5 cm 1 , respectively. The second 19 using the 3drg ( 1 Ag) +- X(3 )transition,

quotes rotationless energies (J 0) for v = 0 - 3 of the 3dlrg ( tAg) state of 85522.4,

87390.8, 89225.2 and 91027.0 cm 1 , respectively. Finally, the third publication 21

again using the 3d7r g ('Ag) 4-X(3 )transition, quotes J= Q rotational energies (J=

2) for v =0 -2 of the 3thrg ( 1 Ag) state of 85527, 87390 and 89226 cm 1 , respectively.

Since, the B value for the 3thrg ( 1 Ag) state is also quoted by the Yale group to equal

1.813 cm1, 24 rotational energy spacing between J= 0 and 2 is therefore equal to the

11 cm-1 . The three different 3dirg ( 1 Ag) state energies thus reported are not consistent

with each other, for example, rotationless energies for v = 0, J= 0 of the 3drg ( 1 Ag)

state are quoted to be 85506.2, 85522.4 and (85527 - 11)85516 cm', depending upon

which publication is referenced. The reference values quoted in Table 4.2 therefore

correspond to the third publication 2 ' since these values are close to the experimental

values and compare like rotational levels, X= 2. A similar discrepancy between

reference values is also observed for the 3d g (3A2) states and the reference values

quoted in Table 4.2 are from the same reference as the 3d7rg ( 1 A2) state values.

Table 4.3. shows the 1g - 3Eg splittings for n = 3 - 8 of the nd'rg states of 02

derived from the experimental energies in Table 4.2. The splittings decrease, as n

Chapter 4. OODR study of the nd.ir / nso Rydberg state of 02 excited via the b ( 'Eg4) state 123

increases, from 450 cm-1 for the 3thrg state to 200 cm', the ion-core splitting, for

6 - 8d7tg states. The intensities for n = 3 - 8 of the ndffg (g) and (3) Rydberg

states, Fig. 4.2, also show a similar effect. At low n the 1g states are of greater

intensity than the 3jj states, which can be clearly seen for v = 1 of the 3dlrg in Fig.

4.1. As n increases to n = 8 the 1 > g and states are a 50/50 mixture of the singlet

and triplet component, shown by an equal intensity for both states in Fig. 4.2. These

observations confirm that the Rydberg states become more accurately described by a

(Z,to) coupling scheme as n increases.

- splittings / cm' V 3d2tg 4d7tg 5d2 g 6d2Zg 7dltg 8dffg

0 445.8 265 220.2 204 200.7 200.7 1 1 453 265.1 219.7 202 201.8 198.2 2 451.8 263.4 215.3 - - -

3 454.8 265.6 - - - -

.4 457.7 - - - - -

5 455.3 - - - - -

6 459.6 - - - - -

Table 4.3.: The 'Z - % splittings, in cm', for n = 3 - 8 of the nd,r5 states of 02. The splittings should

be compared with the 02 (2111/2g) - 02 (113/2g) ion-core spin-orbit splitting of- 200 cm -1 .

Although no rotational analysis has been undertaken, the ndirg Rydberg states

Of 02 appear to be ideal as intermediate states used to study the b( 1 E) valence state

distribution produced from the photolysis of ozone. One good example would be the

3dlrg ( 1 Eg ) state, shown in Fig. 4.1(a). The rotational structure observed from the

ground state using (2+1) REMPI appears to be strong, sharp and regular showing that

the state is probably not perturbed. OODR studied using the 3d7rg ( 1 g ) state would

probably give a clear rotational distribution.

Chapter 4. OODR study of the nd irg Rydberg state of 02 excited via the b (gf) state 124

4.5. The nscrg states, for n = 3 9.

4.5.1. Vibronic data.

Figs. 4.3 - 4.6 show the nsag states (n = 4,5,6,7 and 9) in greater detail. The

unlabelled, but previously assigned signals, are due to nthrg states, as shown in Fig.

4.2. The flSag states were identified mainly on the basis of their effective principal

quantum numbers, n, and quantum defects, ö = n - n, as calculated from the

Rydberg formulae (43). For the nscrg states ö was found to be 1.20 ± 0.03.

The ns7rg states, n ~! 5, should strictly be labelled according to the (,to)

coupling scheme, the (AM coupling labels, appropriate for the 3s states, are retained

in Figs. 4.3 - 4.6 and Table 4.4 for convenience. The experimental energies for flS2Z g

states quoted in Table 4.4 refer to the lowest Jlevel of the Rydberg states observed

via ib =0, v =0 of the b() states. i.e. the S-branch, 3=2. The literature values in

Table 4.4 are reported 19 to refer to the rotationless level of the states, X = 0, even

though the J' 0 level does not exist.

Chapter 4. OODR study of the nd2rg / nso Rydberg state of 02 excited via the b ( 1 Eg+) state 125

3r11 In 1

*

4s u'O

I

WOW I • I I

83340 83430 83520 83610

LJL • I I I I

85230 85320 85410 85 00

4s U=2

I I I I I

87040 87120 87200 87280 87360

4s U=3

I • I I • I

88850 88950 89050 89150

4s U=4

90600 90700 90800 90900

Energy I cm Fig. 4.3.: The [1 +(2')± 1] OODRJREMPI spectra of the 4so Rydberg states, marked with asterisks,

recorded via J = 0 of the b(' ) state. The unlabelled, stronger, signals are due to nd states as observed

in Fig. 4.2.

Co

D) (I)

0 + c.'1

0

Chapter 4. OODR study of the ndir / nso Rydberg state of 02 excited via the b (Ig4) state 126

3iil in

*

5So'=O

*

I I I

* I a

89700 89800 89900 * 90000

5s u'=1

91600 91700 91800 919 *

5s u'=2

_JL JL I • I a I • I

93400 93500 93600 93700 *

5s o'=3

I • I I I

95200 95300 95400 95500

Energy I cm-1

Fig. 4.4.: The [1+(2')+l'] OODRIREMPI spectra of the 55CTg Rydberg states, marked with asterisks,

recorded via J= 0 of the b(' E + ) state. The unlabelled, stronger, signals are due to nd states as observed

in Fig. 4.2.

C 0)

C,) c 0

+ 04

0

El

Chapter 4. OODR study of the nd7rg / nso Rydberg state of 02 excited via the b (g4) state 127

3

I I

*

6su'=O A ' L

92560 92640 92720 92800 92880

6su 1 j

94400 94500 94600 94700

6s u'=2

J~LT 11 1 1 1

96300 96400 96500 96600

Energy / cm -1 Fig. 4.5.: The [1+(2')±l'] OODRJREMPI spectra of the 6sok Rydberg states, marked with asterisks,

recorded via J = 0 of the b(' ) state. The unlabelled, stronger, signals are due to nd states as observed

00

70- C 0)

Cl)

C 0

+ 04 0

in Fig. 4.2.

Chapter 4. OODR study of the nd / nso Rydberg state of 02 excited via the b ( 1 Eg4) state 128

3n1 11_I

I . I

7s u'=l

95900 96000 96100 96200

Co 9su'=O

D) Cl)

+

0

04 LAL 0

95500 95600 95700 95800

97400 97500 97600 97700

Energy I cm -1

Fig. 4.6.: The [1+(2')+1'] OODRJREMPI spectra of the 7so and 9scrg Rydberg states, marked with

asterisks, recorded via J = 0 of the b(' ) state. The unlabelled, stronger, signals are due to nd states as

observed in Fig. 4.2.

Chapter 4. OODR study of the ndirg Rydberg state 0102 excited via the b (g4) state 129

nsog - 1

States 0 1 2 3 4

3scr 3H0 1.86 1.24 - - - - - Lit. 655738 693668 711898

3s 3H 1 1.86 1.24 - - - - - Lit. - 656648 67580b 694458 712658 729906

3so 3fl2 1.86 1.24 - - - - - Lit. 65767a 695508 713758

3scrg h111 1.86 1.24 66357.8 68230.4 70017.5 71950.2 - Lit. 66358c 68226c 70015c 71950c 73743C

4so i-i 2.80 1.20 - 85256.2 - - - Lit. _______

8334493

83361.0*8 85250 87090 88890d,t

906480 4so 2.80 1.20 - 85264.9 87075.8 88898.3 90684.5

Lit. - 833717d

833732

85250d,f 88890d,f

4so fl 2.80 1.20 - - - - Lit. 835349d

83541.1*8 85420 87260d,f 89070d,t

9083329

4scrg 1ii i 2.80 1.20 83586.4 85468.6 87280.7 89094.5 90885.1 Lit. - 83578 85420d,t

(83551.8*8)

87260d,t

5scy 3H 3.79 1.21 89727.8 91602.0 93447.9 95261.4 - Lit.

_____

897187d

897171 9162109

5s , 3.80 1.20 89740.4 91617.5 93459.8 95272.6 - Lit.

_____

897267d 897301 9 91615.1*8

5so 3H2 3.79 1.21 89918.7 91796.8 93639.2 95448.6 - Lit. 899094d

89916 .69 91797.6*8

5scrg 11, 3.80 120 89935.3 91811.6 93655.1 95463.7 - Lit. 899229d

(899260) (91809.0*8)

6scr 3H0 4.80 1.20 - 94451.7 96289.5 - - 6scr 3n, 4.80 1.20 92588.2 94460.8 96298.3 - - 6sq, 3112 4.79 1.21 92766.4 94639.1 96476.4 - - 6so 1111 4.80 1.20 92777.6 94649.9 96487.2 - -

7so 3rJ0 5.80 1.20 - 95961.3 - - - 7sag 3ri, 5.81 1.19 - 95969.5 - - - 7sc 3H2 5.81 1.19 - 96171.7 - - - 7sc& '11, 5.82 1.18 - 96180.5 - - -

9scr 3r10 - - - - - - - 9so 3fl1 7.82 1.18 95552.8 97433.7 - - - 9so 3H2 1 7.81 1.19 1 95747.6 - - - - 9scr, 'H, 7.83 1.17 95756.8 97633.5 - - -

Table 4.4.: See overleaf for legend

Chapter 4. OODR study of the nd'r / nso Rydberg state of 02 excited via the b ('g') state 130

Table 4.4.: The experimental and literature term values, in cm', effective quantum numbers, n", and

quantum defects, ö, for the nso gerade Rydberg states of 02 calculated using ionisation energies of

02 X(2 '1112g) and 02 X( 2 113/2g ) ion-core states found in Table 4.1. Energies relative to X(' Y,9 ), u

=0, J = 0 ground state as defined by Slanger and Cosby 26

* The asterisk is used to indicate that the energy quoted is the energy of the strongest feature of the

band and not necessarily the origin.

(.....) tentative assignment. a estimated in Ref. 10 from low-temp-REMPI spectra of Ref. 9. b Ref. 3.

cRef 13.

d Values taken from Ref. 10. e estimated in Ref. 10 from perturbation of 4so (3H2), v = 0 state.

"Mean energy of strongest features, estimated by Ref. 10. from R.JYokelson, Y. Wang, C.A -. -

Woodward and W.A. Chupka (unpublished). AND Y. Wang Ph.D. thesis. Yale University,

(1999).Uncertamties ±20 cm'

5 Ref. 19.

Several vibrational levels of the flSO g states for n = 4 - 9 are identified in Figs.

4.3 - 4.6 and Table 4.3. Previously only v= 0 of the 4s cluster and v= 0 and I of the

5s cluster have been reported 10. In the present work, the 7SO g V = 0 and 8SOg V =0

and 1 could not be observed due to overlapping nd g structure. The 4SCTg states are

very weak compared to the higher nso g states as a result of predissociation, which

will be addressed further in Section 4.6. The lower component of the 4SO g, V =0 pair

is not observed as it is overlapped by a one-colour signal from the 02 ground state.

The term values presented in Table 4.3 are relative to X( 3 E), v =0, J =0 ground

state 26

The effects of a transition from a (A,S) coupling scheme to a coupling

scheme can be observed if the 31-Ig - 1 Ug splittings are considered, as shown in Table

4.5. As previously explained, at low n, (A,5) coupling is dominant and the splitting is

Chapter 4. OODR study of the nd / nso Rydberg state of 02 excited via the b ( 'Eg ) state 131

much higher than that observed in the ionic core. When n increases, a transition from

(A,S) coupling to (,w) coupling occurs and the splitting energy converges upon that

of the ionic core. The 3flg - 1 flg band intensities show a similar pattern to that of the

ndirg states. At low n (i.e. Fig. 4.4) the 'Hg band possesses the greater intensity and at

high n (i.e. Fig. 4.6) the intensities of the two bands become similar since the singlet

and triplet states are thoroughly mixed.

3iIg - 1J-j splitting I cm'

V 3sa. 4scrg 5scrg 6s a, 7sa 8scr 950g

0 694 - 194.9 189.4 - - 204 1 1 646 203.7 194.1 189.1 211 - 199.8 2 570 204.9 1 195.3 1 188.9 1 - - -

3 685 196.2 191.1 - - - -

4 753 200.6 1 - - - - -

Table 4.5.: The 'flg - 'Hg spin-orbit splitting, in cm', for n = 3 -9 of the nso states of 02. The

splittings should be compared with the 02 (2H1/2g) - 02 (2113/2g) ion-core spin-orbit splitting of— 200

cm- 1 . ' data from values in Table 4.4.

4.5.2. Rotational data.

If the 5SCY g states are considered in greater detail it can be seen in Fig. 4.4 that

v = 0 - 3 of the 5s clusters consist of two closely spaced doublets separated by - 200

cm t , the ion-core X(2 r'3/2g) - X( 2 IT1, 2g) splitting, the weaker partner being to lower

energy by 9- 12 cm-1 . Excitation of v = 0 and 1 of the 5SCTg states from the ground

state has been reported by Yokelson et al. 19 They also observed four electronic states

for each vibrational level, which they assigned to 3iio" In, , 3H2 and components,

in order of ascending energy. In contrast to spectra recorded via the b(' E) state, the

Chapter 4. OODR study of the ndir / nso Rydberg state of 02 excited via the b ('g4) state 132

lower energy of the states with a common ionic core carried the greater intensity. This

is a consequence of the predominantly triplet 3fl + and 3 112 states being excited from

the triplet ground state and the 3H 1 and 1fl I components observed as a result of spin-

orbit coupling. In excitation from the singlet b(') state the reverse is true and the

singlet state is observed more strongly.

By varying the pump wavelength it was possible to rotationally analyse both

the both v =0 and 1 of the 3 112 and 1 11 1 components of the 5SCY g state. The resulting

spectra are shown in Figs. 4.7 and 4.8. Different Jb levels of the intermediate

b(' E' ) valence state were pumped and the resulting rotational levels for both

components can clearly be observed. The unmarked peaks correspond to the 'iii state,

peaks marked with asterisks correspond to the 3r,2 state and peaks marked with

daggers (t) are unassigned. The intensities of the rotational branches of the 'H 1

component are similar to those observed for v = 0 of the 3so d(' Hg) state 16 and

followed the expected pattern for a A92 = 1 transition predicted from the HonI-London

factors for a two-photon transition 27 The observed relative intensities of the

rotational branches of the 3F12 component are similar to those observed for the 'H

component and follow the expected pattern of a A) = 1 transition rather that that of a

zfl =2 transition. In particular, AQ 2 transitions should have a strong Q-branch, but

no Q-branch lines are observed.

Chapter 4. OODR study of the nd,r / nsa Rydberg state of 02 excited via the b ('g4) state 133

5s (1) I I I I I

1234 5 6 7 8 9 10 ILl I I I I 1* 23 4 5 6 7 8 95s (2) o'=O

_JLJb = 2

= 4

WAJ = 6

= 8 ~ok

89850 89900 89950 90000 90050 90100 90150

Energy / CM -1 Fig. 4.7.: The [1 +(2')±l] OODRIREMPI spectrum of v = 0 of the 5scr (112) (asterisked) and 5scrg

( 1H1 ) (unlabelled) states recorded via single rotational levels of the b(' ) valence state. Peaks

labelled t are unassigned. The ladders show the rotational energy levels.

MI

EO

(0

(I)

0

+ 04 0


u'=i I I I I I 15s ( h III i )

1=12345 6 7 8 9 10

5s( 3 ri 2)

1=234 5 6 7 8 9

j-,-A4LJb = 2

Jb = 4

_cUJLLJ_..J L "b = 6

''b= 8AJJL 91800 91850 91900 91950 92000 92050

Energy / CM -1

Fig. 4.8.: The [1+(2')+l] OODRJREMPI spectrum of u 1 of the 5so(H2) (asterisked) and 5scrg

( 1 111 ) (unlabelled) states recorded via single rotational levels of the b( 1 )

valence state. Peaks

labelled t are unassigned. The ladders show the rotational energy levels.

MI

Eq

Cu C 0)

Cl)

0 +

04

0

Chapter 4. OODR study of the nd / nso Rydberg state 0102 excited via the b (g) state 135

The two-photon spectrum of the 3U0-- state excited from the v =0, Ji 0 of the

intermediate b(' E) valence state should contain Q- and S-branches, the Q-branch

being of greater intensity if the linestrengths are those of a AD =2 transition. While

only one branch can be observed in Fig. 4.4, this observation on its own does not

necessarily exclude this assignment since it is to be expected that the intensity of the

S-branch will be weak and may be undetectable. Therefore, a polarisation study of the

3U0+ and 3fli components of the 5sa state was also performed and the resulting

spectra are shown in Fig. 4.9. Fig. 4.9(a) and (b) illustrate the effect of linear and

circular polarisation upon a typical Q = 0 transition, to v =0 of the 5ds'rg (') state.

Under linear polarisation conditions, Fig. 4.9(a), both the Q- and S-branches are

observed, the Q-branch being of much greater intensity than the weaker S-branch.

Upon changing the probe polarisation from linear to circular, Fig. 4.9(b), the intensity

of the Q-branch is dramatically reduced, whereas the intensity of the S-branch

increases by a factor of 1.5. The suppression of the Q-branch intensity and increase in

S-branch intensity is in line with the two-photon Hönl-London factors 27 The linearly

and circularly polarised spectra of v =2 of the SSCYg (I1l +) and 5scr (JIl) states are

shown in Figs. 4.9(c) and (d). There is no change in relative intensities of the two

states, both increasing by a factor of- 1.5, which is to be expected for a AK2 =

transition.

93430 93440 93450 93460 93470 93480

C 0)

U) C 0

+ ('1

0

Chapter 4. OODR study of the nd7rg /ns ag Rydberg state of 02 excited via the b ('g4) state 136

Energy I cm-1

Fig. 4.9.: The [1+(2')+1] OODRJREMPI spectrum of v= 0 of the 5thrg () states recorded with (a)

linearly and (b) circularly polarised probe laser radiation and the [1 +(2')+l] OODRIREMPI spectrum

of u = 2 of the 5scYg (11) and 5scrg (11) states recorded with (c) linearly and (d) circularly polarised

probe laser radiation.

Chapter 4. OODR study of the nd / nso Rydberg state of 02 excited via the b (') state 137

Both of these observations can be explained by invoking an S-uncoupling

interaction between the pairs of states with a common ionic core. Similar phenomena

were observed in the (2+1) REMPI spectra of the 3d5g ( 1 (1)g) and (3(1)g) states, excited

from the a( 'z g )state by Park etal. 25

Strong transitions were observed to both the 1 03 and 3 cI)3 components which

were seen with similar intensities and were separated by the spin-orbit splitting

similar to that of the ground state of the ion. In addition, two weak components, 34 4

and were observed 4 cm - 1 to low energy of the (1)3 and 3 a3 components,

respectively. It was proposed by Park et al. 25 that the weak components were

observed due to an S-uncoupling interaction with the allowed 1 03 component. The

rotational structure for both the weaker components observed from the a('Ag) state

(a" =2) appeared to follow the predicted pattern for a M) = 1 transition. For

example, the 3 02 component possesses a strong P-branch, which is forbidden in a AD

=0 transition and the J' =2 level is absent, indicating that 9 2'= 3 since J' ~! 92 . Both of

these observation indicate that the component carries no oscillator strength and is

only observed due to coupling to the 33 component. The observed rotational

transitions therefore possess Hönl-London factors dictated by the component which

carries the oscillator strength, even though it may only be a minor component of the

coupled state. Thus, the K2 value of the major component of the mixed state, i.e. 92 '=

2 in this example, does not define the observed HOnl-London factors.

Park et al. 25 also noted that the observed rotational energy levels of the

and 33 states do not fit very well to the usual expression T + BJ(J + 1) - DJ2(J + 1) 2

at low .J'. These bands were best fitted by the expression T + 7J + BJ(J + 1), which

Chapter 4. OODR study of the ndffg / nscr5 Rydberg state of 02 excited via the b (g4) state 138

shows that there exist energy shifts proportional to J. The effective energy shifts are

approximately equal in magnitude, but opposite in sign for these two states. The same

is true for the 34 and 1 03 states, except that the magnitude of the energy shifts are

smaller than those of the 3 02 and 33 states.

It is apparent that a similar S-uncoupling of the 5SCIg states is observed in the

presented work. The 5SO g ('H,) and (H,) states undergo spin-orbit coupling, and at

the 5SCTg level are split by the ion-core splitting of— 200 cm -1 . The spin-orbit coupling

is followed by an S-uncoupling interaction between the 5SO g ('H,) - (H2) and the

(3fl,) - ( H+) states. Two strong and two weak transitions are observed to the four

states, all of which have linestrengths of a A92 = 1 transition. These doublets are seen

in most of the higher flSO g spectra shown in Figs. 4.4 - 4.6. No unambiguous doubling

of the 4St7g can be identified in Fig. 4.3. This could be either because the coupling is

weaker or because the signal-to-noise is much lower. The 3H, - 3fl0, doublet observed

at the 4SO g v = 1 level is anomalous as it undergoes heterogeneous interaction with

the close lying 5dirg (3) state 19

The 5scrg ('H,) - (3 n2) doublet has been rotationally analysed, using OODR

via v = 0 of the b(') state, for v =0 and 1, Figs. 4.7 and 4.8. Rotational spectra for

v = 2 of the 5scrg (3H 1 ) state has also been recorded in spectra studying v =0 of the

d(' Hg ) state, see Figs. 3.13 and 3.14. In the 3fl1 spectra no doublet state transitions

were observed, probably due to the fact that the spectra were maximised to record

the d( Hg ) state signal and therefore the 5SO g signal is only weakly observed. The

rotational energies of the rotational analysed 5SO g states are shown below in Table

A,

Chapter 4. OODR study of the ndirg / nso Rydberg state of 02 excited via the b ( 1 Eg+) state 139

5scr u=0 1 171 1 , 5sau=O fl2, 5sa v=1 '11k , 5s u=1 5so v=2 Energy I cm' Energy / cm' Energy / cm' Energy / cm' Energy / cm- '

0 - - - - -

I - 89927 - 91804.5 93452.9 2 89918.7 89935.5 91796.8 91811.6 93459.8 3 89927.3 89945.7 91805.7 91822.3 93471 4 89939.7 89960.4 91817.8 91836.4 93485.7 5 89955.6 89978.7 91833.2 91854.1 93503.3 6 89976 90001.3 91851.7 91875.3 93525.2 7 89996.6 90024.6 91873.5 91899.9 93548.9 8 - 90052.8 91898.5 91928.1 93576.4 9 90053.2 90084.4 91926.7 91959.7 93607.5 10 - 90119.3 - 91994.9 93641.1

Table 4.6.: Rotational term values, in cm', for v =0 and lof the 5so (3H2) and ('Hi) states and v = 2

of the 5s0rH state. The energy values for the Sscrg (3fl,) state have been taken from spectra recorded

to monitor v = 0 of the d(' H g ) state: Energies relative to X( 3 E), u = 0, J =0 ground state.

Rotational constants have been calculated from the rotational term values

given in Table 4.6 using the expression T+ y.J+ BJ(J+ 1) and are presented in Table

4.7. The errors in B and yare typically ± 0.03 cm' and ± 0.3 cm 1 , respectively. The

values for v = 0 of the 1712 and 'Hi states are in reasonable agreement with those of

Yokelson et al. 19

Experimental. Literature values. State B B

cm' / cm' / cm' / cm

112, Ssag t'O 1.71 -1.3 1.628 -0.58

1 171 1 , 557g t'O 1.68 1.2 1.719 0.75

1712, Ssag v=1 1.61 -0.8

1 11,, 5so v=1 1.75 0.2

'H,, SSOg vr2 1.64 1.3

Table 4.7.: Rotational constants, in cm', of some of the 5so Rydberg states of 02. The literature

values were taken from Yokelson et al.


The rotational energies for the 5sag states presented in Table 4.6 can also be

fitted using the conventional T + BJ(J + 1) expression without significantly increasing

the standard deviation of the fit. For example, the data for v =0 of the Sscrg (31712) and

('liii) states could also be fitted using B values of 1.78 and 1.59 cm -1 respectively.

4.6. Predissociation of the 02 Rydberg states.

The numerous REMPI studies of the 3SO g Rydberg states of 02 have been

reviewed by Morrill etal. 10 The 3so C(3 Hg ) and 3sa d('flg) states interact with

several valence states. Both states are crossed by the repulsive 1101 1 g) state, which

interacts strongly with the C(3 Ll g ) state. The primary interaction of the d(flg) state

is with the II ( 1 11g) valence state which is bound in the region of the crossing, but

which has a much larger equilibrium bond length. The crossing between

the d('[Ig ) and the II ( 1 11g) states occurs between v = 2 and 3 of the Rydberg state

which are most strongly perturbed.

All of the interactions have been modelled by Lewis et al. ' using a coupled

Schrodinger equation (CSE) approach. This method accurately predicts the line

positions for the first four vibrational levels of the d('Hg ) state, even in regions of

strong perturbation.

In addition, it was predicted that interactions of the bound Rydberg states with

the repulsive valence states would result in an oscillatory broadening of the rotational

lines, e.g. predicted linewidths for J = 1 of v = 1, 2 and 3 of the C(3 [Ig ) state were

150, 1.5 and 77 cm, respectively. Experimental linewidths of several vibrational

Chapter 4. OODR study of the ndrg / nso Rydberg state of 02 excited via the b ( 1 Eg+) state 141

levels of both the C( 3 ITl g )and d('FIg ) states have been reported 10. The calculated

linewidths reproduce the trends in experimental values well.

In the higher ns clusters the ii 1 and I1i states are mixed and both interact

with the repulsive II (3171) state and the II ( 'I-1g) state above its dissociation limit. The

effect of these interactions on the linewidths has also been simulated by Morrill et al.

'° The predicted linewidths for v =0-4 of the 4SCTg (3fl1) state vary between 3.5 and

6.5 cm-1 while those of the 4SCFg ('H 1 ) state vary between 1.9 and 8.3 cm-1 . These

values are consistent with those observed (- 6 cm') in the spectra in Fig. 4.3,

although the spectral quality was not high enough to observe any oscillation in the

values. v = 1 of the 4SO g (1II) state is anomalously sharp as it interacts

heterogeneously with the close lying 3dlrg (3g) state 19

It was also predicted that the linewidths of v =0 and 1 of the 5s cluster should

be less that 1 cm'. This is consistent with the observed values of— 2 cm'. which

probably includes a small amount of power broadening. Similar narrow linewidths are

observed for the higher flSCTg states and explain the relative ease with which they are

seen, especially compared to the 4SCTg states.

The same dissociation pathway is also available to the ndirg ('H i) states and

must, in part, explain why these states are not observed with any obvious intensity in

two-photon spectroscopy from the X( 3 ), a('L 5 ) and b(' ) states. This is

especially true of the 3d complex which are nearly isoenergetic with the heavily

predissociated 4s complex. From comparison with the higher ns complex it is

expected that some of the higher ndir ('H') states should be much less predissociated.

Chapter 4. OODR study of the ndir5 / nscrg Rydberg state of 02 excited via the b ( g4) state 142

4.7. Conclusions.

The OODRIREMPI excitation scheme previously used to study the

d(1 Hg ) state has been extended to higher energy to investigate the higher energy the

nds'r/ flSO g gerade Rydberg states of 02 excited via single rotational levels of the

b(' ) valence state. The use of OODR spectroscopy allowed higher vibrational

levels, than previously observed, of many of the ndirl flSO g states to be identified due

to the simplification of the recorded spectra. In general, the observed energies agree

favourably with previous studies and the newly observed levels were consistant with

these values, as shown by consistent quantum defects over the entire n-Rydberg

series.

Both the nso g (H)- ( 1H 1 ) Rydberg states and fldEg (3) - ( 11) Rydberg

states show evidence for a transition from (AS) coupling scheme to a (,a) coupling

scheme as the principal quantum number, n, increases to n > 5. Evidence for this

transition includes linestrengths and splitting energies, which become similar to that

of the ground state of the 02 + ion, the 02+ X( 2 I1 1125 )and 02+ X(2 113i2g )ion-core

states. The flSag states also show evidence for an S-uncoupling interaction between

the 'IT 1 - 31T12 and the 3fl 1 - 3fl0+ states. A direct result of this interaction was that the

rotational linestrengths in the linearly and circularly polarised spectra of transition to

the nscrg (3fl2) and (11 +) states are those of An = 1 transitions not of A) =2 and 0.

Although no rotational analysis has been undertaken, the nd2rg Rydberg states

Of 02 appear to be ideal as intermediate states used to study the b(' Y-') valence state

distribution produced from the photolysis of ozone. One good example would be the

Chapter 4. OODR study of the ndrg / nso Rydberg state of 02 excited via the b ('g) state 143

3diz (g) state. The rotational structure observed from the ground state using (2+1)

REMPI appears to be strong, sharp and regular, showing that the state is probably not

perturbed. OODR studied using the 3drg ( 1 g) state would probably give a clear

rotational distribution of the b(' ) valence state.

4.8. References.

A. Sur, C.V. Ramana and S.D. Colson, J. Chem. Phys. 83,(1985) 904.

A. Sur, C.V. Ramana, W.A. Chupka and S.D. Colson, J. Chem. Phys. 84,(1986)

W.J. van der Zande, W. Koot, J.R. Peterson and J. Los, Chem. Phys. Left. 140,

(1987) 175.

P.J.H. Tjossem and K.C. Smyth, Chem. Phys. Left. 144, (1988) 51.

W.J. van der Zande, W. Koot, J. Los and J.R. Peterson, J. Chem. Phys. 89, (1988)

6758.

J.A. Stephens, M Braunstein and V. McKoy, J. Chem. Phys. 89,(1988) 3923.

W.J. van der Zande, W. Koot and J. Los, J. Chem. Phys. 91, (1989) 4597.

A. Sur, R.S. Friedman and P.J. Miller, J. Chem. Phys. 94,(1991) 1705.

A. Sur, L. Nguyen and N. Nikoi, J. Chem. Phys. 96,(1992) 6791.

J.S. Morrill, M.L. Ginter, B.R. Lewis and S.T. Gibson, J. Chem. Phys. 111 (1999)

173.

B.R. Lewis, S.T. Gibson. S.S. Banerjee and H. Lefebvre-Brion, J. Chem. Phys.

113, (2000) 2214.

Chapter 4. OODR study of the ndirg / nso Rydberg state of 02 excited via the b ('g4) state 144

R. Ogorzalak Loo, W.J. Marinelli, P.L. Houston, S. Arepalli and R.W. Field, J.

Chem. Phys. 91, (1989) 5185.

R.D. Johnson III, G.R. Long and J.W. Hudgens, J. Chem. Phys. 87, (1987) 1977.

B.R. Lewis, S.T. Gibson, J.S. Moth!! and M.L. Ginter, J. Chem. Phys. 111 (1999)

ILTel

P. O'Keeffe, T.Ridley, K.P. Lawley, R.J. Donovan, H.H. Telle, D.C.S. Beddows

and A.G. Urena, J. Chem. Phys. 113 (2000) 2183.

T.Ridley, K.P. Lawley, H.A. Sheard and R.J. Donovan, J. Chem. Phys. 116 (2002)

451.

P. O'Keeffe, T.Ridley, H.A. Sheard, K.P. Lawley, R.J. Donovan and B.R. Lewis,

J. Chem. Phys. 117 (2002) 8705.

H. Park, P.J. Miller, W.A. Chupka and S.D. Colson, J. Chem. Phys. 89,(1988)

6676.

R.J. Yoke!son, R.J. Lipert and W.A. Chupka, J. Chem. Phys. 97, (1992) 6153.

S.T. Pratt, J.L. Dehmer and P.M. Debmer, J. Chem. Phys. 97, (1990) 3072.

R.J. Yoke!son, R.J. Lipert and W.A. Chupka, J. Chem. Phys. 97, (1992) 6144.

W.L. G!ab, P.M. Dehmer and J.L. Dehmer, J. Chem. Phys. 104, (1996) 4937.

H. Park, L. Li, and W.A. Chupka, Chem. Phys. Left. 162, (1989) 317.

H. Park, L. Li, and W.A. Chupka, J. Chem. Phys. 92, (1990) 61.

H. Park, L. Li, W.A. Chupka and H. Lefebvre-Brion, J. Chem. Phys. 92, (1990)

5835.

T.G. Slanger and P.C. Cosby, J. Chem. Phys. 92, (1988) 267.

R.G. Bray and R.M. Hochstrasser, Mo!. Phys. 31, (1976) 1199.

Chapter 5. The wavelength dependence of the ionisation and dissociation products ofSO 2 145

Chapter 5.

The wavelength dependence of the ionisation and

dissociation products of SO 2 .

5.1. Introduction.

The photodissociation of 03 and the spectroscopy of its 02 photo-fragment is

extended in the present work to include sulfur dioxide, SO2. As previously stated, SO2

is isovalent with 03, they share similar ground states and equilibrium geometries. The

most significant differences between SO2 and 03 are their dissociation energies.

The six lowest dissociation channels of 03 and SO2 are shown in Table 5.1

along with their thermodynamic thresholds, which can be accessed from the (0,0,0)

levels of their ground states.

Channel Threshold Channel Threshold

/mn

03 X( 1A 1 ) -+02 X( 3 E;) ±0 ( 3P) 1178 SO2 X('A 1 ) - SO X( ) +0 (P,) 219

03 X( 1A 1 ) -+02 a('A R )+ 0( 3P) 612 S0 2 X('A 1 ) -> SOa('A)+ O(3P) 194

03 X('A 1 ) -3 02 b(' )+ 0(3P) 463 S02 X('A 1 ) -* SO b( 1 E)+ O(3P) 178

03 X( 1A 1 )-* 02 X( 3 )± O('D) 411 SO2 X(A 1 )-* SOX( 3 r) + 0('D) 162

03 X('A)-* 0 2 a('A g )+ O('D) 310 502 X('A 1 )-* SOa('A)+ O('D) 148

03 X('A 1 )-* 02 b( )+ O('D) 267 SO2 X('A 1 ) SOb()+ 0('D) 139

Table 5.1.: The thermodynamic thresholds, in run, for the six lowest dissociation channels of 0 3 and

SO2.

Chapter 5. The wavelength dependence of the ionisation and dissociation products ofS0 2 146

Since the dissociation threshold of 03 is so low, the absorption of a single

photon of visible (770 - 390 run), UV (390 - 200 nm) and VUV (<200 nm) laser

radiation by 03 provides enough energy to dissociate it. Whereas, SO2, which

possesses a much higher dissociation threshold, requires a high energy UV or VUV

photon to induce one-photon dissociation. Using commonly used laser wavelengths

(visible and UV) dissociation of SO2 must be achieved by the absorption of at least

two photons. Therefore, the dissociation (and spectroscopy) of SO2 can be greatly

altered if the first absorbed photon is resonant with an electronic state below the

dissociation threshold.

One-colour laser resonant enhanced multi-photon ionisation (REMPI)

experiments performed upon jet cooled SO2 yield different ion species depending

upon the wavelength of the incident radiation. The major ion-channels observed in

one-colour REMPI experiments are SO and S. The rest of this Chapter provides an

overview of the interesting aspects of the one-colour spectroscopy of SO2.

Four specific wavelength regions in the one-colour REMPI spectrum of SO2

are of spectroscopic importance, these being; 212 - 230 nm, 245 -295 run, 340 -410

nm and 410 - 455 nm. The four regions correspond to (a) one-photon excitation of the

SO2 C('B 2 )+-X('A 1 )transition, (b) one-photon excitation of the SO d(H) — a('A)

transition, (c) the one- and three-photon excitation of the SO2 a( 3B 2 )4— X(A 1 ) and

SO2 G +— X('A 1 ) transitions and (d) the two-photon excitation of the SO2

C('B2 ) +— X('4) transition, respectively.


5.2. One-colour REMPI spectroscopy of S02 in the 212 - 230

nm wavelength region.

One-colour REMPI spectra of SO2 in the 212- 230 nm wavelength region

recorded by collecting the SO are shown in Figs. 5.1 and 5.2. 212 - 230 nm photons

are able to pump the one-photon C(B2 ) +- X('4) transition in SO2. A further photon

absorption from the C(B2) state results in excitation to unknown state(s) in the 86957

- 94340 cm-1 energy region, relative to v =0 of the ground state. The two spectra were

recorded using defocussed (Fig. 5.1) and unfocussed (Fig. 5.2) laser radiation. No ion-

signal was observed in the SO2, S/02 and O ion channels at these wavelengths

using either focussing condition. The resonant structure observed in the SO ion-

channel corresponded to the one-photon C('B2 ) - X('A 1 ) transition.

The assignment of the (v I ,v2,v3) band structure of the C(B2) state, indicated in

Figs. 5.1 and 5.2, has been performed by many groups, the most thorough assignment

has been performed by Yamanouchi et al. 6 Tnitions are observed from the (0,0,0)

level of the SO2 i('A1 )state to the (0,n,4)[n = 0 - 1], (1,n,0) [n = 1 - 4], (1,n,2) [n = 0

- 71, (2,n,0) [ii = 0 -2] and (3,n,0) [n = 0 - 31 levels of the C(B2) state in Fig. 5.1 and

(1,n,0) [n = 3 -4], (l,n,2) [n = 1 - 7], (2,n,0) [ii = 0 -2] and (3,n,0) [n = 0 - 31 levels of

the C('B2 ) state in Fig. 5.2. It is apparent that transitions are observed to several V2

bending series (v2 = 370 cm) and that only even values of the v3 asymmetric stretch

are observed.


I I I I I I I I SO2 (1 B) (1,n,2)

172 162 152 142 132 122 112 102

0) Cl) C 0

+ 0 Cl)

s02 c('B) (3,n,0)

330 320 310 300

SO2 (1 B2) (2,n,0)

PH I 220 210 200

I I SO2 ('B) (1,n,0)

140 130 120 110

022

SO2 (B) (0,n4)

14 004

212 214 216 218 220 222 224 226 225 230 232 234

One-photon Wavelentgh / nm

Fig. 5.1.: One-colour (212 -232 nm) REMPI spectrum ofjet cooled S0 2 X('A1 )state recorded by

collecting the SO ion-channel. The spectrum shows resonances attributed to the one-photon

C('B2 ) <-- .V(A 1 ) transition in SO2 . The spectrum was recorded using defocussed laser radiation.

SO2 6(B) (1, n,2)

172 162 152 142 132 122 112

SO2 C(1 B) (3.n,0) SO2 ö(1 B2) (1,n,0)

F-I 330 320 130 110

SO2 1B2) (2,n,0)

220 210 200

212 214 216 218 220 222 224 226 228 230

One-photon Wavelength / nm

Fig. 5.2.: One-colour (212.5 -229.5 mn) REMPI spectrum ofjet cooled S0 2 X('A1 )state recorded by

collecting the SO ion-channel. The spectrum shows resonances attributed to the one-photon

C('B2 ) - X('A 1 ) transition in SO2 . The spectrum was recorded using unfocussed laser radiation.

Cu C 0)

Cl) C 0

+ 0 U)


Although no high-resolution REMPI spectra are presented, Yamanouchi et al.

have reported high-resolution ( 0.08 cm') laser induced fluorescence (LIF) spectra

for 33 (vl,v2,v3) C('B2 ) +- (0,0,0)X('A 1 ) transitions encompassing the majority of the

transitions observed in Figs. 5.1 and 5.2. The reported rotational analysis of these

spectra show that v3 :;e 0 vibronic levels are perturbed by a Coriolis interaction with

nearby vibrational levels possessing odd V3 quanta.

Due to the focussing conditions used whilst recording the REMPI spectra

(Figs. 5.1 and 5.2), only allowed single photon transitions are likely to be observed,

since, due to the low photon intensity, multi-photon and forbidden transitions are

highly improbable. This fact not only restricts the absorption of radiation from the

SO2 X( 1A 1 ) state to the allowed one-photon C('B2 ) — X( 1A1 ) transition, but also

restricts further photon absorption from the C(B 2 ) state, and states within SO, to one-

photon allowed transitions. The one-colour, three photon, thermodynamic threshold

for formation of SO X( 21, 2 ) from SO2 X('A 1 ) occurs at 232.7 urn ([Do (SO2

X(1A1) _> So X( 3 ) + 0 (3P2)) 45725.3 + IP (SO) 83155] - 3), therefore a

minimum of three photons of wavelength 212 - 230 nm are energetically required to

produce S0 X( 2P12 ) from SO2 X( 1A 1 ).

The SO X( 3 Z-) + 0 (3P2) dissociation limit at 45725.3 cm 2 occurs at an

energy which excites the (1,4,2) band of the C('B2 ) state. For an SO ion-signal to be

observed (Figs. 5.1 and 5.2) at energies below this threshold (i.e. at wavelengths

greater than 218.6973 urn) a second photon must be absorbed by SO2 to excite S02 *

prior to dissociation.


The most likely mechanism for the production of S0 from SO2 requires the

second absorbed photon to be resonant with a repulsive state, which is responsible for

dissociating SO2 to SO* which can then be ionised by a third absorbed photon. This

mechanism has the advantage that all the transitions are single-photon transitions and

can therefore be excited by the unfocussed radiation. Dissociation of S02* can also

produce lower energy states of SO, which can be ionised via two photon absorption in

the SO channel, assuming that the two photons are absorbed resonantly (see Section

5.4)

A less likely mechanism for the production of S0 involves the absorption of

a third photon by S02* to produce S02t S0 may be formed subsequently, either by

predissociation of the S02, which is very unlikely, or by absorption of a further

photon to excite a repulsive state of S02, which can dissociate directly to S0.

Although either mechanism could be responsible for the production of S0 from

SO2, the experimentally observed lack of S02+, argues against them, since one would

still expect to observe some residual S02. A second drawback of the mechanism is

that it requires a two-photon (probably non-resonant) ionisation process in SO2, which

is unlikely due to the focussing conditions used.



Transitions attributed to the SO2 C(B2 ) state can also be observed in the 410 -

455 nm wavelength region recorded in the S0 and S ion-channels, as shown in Fig.

Chapter 5. The wavelength dependence of the ionisation and dissociation products ofS02 151

5.3. 410 - 455 rim photons are able to pump the two-photon C('B2 )'—'c--X('A 1 )

transition in S02. At the one-photon level resonance cannot be achieved with another

electronic state from the (0,0,0) level of the ground state, since the photon does not

possess enough energy to pump any electronic transition. Excitation at the three and

four photon level is possible, although, this results in excitation to unknown state(s) of

SO2.

The transitions, observed in Fig. 4.3, therefore correspond to the two-photon

allowed C(B2) +-+-- X(A 1 ) transition. The spectra were recorded using tightly

focussed laser radiation due to the coherent nature of the two-photon transition.

SO2 C632) (1 .n.2)

1.10,2 192 182 172 162 152 142 132 122 112 102

SO2 6(1 B2) SO2 C(1 B,) (2.n.0) (3,n,0) [J I

330 320 310 300 220 210 SO2 6(1 B,) (1,n,0)

140 130 SO+ ion

I • • I • I • I • I • I • I •

410 415 420 425 430 435 440 445 450 455

One-photon Wavelength I nm

Fig. 5.3.: One-colour (412.5 - 453 nm) REMPI spectrum ofjet cooled SO 2 X(A1 ) state recorded by

collecting the SO and S ion-channels. The spectra show resonances attributed to the two-photon

C('B2 X( 1A) transition in SO2 . The spectrum was recorded using tightly focussed laser

radiation.

Co

0) (1) C 0

+ 0 co U)


Transitions are observed from the (0,0,0) level of the SO2 X('A 1 ) state to the

(1,n,0) [n = 3 - 4], (1,n,2) [n = 0 - 10], (2,n,0) [n = 1 - 2] and (3,n,0) [n = 0 - 3] levels

of the C( B2 ) state in the SO ion-channel. The strongest of these resonances are also

seen in the S ion-channel.

If the relative intensities of the resonances in the SO and S ion-channels are

compared, it is apparent that some of the S resonances are intensified. For example,

the (1,7,2) band appears as two - equal intensity bands in the SO ion-channel (the

(1,7,2) band appears as two bands a result of a Coriolis interaction). The two (1,7,2)

bands in the S ion-channel possess different intensities, the higher wavelength band

being x5 the intensity of the lower wavelength band. This observation can be

explained thus: part of the S ion-signal originates from the dissociation of S0* to

form S +0, the S atoms then absorb further photons to produce S. If these further

absorbed photons in the S channel correspond to a transition in sulphur then the

amount of S produced will increase. The intensification of the S signal at 427.509

nm is due to a three-photon atomic transition, S 5s (5S3) —*--- S 3 p (3P2),

(70 173.968 cm) as shown in Fig. 5.4.

U) 4-a

C

C D)

C') C 0

+ Co

Chapter 5. The wavelength dependence of the ionisation and dissociation products ofSO 2 153

(3+1) 3D°3+-+-+--'P 2

(3+1)

Li(3+1)

(b) Atomic Sulfur absorption

- _ S signal (a)

425 430 435 440 445 wavelength/ nm

Fig. 5.4.: A comparison of the S signal which results from the one-colour, 425- 444 nm, REMPI

spectrum ofjet cooled SO 2 X( 1A 1 ) state (a) and a (3+1) REMPI spectrum of the atomic resonances of

sulfur in the 425 - 442 am region excited in a two-colour experiment (b).

The 5D <--<--- 'D transition gives rise to a further example of this type of

enhancement of the S signal. The two-colour spectrum shown in Fig 5.4(b) was

recorded by exciting a strong C('B2 ) +— X( 1A 1 ) one-photon transition (and

subsequent dissociation following absorption of a second photon) with the unfocussed

pump laser and scanning the focussed probe laser over the one photon range shown.




One-colour REMPI spectra ofjet cooled SO2 X(A 1 ) state in the 245 - 295 nm

wavelength region recorded by collecting the SO and S are shown in Fig. 5.5. No

significant SO2 ion-signal was observed within the entire wavelength region.

(b) )LLL D Co C C)

C,) C

0

(aJLj ° 245 250 255 260 265 270 275 280 285 290 295

Wavelength I nm

Fig. 5.5.: The one-colour REMPI spectra ofjet cooled SO 2 X(A 1 ) state in the 245 - 295 nm

wavelength region recorded by collecting the (a) SO and (b) S' ., under defocussed conditions.

The features in the SO ion-channel relate to resonances within the SO

channel. Speth at a1 2 have reported (1+1) REMPI spectrum of the SO a() state

(produced via photodissociation of SO2) in the 245 - 280 nm wavelength region. They

observed transitions from the SO a(A) state (a single transition was also reported to


be observed from the SO b(' state) to three SO singlet state at energies of 44551.1,

51326.6 and 52031.2 cm-1 (relative to v =0 of the SO X(3 y-) state), which they

assigned to v = 0, 9 and 10 of an unknown 1 171 state. The 'fl state were later re-

assigned, by Bonn and Ornallas 3 and Archer et al. ', to v =2 of the SO d( lIE) state

and v = 0, 1 of the SO e('lIE) state, respectively.

SO a('i)

vibrational level

2hv threshold

for SO a(A)

formation from

2hv threshold

for SO a(L1)

ionisation

s02_1('4)

e(1T1), v=0

() transition

e( 1 11), v 1

-a()

transition

d('171), v=2

transition

mm mm mm mm mm

0 387.7 259.2 219.9 216.6 265.2 1 379.6 262.9 225.4 221.8 273.2 2 371.9 266.8 231.0 227.3 281.6 3 364.6 270.6 236.9 233.0 290.4 4 357.7 274.6 243.0 238.9 299.6 5 351.1 278.6 249.4 245.1 309.3 6 344.9 282.6 256.0 251.4 319.5 7 338.9 286.8 262.8 258.0 330.3 8 333.2 291.0 270.0 264.9 341.6 9 327.8 295.2 277.4 272.1 353.6 10 1 322.6 1 299.6 285.1 279.5 366.3 11 1 317.7 1 304.0 293.2 1 287.3 379.8

Table 5.2.: The rotationless two-photon thresholds, in nm, for formation and ionisation of u = 0 - 11 of

the SO a('A)state. The SO (v0, 1) e('1Tl)-(v=0- 11) a('A) and the SO(v2) d( 1 11)-(v

=0 - 11) a('A) rotationless transition wavelengths are also shown.

The two-photon thresholds for the formation of rotationless SO a() state via

the SO2 Y(A 1 ) state and the two-photon thresholds for the ionisation of rotationless

SO a(A) state are reported in Table 5.2 for v =0 to 11, the limit of the known

molecular constants of the SO a('A) state. The SO (v = 0, 1) e('ITl) - (v = 0 - 11)

a(A) and the SO (v =2) e(' [I) - (v = 0 - 11) a('i) rotationless transition

wavelengths can also be seen in Table 5.2. For a (1+1) REMPI transition to be

Chapter 5. The wavelength dependence of the ionisation and dissociation products of SO 2 156

observed the wavelength of the transition must be lower than both the two-photon

formation and ionisation thresholds. It should be noted that the high rotational levels

(2,0) band of the d('[I) - a(1A) transition can be observed, even though the

transition appears to be disallowed by Table 5.2. Vibrational levels of the e(n) state

with v ~! 2 can not be observed, since these levels lie above the S ('D) +0 (3Pj)

dissociation limit and are therefore predissociated.

I I t I i I 111111 IIIiIIiIiIiIIiiiIIiR

40 - . 30 - 20 .100

I I I I I I I I I I I I I I II I I I I I 111111 111111 30 20 10 0

30I I I I I ii IIIII II I P

20 10 1

WE

Cu C 0)

C') C 0

+

0

36600 36620 36640 36660 36680 36700 36720 36740 36760 36780

One photon Wavenumber I cm

Fig. 5.6.: The one colour REMPI spectrum ofjet cooled SO2 showing the SO (v = 2) e('H) +- (v= 9)

a('A) transition. The rotational analysis is taken from Ref. 2.

The SO (v =2) e(' H) - (v =9) a(A) transition recorded in the SO

channel is shown in greater detail in Fig. 5.6. Rotational analysis of the band has been

achieved using the a(A) and e(' 1_1) state constants of Speth at a12

Chapter 5. The wavelength dependence of the ionisation and dissociation products of SO2 157

The spectrum obtained by collecting, S, shown in Fig. 5.5(b) is significantly

different from that obtained by collecting, SO P, although some bands are common to

both spectra. In particular, two intense peaks observed at 263 and 283 nm are not

observed in the SO spectrum. One would be tempted to assign the sharp resonances

within the S4 spectrum to multi-photon atomic resonances. Such atomic resonances

have been shown to occur within the 245 - 295 nm region 5, but are absent in Fig. 5.5

since the spectra were recorded under defocussed conditions. The intense resonances,

in Fig. 5.5(b), will be shown to be due to a newly observed ion-pair state. OODR

excitation of this ion-pair state in SO will be discussed in Chapters 7 an 8.



The three-photon threshold for the ionisation of SO2 (IP (SO2) = 12.349

±0.003 eV) 6 occurs at 301.2 nm and the four-photon threshold for the ionisation of

SO2 occurs at 401.6 nm. Therefore, four photons with a wavelength between 340-400

nm are required for ground state of SO2 to be ionised. Within this energy range lies

(1+3) REMPI via the SO2 21( 3B1 ) state, (1+3) REMPI via the S02 A('A 2 ) state, (2+2)

REMPI via high vibrational levels of the SO2 C('B2 ) state and (3+1) REMPI via the I,

II and III SO2 Rydberg states.

The one-colour REMPI spectra of jet cooled SO2 X(A 1 ) state in the 340 - 410

nm wavelength region recorded by collecting SO2, SOP, S4/024 and O are shown in

Chapter 5. The wavelength dependence of the ionisation and dissociation products of SO 2 158

Fig. 5.7. No significant resonances were observed at wavelengths < 360 nm in the

SO2 ion channel and> 390 rim in the SOP, S/02 and O ion-channels.

One important spectroscopic observation within the 340 - 410 nm wavelength

region is that the parent SO2 ion is observed. The parent SO2 ion is rarely observed

in REMPI spectroscopy of 502 inevitably due to dissociation of highly excited S0 2*

to form SO and 0 fragments. Within the 340 - 410 nm wavelength region for an SO2

ion-signal to be observed from the ground state a resonance is required at the three-

photon level with a singlet Rydberg state. Resonances in the SO2 ion-channel

corresponding to triplet Rydberg states can also be observed in the one-colour REMPI

spectrum if an accidental resonance is also achieved with the S02 (3B 1 ) state.

The S0, S and 0 ion-channels show resonances which can be attributed to

(1+3) REMIPI via the SO2 (3B 1 ) state, (1+3) REMPI via the SO2 A('A2 ) state and

(3+1) REMPI via the II SO2 Rydberg state. Weak resonances can also be attributed to

(2+2) REMPI via high vibrational levels of the SO2 C(B 2 ) state in the SO ion

channel at- 385 -390 rim.

The 340 - 410 nm wavelength region will be discussed in greater detail in

Chapter 6 concentrating upon the Rydberg states structure observed in the SO2 ion

channel.

Chapter 5. The wavelength dependence of the ionisation and dissociation products of SO 2 '59

+ c.'l 40

0 20

0 100

80

60

(l) 40

20

- 0

80

+ 60

40

20

0

8

6

+ 0

2

0 340 350 360 370 380 390 400 410

Wavelength / nm Fig. 5.7.: The one-colour REMPI spectra ofjet cooled S0 2 X('A 1 ) state in the 340 - 410 run

wavelength region recorded by collecting SO 2 , SOP, S/02 and 0 ions. Note, no significant

resonances were observed at wavelengths <360 rim in the SO2 ion channel and > 390 nm in the SO 4,

S4/02 and 0 ion-channels. -

Chapter 5. The wavelength dependence of the ionisation and dissociation products of302 160

5.6. A summary of the one-colour REMPI spectroscopy of

so2 .

Four specific wavelength regions in one-colour REMPI spectrum of SO2 have

been surveyed; (a) 212 - 230 run, (b) 245 - 295 rim, (c) 340 - 410 nm and (d) 410 -

455 nm. The four regions are dominated by; (a) one-photon excitation of the SO2

C('B2 ) +- X('A) transition, (b) one-photon excitation of the SO d( 1 [I) -

transition, (c) the one- and three-photon excitation of the S02 (3B2 ) - X('A 1 ) and - -

SO2 H - X(A 1 ) transitions and (d) the two-photon excitation of the SO2

C('B2 ) +— X('4) transition, respectively.

It is apparent from the REMPI spectroscopy of SO2 that the relative ion yields

Of SO2, SOP, S and O are dependent upon the excitation wavelength used. For

example, an SO2 ion-signal is only observed in the 360 - 410 nm wavelength region

and is absent at other wavelengths. Understanding the excitation and dissociation

processes involved at a particular wavelength should help identify new electronic

states of SO2 and SO and the dissociation pathways between them. The 340 -410 nm

and 245 - 295 nm regions will be discussed in greater detail in Chapters 6 and 7,

respectively.

Chapter 5. The wavelength dependence of the ionisation and dissociation products ofSO2 161

5.7. References.

K. Yamanouchi, M. Okunishi, Y. Endo and S. Tsuchiya. J. Mo!. Struct. 352-353

(1995) 541.

R.S. Speth. C. Braatz and E. Tiemann. J. Mol. Spectro. 192 (1998) 69.

A.C. Bonn and F.R. Ornel!as. Chem. Phys. 96 (1992) 8054.


J.R. App!ing, M.R. Harbol, R.A. Edgington and A.C.Goren. J. Phys. Chem. 97

(1992) 4041.

L. Wang. Y.T. Lee and D.A. Shirley. J. Chem. Phys. 87 (1987) 2489.

Chapter 6 The I, Hand III Rydberg states ofS02 162

Chapter 6.

The 1,11 and III Rydberg states of SO2 .

6.1. Introduction.

The Rydberg states of SO2 within the 73,000 - 87000 cm 1 region have been

studied using both one-colour (3+1) REMPI and two-colour OODRIREMPI

experiments in which SO2, SOP, S/02 and O were collected.

As previously stated in Chapter 5, one important spectroscopic observation

within the 360 -410 nm wavelength region is that the parent SO2 ion is observed. It

has been found experimentally that for an SO2 ion-signal to be observed from the

ground state a resonance is required at the three-photon level with a singlet Rydberg

state. Resonances in the SO2 ion-channel corresponding to triplet Rydberg states can

also be observed in one-colour REMPI spectroscopy if an accidental resonance is also

achieved with the SO2 a(3B 1 ) state.

The three-photon threshold for the ionisation of SO2 UP (SO2) = 12.349

±0.003 eV) 1 occurs at 301.2 nm and the four-photon threshold for the ionisation of

SO2 occurs at 401.6 nm. Therefore a minimum of four photons with a wavelength

between 345-400 nm are required for the SO2 k(A 1 ) state to be ionised. Within this

energy range lies (1+3) REMPI via the SO2 (3B 1 ) state, (1+3) REMPI via the SO2

A('A 2 ) state, (2+2) REMPI via high vibrational levels of the SO2 C( 1 B 2 ) state and

(3+1) REMIPI via the I, II and III SO2 Rydberg states.

Chapter 6. The I, II and III Rydberg states of S02 163

The electronic assignments of the Rydberg states observed within the 3 45 -4 10

rim region are either unknown or tentatively assigned. The three SO2 Rydberg states

observed in the 360 -410 nm region will therefore be denoted by I, II and III from

lowest to highest energy, respectively.

6.1.1. Previous studies.

6.1.1.1. The SO2 21( 3B1 ) state and SO2 ;i(,42 ) state.

The lowest energy absorption in the electronic spectrum of SO2 from

the X( 1A 1 ) ground state is attributed to the ( 3B 1 ) - X( 1A 1 ) transition. The singlet-

triplet transition is spin forbidden and is therefore only observed weakly in the SO2

absorption spectrum. Brand et al. 2 therefore had to use a high concentration (800

Ton) of SO2 to obtain a medium-resolution absorption spectrum of the

SO2 21( 3B 1 ) state at room temperature. Although the spectrum was recorded at

medium-resolution, identification and assignment of the band structure for the SO2

21( 3B 1 ) - k(A 1 ) transition was possible. Both Brand et al. 23 Merer 4 were also

able to partially resolve the rotational structure for the (000)-(000), (010)-(000),

(100)-(000) and (1 10)-(000) transitions. Perturbation of the (I 10) band of

the a(3B 1 ) state was identified by both authors and attributed to vibronic interaction

with a (3A') state.

Brand and Names 3 also studied the SO2 A('A 2 ) - X(A 1 ) transition, which is

electronically forbidden, again using absorption. Isotopic shift data was used to

Chapter 6 The I, II and Ill Rydberg states ofS0 2 164

identify and assign the band structure. Hamada and Merer 5 concluded that the

transition is electric-dipole forbidden unless odd quanta of V3 (b2) are excited,

producing B1 vibronic levels. A complete vibrational assignment of the SO2 A('A 2 ) 4-

X('A 1 ) transition is reported by Hegazi et al. 6

The first REMPI study of the SO2 ( 3B 1 ) - X( 1A) transition was reported in

1980 by Colson et al. ' Linearly and circularly polarised radiation (360-390 run) was

used to produce the REMPI spectra. Both a static cell and a jet-cooled molecular

beam of SO2 were used; the static cell contained only 10 Torr compared with 800

Ton for the absorption stud y 2 The spectra showed the SO2 ( 3B1 ) e-

1('A 1 ) transition in greater detail than absorption. Resonances at the three-photon

level were also observed and attributed to a Rydberg state in SO2.

6.1.1.2. Photoelectron study of the three lowest-lying electronic states

of the SO2 ion-core.

The potential energy surface for an unperturbed Rydberg state should be

similar to that of the ion-core state on which the Rydberg state is based. A study of the

vibrational structure for various electronic states of an ion should therefore be useful

when determining the core structure of Rydberg states.

The three lowest-lying electronic states of the SO2 ion-core have been studied

by. Wang etal. 1 using photoelectron spectroscopy of jet cooled SO2. The three

lowest-lying electronic states of SO2 are X( 2A 1 ), A( 2A 2 ) and g( 2B2 ) ion-core states.

Chapter 6 The I, II and III Rydberg states ofS0 2 165

The vibrational structure of the SO2 X( 2A 1 ) ion-core state can be assigned to

a single V2 progression. The vibrational spacings of the progression are regular for V2

= 0 - 7, peaking at V2 3, and a least-squares fit of G(v2 + 1/2) vs v2 over this range

yielded the following spectroscopic constants: a'e = 404 ± I cm and (DeZe = 1.5 ± 0.3

cm.

The vibrational structure of the SO2 A( 2A) and B( 2B ) ioncore states

overlap but, due to the high spectral resolution achieved by Wang et al. 1, vibrational

progressions in both the A( 2A 2 ) and §(2 B2 ) ion-core states can be assigned. The

vibrational structure of the S02 A( 2A 2 ) ion-core state can be assigned to several

vibrational progressions: (0,0,v3) for V3 = 0 - 2, (0,v2,0) for V2 0 - 3, (1,0,V3) for v3 =

0 - 2 and 4 and (0,v2,4) for V2 =0 -4. The vibrational spacings for the progressions are

irregular and no toe or coeXe values could be obtained, typical spacings include: vi

(1,0,0)-(0,0,0) = 981 cm -1 , V2 (0,1,2)-(0,0,2) = 353 cm and v3 (0,0,1)-(O,0,0) = 202

cm

The vibrational assignment for the B( 2 B2 ) ion-core state is reasonably

straightforward: Two vibrational progressions can be observed: (0,v2,0) for V2 = 0- 13

and (0,v2,2) for v2 = 0 - 10, peaking at v2 = 3 and 5, respectively. A least-squares fit of

G(v2 + 1/2) vs V2 yielded the following spectroscopic constants: co,, =489±5 cm-1 and

O-)e%e = 2 ± 1 cm'.

The bending mode frequencies and ionisation thresholds for the three SO2

X( 2A 1 ), A( 2A 2 ) and B(2B2 )ion-core states are summarised in Table 6.1. These

values can be used to assign the ion-cores for Rydberg states observed in the

spectroscopy of SO2.

Chapter 6. The I, Ii and III Rydberg states ofS0 2 166

Ion-core state 4 (v2) / cm' v2 levels observed Ionisation threshold / cm'

X(2 A,) 404 0-7 99,601.4

A(2A2) 353 [(0,1,2)-(0,0,2) spacing} 0-2 104,755.2

B( 2 B2) 489 0 - 13 107,578.2

Table 6.1.: Vibrational frequencies, in cm', for the v2 bending mode and the number of v 2 levels

observed in a single vibrational progression for the SO 2 1(2A 1 ), A( 2A 2 ) and B(2B2 ) ion-core states.

The ionisation thresholds, in cm', for the SO 2 ion-core states are also given '.Note; an (0e (v2) value

cannot be calculated for the A( 2A 2 ) state due to its irregular progression, a value for the (0,1 ,2)-(0,0,2)

spacing has been given instead.

6.1.1.3. VUV absorption experiments from the SO 2 X(' A 1 ) state to the

singlet 1,11 and III Rydberg states.

Two previous studies encompassing the 136-120 nm region (corresponding to

three-photons of 360-400 nm radiation), published in the 1960's by Golomb etal. 8

and Watkins, 9 observed the 'I, 'II and LIII Rydberg states via one-photon absorption.

Golomb et al. 8 tentatively assigned the absorptions as being due the lowest members

of three Rydberg series whose higher members were seen to converge upon three

different ion-core states.

Watkins' 9 one-photon absorption study showed that the transitions within the

region 136-120 nm region arose from different Rydberg series than those assigned by

Golomb et al. 8 Watkins 9 observed three singlet Rydberg states within the 136-120

nm wavelength range in both S' 602 and S' 802, labelled as the 'I, 'II and 'III states.

Two vibrational progressions were observed, (0,v2',O) and (1 ,v2 1,0), for the I state,

Chapter 6. The 1, II and III Rydberg states ofS0 2 167

where v2' = 0 - 2. A single vibrational progression, the (0,v 2 1 ,0), was observed for the

II and ljjj states, where V2' = 0 - 4 and 1 - 6, respectively. The transition energies for

the observed vibrational levels were reported to be reproduced by Equations (6. 1),

(6.2) and (6.3) for the 1, 'II and 'III states, respectively, in S' 602.

v( !J) / cm-1 = (74599 + 1068 v 1 '+ 351 v2' - 2 V2' -12 V! ' V2 ') (6.1)

v (UJJ) / cm-1 = (78722 + 399 V') (6.2)

v (!JJJ) I cm-1 = (81087 + 398 V2 ') (6.3)

Isotope shift data was used by Watkins 9 to extrapolate to the unobserved

(0,0,0) !jjj state band position at 81087 cm -1 , as given by Equation (6.3). No attempt

was made to assign the electronic structure of the observed Rydberg states.

Photoabsorption and fluorescence cross sections of SO2 were measured using

synchrotron radiation in the 106-133 nm region by Suto et al. 10 This low-resolution

absorption spectrum in the 119-133 nm wavelength range showed structure due to the

lj !jj and !jjj Rydberg states. The authors made no attempt to identify or assign the 'I

state resonances, presumably due to the weak signal intensity and limited vibrational

information in their low-resolution absorption spectrum. Resonances to the ljj

Rydberg state were observed strongly. A vibrational progression in the (0,v2',O) band

series was observed for V2' 2 to 9, peaking at V2' 4, with V2 ' 380 cm-1 . Comparing

the spectral assignment with that of Watkins 9 for the 'II state, it is clear that the v2'

assignment of Suto et al. 10 is higher by two quanta.

Chapter 6 The!, II and III Rydberg states ofS0 2 168

Quantum defect calculations performed by Suto et al. 10 showed that the 1II

Rydberg state possessed an n = 2.27, assuming that the Rydberg state converges

upon the SO2 X( 2A 1 ) ion-core state. If the vibrational assignment of Watkins 9 is

correct, n increases to n = 2.31. The Ijj Rydberg state was stated to result from a

transition from the 8a1 orbital (the V(A,) ground state HOMO) to the 4s orbital. A

quantum defect of 1.73 (or 1.69) for n =4 was noted to be comparable with those of

the ns orbitals of atomic 0 and S, which are 1.2 and 2.0, respectively.

The 'III Rydberg state observed weakly in the 119-122.5 rim region was

quoted to possess a v'2 of 390 cm-1 , although no vibrational analysis was published,

Suto et al. 10 commented that it was likely that this Rydberg state also converged upon

the SO2 X( 2A 1 ) ion-core state.

6.1.1.4. REMPI studies of the IlRydberg state excited from the SO 2

X(A l ) state.

Colson et al. 7 used static gas transient lensing (TL) and jet cooled REMPI

experiments to study the â(3B 1 ) - X('A 1 ) one-photon transitions in the 360 - 390 nm

region. TL experiments upon a static cell of SO2 at room temperature resulted in a

simple one-photon absorption spectrum of the ( 3B1 ) +- X('A1 )transition. Since the

spectrum was recorded at room temperature high rotational levels of the ( 3B 1 ) were

observed resulting in broad vibrational bands. REMPI studies upon the same static

cell set-up yielded a similar ( 3B 1 ) state spectrum with three additional resonances

attributed to three-photon excitation to a Rydberg state. Colson et al. 7 citing Golomb

Chapter 6. The I, II and HI Rydberg states ofS0 2 169

et al. 8 tentatively assigned the observed Rydberg states to be based upon the

S02X( 2A 1 ) ion-core state. The Rydberg resonances observed by Colson et al. 7 are

the (0,0,0), (0, 1,0) and (0,2,0) bands of the 'II state using the vibrational numbering of

Watkins . One important observation made by Colson et al. 7 was that the Rydberg

state resonances (the ljj state) were not observable under circular polarising

conditions. The REMPI spectrum recorded under nozzle beam conditions showed

significantly narrower peaks due to rotational cooling. Only a small portion of the jet

cooled REMPI spectrum is reported. The (0,0,0) band of the 'II state can clearly be

observed in the jet cooled MPI spectrum at 380.5 nm.

Zhang et al. 11 reported one-colour REMPI spectra of a diffuse beam of SO2

recorded by monitoring SO2, SOP, S and O in the 365 - 405 nm region. The SOP ,

S and 0 ion-channels show resonances which can be attributed to the S02 2i( 3 B1 ) 4-

X('A 1 ) transition. Due to the use of a diffuse molecular beam, several intense hot

bands can be observed which are not observed in jet cooled REMPI spectra 7"2

The SO2 ion-channel showed a spectrum which was significantly different to

those recorded in other ion-channels. Zhang et al. 1 1 incorrectly assigned the

resonances to two-photon SO2 C('B2 )4— X(A 1 ) transitions. The resonances, in fact,

correlate with the 'IlRydberg state at the three-photon level. Hot bands can also be

observed in the Rydberg state resonances, again due to the use of a diffuse beam.

Although no analysis of the Rydberg state structure observed by Zhang etal. "is

performed, the SO2 spectrum can be used to identify hot band positions in 'II

Rydberg state spectra.

Chapter 6 The I, II and HI Rydberg states ofS0 2 170

Xue et al. 12 reported the one-colour REMPI spectrum of jet cooled SO2

recorded by monitoring the SO2 ion channel in the 373 - 381 nm region. The one-

colour, three-photon REMPI spectrum of the II Rydberg state shows peaks

corresponding to the (0,0,0), (0, 1,0) and (0,2,0) vibrational levels. Further resonances

were attributed, by Xue el a! 12 to the a(3B 1 ) - X('4) transition observed due to

accidental resonance with the 3jj state (0,2,0) at 378.1 run, the 'II state (0,3,0) at -

376.7 nm and (0,4,0) at 374.8 run, although these accidental resonances were not

identified in the work of Xue el a! 12 The resonance at 379.7 nm reported by Xue et

al. 12 unidentified.

Based upon the observed bending frequency, 399 cm, the II states were

12 reported to converge upon the SO2 X( A 1 ) ground state. Xue et al. calculated the

effective principle quantum number to be n = 2.31. The quantum defect was

therefore quoted to be either 8 = 1.69 or 0.69 depending upon whether n equals 4 or 3.

Quantum defects for the 3d, 4s and 4p orbitals were estimated from both atomic sulfur

and other sulfur containing molecules, typical values for the quantum defect were

estimated to equal 0. 1, 2 and 1.65, respectively. The 'IlRydberg state was therefore

assigned to a 4pa I orbital since the quantum defects for the 'II state closely matched

the estimated value of a 4p orbital.

Chapter 6. The I, Hand III Rydberg states ofS0 2 171

6.1.1.5. OODR studies of the II Rydberg state excited from the SO 2

x('A) state via the (0,0,0) band of the a(3B1 ) valence state.

OODR experiments recorded by collecting the SO2 and SO ion-channels

were also carried out by Xue etal. 12 using the (0,0,0) band of the ( 3B1 ) state at

387.99 nm as an intermediate state via a [(1)+2'+1'] process. An accidental two-

photon double resonance with an unknown vibrational level of the C(B 2 ) state at

51600 cm' was also reported to be observed via a [2+(1')+1'] OODR process. The

double resonance study via the ( 3B1 ) state showed a vibrational bending progression

in the 3 1 Rydberg state for V2' = 0 in the SO ion-channel and v2' 2 - in the SO2

ion-channel.

Based upon the observed bending frequency, 390 cm', the 311 state was

reported to converge upon the S0 2 X( 2A 1 ) ground state. Xue etal. 12 the

effective principle quantum number to be n = 2.28, giving 6 = 1.72 if n = 4. The 311

state was assigned as a 4p Rydberg state and the singlet-triplet splitting of 526 cm'

was found to be consistent with other known 4p singlet-triplet splitting, such as the

870 cm' splitting of the 4p Rydberg states of N2.

6.2. Experimental.

A one-colour REMPI experimental set-up was used to study the (3+1) REMPI

Of SO2 seeded in He (5% [SO2], 95% [He]) by 345 - 410 nm radiation recorded by

collecting the SO2, SOP, S/02 and O ion-channels. Three laser dyes DMQ, QUI

Chapter 6. The I, II and HI Rydberg states ofS0 2 172

and PBBO were used to generate the required laser wavelengths 345 - 370 nm, 367 -

395 nm and 393 - 410 rim, respectively.

Two-colour OODR experiments collecting the SO2 ion-channel have also

been recorded, where the pump photon was tuned to both ( 3B1 ) <— k(A 1 ) and the

SO2 C(B) —±- X('A 1 ) transitions. One known vibrational level of the ( 3B 1 ) state

was chosen, the (0,0,0) level, and two unknown vibrational levels of the C(B2 ) states

were chosen, corresponding to two-photon (pump) energies of 51026 and 51794 cm'.

An approximate calibration of the spectra was achieved using the known ( 3B1 ) -

X('A 1 ) energies reported by Brand et al. 2

6.3. Results and discussion.

, 6.3.1. An overview of the S02+.) S0+ 9 S+(02+) and 0 ion-channels

observed via one-colour REMPI spectroscopy in the 345 - 410 nm

wavelength region.

The one-colour REMPI spectra of SO2 recorded by monitoring the SO2, S0,

S/(02) and O ion-channels using 345-410 nm radiation are shown in Fig. 6.1. The

SOj spectra were recorded between 360 and 410 nm. Below 360 nm no significant

502f ion-signal was observed. The S0, S/(02) and O spectra were recorded

between 345 and 390 nm. Above 390 nm no significant ion-signal was observed for

these three ions.

Chapter 6 The I, Hand III Rydberg states ofS0 2 173

It is apparent from Fig. 6.1 that the spectrum observed in the SO2 ion-channel

is significantly different from those observed in the other three ion-channels. The

SO2 ion-channel shows resonances attributed to the three-photon excitation of

the X(A 1 ) state to the 'I, 'II and 'III Rydberg states, as observed by several groups 72

These Rydberg states will be discussed in detail in Section 6.3.2 of this Chapter.

The SOP, S/(02) and O ion-channels primarily show resonances attributed

to the one-photon excitation of the X('A 1 ) state to the i( 3B1 ) state 2 and

the A( 1A 2 ) state 6 Several resonances within the S0 ion-channel between 383 - 390

nm are observed and attributed to two-photon excitation from the X('A 1 ) ground state

to high vibrational levels of the C(B 2 ) state. These resonances are of interest since

they will be used as intermediate states in OODR experiments. A resonance at 355.5

nm marked with an asterisk (*) obseryed in the SO P, S(02) and 0 ion-channels is

attributed to a two-photon transition within the SO radical and will be discussed

further in Section 6.3.5.

174 Chapter 6 The I, H and HI Rydberg states ofS0 2

II I I I II It I I I I I I (B1) I I 'SO9A(1A,)

80

60 +

o 40 (1)

20

0

80

60 + o 40 C,)

20

0

80

60 + +c?cl 40

20

0

8

6

+ 0

2

A

I l

I • I I • I i I • I

H: UT S02 C(1B)

• •

•

340 350 360 370 380 390 400 410

Wavelength I nm Fig. 6.1.: The 340-410 nmREMPI spectra of SO 2 recorded by collecting the SO 2 , SOP, S(02 ) and

0 ion-channels. The vibrational energy levels of the SO 2 (3B1 ), SO2 A(A), SO2 C(1B2 ) and 'I, 'II

and 'III Rydberg states are indicated. The pealc marked with an asterisk (*) denotes a 2+1 REMPI

transition in SO, see text for further details. Note, below 360 tim no significant SO 2 ion-signal is

observed and above 390 nm no significant SO P, S or (Y ion-signal is observed. A y-axis offset has

been used to identify the different dye regions within a particular spectrum.

Chapter 6 The I, Hand HI Rydberg states ofS0 2 175

6.3.2. One-colour (3+1) REMPI study of the I 1311 'III Rydberg

states.

The spectrum obtained by monitoring the SO2 ion-channel within the 'II

Rydberg state region is shown in greater detail in Fig. 6.2. The energy scale used

denotes the three-photon energies at which the observed Rydberg state structure lies

relative to the (0,0,0) level of the SO2 Y(A 1 ) ground state. Asterisks are used within

the 'II state vibrational assignment to indicate hot bands originating from the (0, 1,0)

level of the X('A 1 )

ground state. The one-photon ä(3B1 ) valence state vibrational

level assignments are also indicated since resonances at this level drastically increase

the observed peak intensity of triplet Rydberg states due to accidental resonant

enhancement.

Unlike spectra recorded via other ion-channels, structure relating to

a( 3B 1 ) valence state is essentially absent unless resonance is also achieved at the

three-photon level with a Rydberg state. This can be most clearly observed from the

position for the (0,0,0) level of the ( 3B 1 ) state. This peak is the most intense in

spectra recorded in the SO P, S and 0 channel but is almost absent in that recorded

via the SO2 channel, as shown in Fig 6.1. The small dip observed at the position of

this level, in the SO2 channel, is due to ringing in the SO ion-channel.


CU C

Cl) C 0

CM +

0 Cl)

SO2 a(3 B 1 ) J I I I I 000 010 020 100 030

111020

1111* i"i ii I I I 000* 010 000 020* 010 020 030 040

77000 78000 79000 80000 81000

Three-photon energy I cm

Fig. 6.2.: The one-colour, three-photon REMPI spectrum of the I.311 Rydberg states of SO 2 . Structure

marked with an asterisk (*) denotes hot bands from the (0, 1,0) level of the S0 2 X('A 1 ) ground state.

The 1II Rydberg state will be observed preferentially over the 3 1 Rydberg state

since excitation occurs from the singlet ground state. Only one vibrational level of the

triplet state is observed strongly, the (0,2,0) level, due to an accidental double

resonance with the (0,2,0) level of the ä(3B1 ) state as observed in Fig. 6.2. The triplet

state (0,2,0) assignment and further observed vibrational levels will be discussed later,

in Section 6.3.4, where OODR experiments using the â(3B 1 ) state as an intermediate

state are discussed.

Chapter 6 The I, II and III Rydberg states ofS0 2 177

The 'II state vibrational origin occurs at 380.8 nm corresponding to a three-

photon energy of 78772 cm'. In the one-colour, three-photon excitation spectrum,

Fig. 6.2, a progression in the bending v2' mode is observed possessing an Oie

400 cm for v2' = 0-4. Hot bands are also observed originating from the (0,1,0) level

of the X('A 1 ) ground electronic state to v2' = 0-2 of the II state. The origin (0,0,0)

band assignment, which agrees with that of Watkins 9 and Xue et al. 12 rather than that

of Suto et al. 10 , has been made based upon the fact that 400 cm' below the

assigned (0,0,0) origin at 78372 cm in Fig. 6.2 no unassigned SO2 ion-signal is

observed.

The 'land JJI Rydberg states have been observed for the first time in the one-

colour REMPI spectra recorded by collecting the SOj ion channel, as. shown in Fig.

6.1. Both Rydberg states are observed weakly in the REMPI spectra and have

previously been observed in absorption spectra by Watkins .

The 'I Rydberg state is the lowest energy Rydberg state of SO2 observed in

this study. Single-photon ionisation of the 'I Rydberg state can only theoretically

occur below 400.6 rim based upon the position of the (0,0,0) origin of the

SO2 1(2A1) ion-core state. Therefore the lower vibrational levels of the 'I Rydberg

state must be detected by a (3+2) REMPI process. Even so, six vibrational levels of

the 'IRydberg state can be identified, as shown in Fig. 6. 1, and the vibrational

energies match closely with energies observed in the absorption work 9. A progression

in both v 1 and V2 is observed possessing an co, (v i ) 349 cm' and an w (v2) 1668

cm

Chapter 6 The I, Hand III Rydberg states ofS02 178

The 'III Rydberg state is observed in the lower wavelength region of Fig. 6. 1,

primarily in the SO2 ion-channel. It should be noted that the vibrational origin of the

'III Rydberg state overlaps the (1, 1,0) band of the a(3B 1 ) state, as observed in Fig.

6.5. Six vibrational levels of the 'III Rydberg state can be identified, as shown in Fig.

6. 1, the vibrational energies match closely with energies observed in Watkins'

absorption work. A progression in the v2 vibrational mode is observed possessing an

392 cm'.

6.3.3. A two-colour OODR study of the 'II Ryd berg state.

Two-colour OODR spectra excited via high vibrational levels of the

singlet C(B 2 ) state show further evidence for the vibrational assignment for the 'II

state and are shown in Fig. 6.3. The two vibrational levels of the C(B 2 ) state used in

these experiments, which lie at 51794 cm-1 and 51026 cm', were observed weakly as

(2+2) resonances in the SO ion-channel as shown in Fig. 6.1. These states, which lie

- 6070 cm' and 5300 cm-1 above the ground state dissociation limit for SO2, have

not been assigned in this, or previous work. [(2)+1'+1'] OODR experiments via the

singlet C(B 2 ) state preferentially shows singlet Rydberg state structure due to the

singlet nature of the intermediate state used.

The observed resonance structure observed in Fig. 6.3(a) and (b) can be

assigned to the (0,0,0), (0, 1,0), (0,2,0) and (0,3,0) levels of the 'IlRydberg state. A V2 '

vibrational bending progression can therefore be observed in both Fig. 6.3(a) and (b)

possessing co, 400 cm'. The (0,0,0) band origin of the 11 Rydberg state is observed


in Fig. 6.3(a), due to an accidental double resonance with the (2,0,0) level of the

a(3B 1 ) state excited using a single probe photon at 27620 cm'. The triplet state

(0,0,0) assignment will be discussed later when OODR experiments using the

( 3B 1 ) state as an intermediate state is discussed in Section 6.3.4.

'-S

0) Ci)

0 +

c'1

0 (I)

111 [1'+(2)+l'] 0,0,0

1 111

1 1 11 0,0,0 U, 1,0 0,2,0 0,3,0

78500 79000 79500 80000 80500

Energy I cm 1

Fig. 6.3.: The OODR spectra of the 1.311 Rydberg states of SO 2 recorded using a [(2)+ F+ excitation

scheme via the C('B2 ) state at (a) 51026 cm and (b) 51794 cm -1 . The (0,0,0) origin band of the 311

Rydberg state is observed in (a) due to an accidental double resonance with the (2,0,0) level of

the a(3B 1 ) state.

6.3.4. Observation of the 3jj Rydberg state using OODR

spectroscopy.


An OODR spectrum of the triplet 3i1 Rydberg state excited via the (0,0,0)

band of the '(B1 ) - X(,4,)transition has been measured by Xue etal. 12 The use of

the intermediate a(3B 1 ) state allows the triplet manifold to be probed using a

[ 1 +(2')+ 1'] excitation scheme and the 3jj Rydberg state to be identified. Both the SO2

and SO ion-channels were collected and the resulting spectra showed a vibrational

bending progression with v2' 0 - 5 and an origin of 78246 cm-1 . The v2' = 0 band was

observed as a very weak peak in the SO ion-channel spectrum only, whereas, the V2 '

= 1, 2 and 3 bands were seen as strong medium and weak peaks, respectively, in the

SO ion-channel. The observation of the band origin in the SO ion-channel and not in

the SO2 ion-channel is peculiar. It would be expected that if all other vibrational

members of the 3 Rydberg state vibrational progression were observed in the SO2

ion-channel then the band origin would also be observed in the SO2 ion-channel. The

reasoning behind this lack of (0,0,0) SO2 ion-signal is reported to result from a

significant difference in the ionisation pathway below 380 run, although this was not

fully explained by Xue et al. 12 1n the present work this OODR spectrum has been

repeated and expanded to include a second [l+(1+1')+l'] OODR excitation scheme in

which SO2 was collected.

The OODR spectra of the 1 '311Rydberg states of SO2 recorded (a) via the

(0,0,0) band of the a(3B1 ) state using a [l+(2')+l'] excitation scheme in shown in Fig.

6.4(a). Three [1+(2')+1'] resonances can be observed in Fig. 6.4(a) corresponding to

resonances with the 3 Rydberg state, similar to those observed by Xue et al. 12 These

resonances have been reassigned by lowering the V2 vibrational quanta by one from

the assignment reported by Xue et al. 12 (see below). The resonance at 78636 cm'

Chapter 6 The I, Hand HI Rydberg states of S02 181

observed using both excitation schemes via the (0,0,0) level of the a( 3B1 ) state now

corresponds to the (0,0,0) vibrational origin of the 311 state, not (0,1,0) as assigned

previously. The major reasoning behind this reassignment is that no SOj or SO (not

shown) ion-signal is observed in both excitation schemes one vibrational quanta (-

3"

0,0,0 0,1,0 0,2,0 03,0

1111 I 0,0,0 0,1,0 0,2,0 J* 3

'~l

(0,0,0)

. I • I • I • I

• I • I • I • I • I

RMow 20

(U C

(I) C 0

+ CM

0 C/)

78000 78200 78400 78600 78800 79000 79200 79400 79600 79800 80000

Energy / cm - 1

400 cm") below, the new assigned band origin, denoted using solid arrows in Figs

6.4(a).

Fig. 6.4.: The OODR spectra of the 1.311 Rydberg states of SO2 recorded (a) via the (0,0,0) band of

the Zi(3B1 ) state using a [l+(2')+l] excitation scheme and (b) via the (0,0,0) band of the a( 3B1 ) state

using a [1+(l+l')+l'] excitation scheme. Note: two one-colour signals are observed and assigned in (a)

using asterisks. The supposed 3jj Rydberg state origin of Xue at al 12 is indicated in (a) using a solid

arrow.

The origin band is also observed via a [1+(1+1')+l'] excitation scheme where

the origin of the a( 3B1 ) - X(AI ) transition is pumped and the 311

--- a(B1)


transition is probed using one pump and one probe photon as shown in Fig. 6.4(b).

The [1+(2')+1'] resonances (Fig. 6.4(a)) match perfectly with resonances observed in

the [l+(l+l')+l'] excitation spectra (Fig. 6.4(b)). As before no ion-signal is observed

in either SO2 or SO ion-channel one quantum of V2' to lower energy, consistent with

the new assignment of the origin. The observation of the band origin would be

expected, in the SO ion-channel on the strength of Xue et al. 12 arguments.

Xue et al. 12 also observed the strong peak at 78640 cm' which they assigned

to an unknown vibrational level of the C('B 2 ) state at 51600 cm excited via a

[(2)+l '+1'] process. However, this suggested excitation scheme is questionable since

it has been shown in Section 6.3.3 that excitation via the C('B 2 ) state results in

primarily singlet Rydberg state structure, as observed in Fig. 6.3.

Another observation reported by Xue et al. 12 should also be reassessed.

Linewidths for the 78640 cm' band of the 3jj state detected via the [l+(2')+l'] and

[(2)+l'+l'] (reassigned to [l+(l+l')+l']) excitation schemes were reported to equal

9.75 cm' and 16.39 cm respectively, and were attributed to a difference in the

lifetime of the state accessed at the two-photon level. However, a fundamental error

was made by Xue et al. 12 calculating the linewidths. Since the spectra are

recorded as a function of the probe photon energy and in the [2+(1 ')+1'] (reassigned to

[1+(1+1')+l']) excitation scheme one probe photon is required for the three-photon

resonance, whilst in the [1+(2')+1'] excitation scheme two probe photons are required.

Therefore, one would expect the former to be twice the width of the latter. The only

way to compare linewidths in the two excitation schemes is to express the spectra in

terms of total state energy, as shown in Fig. 6.4, where the spectral linewidths for


corresponding peaks in both schemes are similar. Therefore, discussion of

predissociation lifetimes of both the Rydberg and intermediate C('B 2 ) state by Xue et

al. 12 based upon linewidth observations for the two different schemes is incorrect and

should be ignored.

Finally, two one-colour resonances (marked with asterisks) can be observed

in Fig. 6.4(a), which correspond to the (0,2,0) and (0,3,0) vibrational levels of the 311

and 111 Rydberg state, respectively.

6.3.5. Observation of the 'II Ryd berg state in the SO,S and 0 j.

channels.

It is possible to observe ion-signals relating to transitions to the 1JJ Rydberg

state in the one-colour REMPI spectra not only in the SO2 ion-channel but also the

SO and S ion channels, as shown in Fig. 6.5. The 0 ion-channel shows no Rydberg

state resonances, only showing very weak structure attributed to higher vibrational

levels of the ( 3B1 ) state.

Within the SO+ and S+ ion-channels, Fig. 6.5, it is possible to observe

resonances attributable to both the ( 3B1 ) state and the 'II Rydberg state. The

vibrational intensity distributions of both these two ion-channels do not follow that of

+ i

. + + i the SO2 on-channel. The distributions, observed m the SO and S on-channels,

appear to increase in intensity as v2 increases, for example, the ratio of (0,4,0)1(0,2,0)

ion-signal is greater for SO and S than that observed in SO2. The fact that the

higher vibrational levels of the 1H Rydberg state are observed more strongly in the

Chapter 6 The I, H and HI Rydberg states ofS0 2 184

SO and S ion-channels indicates that the higher vibrational levels are more

predissociative and are therefore observed with a lower intensity in the SO2

channel.

1 0,101 0,2,0 1,0,0 0,3,0 1,1,0

III I I I 0,0,0 0,1,0 0,2,0 0,3,0 0,4,0

3 111 0,2,0

MR so

0

(I)

CD C 0)

C so JL_.A_kJLJL/A

0

S+ 'A~~

0 + A_

78000 79000 80000 81000 82000

- Energy / cm -1 Fig. 6.5.: The 1,3 If Rydberg states and the band origin of the 'III Rydberg state as observed in the one-

colour REMPI spectra of SO2 collecting the SO2 , S0, S(02) and O ion-channels.

Chapter 6 The I, H and HI Rydberg states of S02 185

Therefore, it is possible to conclude that the SO and S ion-signals are not

predominantly formed from direct dissociation of SO2 ions. It is more likely that the

SO and S ion-signals are formed from the dissociation of S02* to SO/SO* which

undergo ionisation. The peak marked with an asterisk (*) in Fig. 6.1 is attributed to a

(2+1) REMPI resonance in SO*, showing evidence for the production of SO*. The (*)

peak is also observed in the S ion-channel showing that a significant proportion of S

ions are produced as secondary ions from the dissociation of SO P .

6.3.6. Determination of electronic assignments for the 1,11 and III

Rydberg states.

Previously, several attempts have been made to determine the electronic

assignments for the 1,11 and III Rydberg states. The majority of the previous work has

concentrated upon the II Rydberg state, since this state is observed with a greater

intensity in both absorption and REMPI experiments.

The I Rydberg state probably converges upon the s0 2 4(2A 2 ) ion-core state,

based upon the observation that the symmetric stretch V1 and bending V2 vibrational

frequencies of the Rydberg state are 1068 cm' and 351 cm', respectively. These

vibrational frequencies are similar to those observed in the photoelectron spectra

reported by Wang et al. 1 for the A2 ion-core state, i.e. v 981cm4 and v2 353 cm- .

A quantum defect, 8, can be calculated based upon the A(A 2 ) ion-core assignment

for the IRydberg state using the Rydberg formula (Equation 4.3). The effective

principle quantum number, n = 1.92 for the I Rydberg state, based upon the assigned

Chapter 6 The I, II and III Rydberg states ofS02 186

A( 2A 2 ) ion-core state. Depending upon the value assigned to the principle quantum

number, n, the quantum defect for the I Rydberg state is calculated to equal either

2.08 (where n =4) or 1.08 (where n = 3). These values can be compared with values

of atomic sulfur for 4s (6 = 2.02 - 2.12), 4p (6 = 1.58 - 1.66) and 3d(6 = 0.14 - 0.35).

Thus the I Rydberg state appears to correspond to a 4s orbital possessing A2 core

symmetry

The II Rydberg state probably converges upon the SO2 X( 2A 1 ) ion-core

ground state, based upon the observation that the bending (v2) vibrational frequencies

of the Rydberg state and ion-core state are similar at 400 cm -1 . No other SO2 ion-

core state shows a single V2 assignment possessing co, 400 cm- , a fact that has not

gone unnoticed by other groups 8, 10,12 The effective principal quantum number, n

2.31 for the II Rydberg state based upon the assigned X( 2A 1 ) ion-core state.

Depending upon the value assigned to the principal quantum number, n, the quantum

defect for the II Rydberg state is calculated to equal either 1.69 (where n =4) or 0.69

(where n = 3). These values can be compared with values of atomic sulfur for 4s (6

2.02 - 2.12), 4p(6 = 1.58 - 1.66) and 3d(6 = 0.14 -0.35). Thus the IlRydberg state

appears to correspond to a 4p orbital. Xue et al. 12 agreewith the 4p orbital, A1 core

symmetry, assignment.

The III Rydberg state also probably converges upon the S02X( 2A 1 ) ion-core

ground state, based upon the observations of the bending v2 vibrational frequencies of

co,. - 398 cm- 1 . The quantum defect for the III Rydberg state has been calculated to

equal either 1.54 (where n =4) or 0.54 (where n = 3). These values can be compared

with values of atomic sulfur for 4s (6 = 2.02 - 2.12), 4p (6= 1.58 - 1.66) and 3d (6=

Chapter 6 The 1, 11 and III Rydberg states ofS0 2 187

0.14 - 0.35). The III Rydberg state appears to correspond to a 4p orbital possessing A1

core symmetry similar to that observed for the II Rydberg state.

It is possible to assign two Rydberg states to the same orbital, 4p, and core

symmetry, A, since the symmetry of the Rydberg electron has not been determined.

i.e. the 4p cluster consists of three p orbitals, the 4Px, 4py and 4Pz, each orbital will

have its own symmetry, A 1 , B or B2, which is combined with the core symmetry to

yield the overall Rydberg state symmetry. At this time its is not possible to determine

the symmetry of the Rydberg electron.

6.4. Conclusions.

The most important experimental observation, made in the one-colour study of

SO2, within the 345-411 nm region is that a strong SO2 ion-signal is observed. This

is important because in almost every other wavelength region no SO2 is observed.

For an SO2 ion-signal to be observed resonance at the three-photon level is required.

At these energies resonances have been observed to three singlet and one triplet

Rydberg states. The states were labelled lj , ljj 11 and 'III.

Using both photoelectron data pertaining to the SO2 ion-core state and

quantum defect calculations the observed Rydberg states were shown to correlate to I

(A 2 core; 4s orbital), II ( A 1 core; 4p orbital) and III ( A 1 core; 4p orbital).

The 'hand 311Rydberg state have been studied using OODR spectroscopy via

the C(B 2 ) state and the a( 3B1 ) state, respectively. Evidence was also obtained which

Chapter 6 The I, II and HI Rydberg states of SO 2 188

suggested that the Rydberg states are predissociative, especially at high vibrational

levels.

6.5. References.

L. Wang, Y.T. Lee and D.A. Shirley. J. Chem. Phys. 87, (1987) 2489.

J.C.D. Brand, V.T. Jones and C. Di Lauro. J. Mo!. Spectrosc. 45, (1973) 404.

J.C.D. Brand and R. Names. J. Mol. Spectrosc. 46, (1973) 194.

A.J. Merer. Discuss. Faraday Soc. 35, (1963) 127.

Y. Hamada and A.J. Merer. Can. J. Phys. 52, (1974) 1443.

E. Hegazi, F. Al-Adel, A. Hamdan and A. Dastageer. J. Chem. Phys. 98, (1994)

12169.

S.D Colson, W.Y. Cheung. J.H. G!ownia and S.J. Riley. Chem. Phys. Left. 76,

(1980) 515.

D. Go!omb, K Watanabe and F.F Marino. J. Chem. Phys. 36, (1962) 958.

I.W. Watkins. J. Mol. Spectrosc. 29, (1969) 402.

M. Suto, R.L. Day and L.C. Lee. J. Phys. B: At. Mol. Phys. 15, (1982) 4165.

L. Zhang, L. Pei, J. Dai, T. Zhang, C. Chem, S. Yu and X. Ma. Chem. Phys. Left.

259, (1996) 403.

B. Xue, Y. Chen and H-L. Dai. J. Chem. Phys. 112, (2000) 2210.

Chapter 7. Observations of an ion-pair state of SO using OODR via the coupled B () state 189

Chapter 7.

Observation of the a (H) ion-pair state of SO

Mowing the OODR excitation of the X( 3 )ground

state via the coup1edB( 3 ) state.

7.1. Introduction.

The detection and characterisation of ion-pair (i.p.) states in molecules is of

interest as theory predicts their presence, but they are hard to observe experimentally,

especially via one-photon absorption techniques. This is because ion-pair states have

high Te values, requiring VUV or multi-photon techniques, and large r e values

resulting in low Frank-Condon overlap with lower energy valence states which tend to

possess considerably smaller r e values. A further experimental difficult concerning

REMPI spectroscopy is that ionisation of ion-pair states to give the parent ion is

difficult due to similar Frank-Condon overlap arguments, the parent ion usually

having a similar re to the ground state.

Previously, the majority of the experimentally observed ion-pair states were

observed in halogens, inter-halogens molecules and alkali-metal dimers as reviewed

by Lawley and Donovan'. Only one, group XVI, ion-pair state has been previously

observed, in the S2 radical by Cooper and Western 2 Ground state S2. X(' F,9 ), was

prepared by passing an electric discharge through a mixture of 5% H2S in Ar, prior to

expanding the mixture through a pulsed nozzle. The jet cooled beam of S2 X('1 9 )

Chapter 7. Observations of an ion-pair state ofSO using OODR via the coupled B (3) state 190

radicals were then excited using [1 +(I ')+ 1'] OODR excitation scheme, via the coupled

S2 B"(3H)/B(3;) states, to the newly observed ( 5 E) ion-pair state in S2. The

excitation scheme used by Cooper and Western 2 is remarkably similar to that used in

the present study to detect the ion-pair state in SO.

In the present study [1+(1')+2] and [1+(2')+2] OODR excitation schemes are

used to study, the arbitrary labelled a, a (171) ion-pair state of SO via the coupled

B( 3 ) state, which is optically pumped from the SO X( 3 ) state, generated from

the two-photon photolysis of SO2

The OODR excitation scheme is shown schematically in Fig. 7.1. Ground state

SO2 is first excited using a minimum of two pump photons, whose wavelength has

been chosen such that they excite the one-photon v 1; SO B( 3 ) +- v = 5;

SOX(3 )transition at - 274.4 nm, as shown in Fig 1(a). The most probable

excitation/dissociation scheme in SO2 involves the absorption of two pump photons to

high vibrational levels of the SO2 C('B2 ) state 27160 cm -1 above the SO2 -+ SO

X()+ 0 (3 P2) dissociation threshold (45725.3 cm') 'at 72886 cm -1 .

Dissociation at the two-photon level is sufficient to energetically allow v = 5 of the

SO X(3 ) state to be produced which possesses 5562 cm 1 of internal energy 2

A second, probe laser was then used to excite a one-photon transition in SO

from v = 1 of the coupled SO B( 3 ) state to multiple vibrational levels of the newly

observed ion-pair state. Resonances between these two states are observed most

prominently in the S ion-channel. The exact ionisation process producing the S ions

is unknown at this time, but due to energetic constraints at least two further

(pump/probe) photos are required for S production from the ion-pair state level.

30 '.

0 A SO(i.p) 9

Repulsive state

23A1 3'A'

tTTT:I v i

Coupled

1 A SOB('E) £ A 2

V I -

5 so X el

(a)

(b) uDoso , o p))

Chapter 7. Observations of an ion-pair state ofSO using OODR via the coupled B () state 191

Although, the ion-pair state structure can also be observed in the SO ion-channel it is

easily obscured, especially at higher energies, by resonances attributed to 1+1 and

1+1' transitions from the SO a('A) state to higher energy valence states, as observed

by Speth et al. 2 and Archer et al.

S ions ions

fly lily

130

120

110

,--100

E C.) 90

C.,

C) 80

70 >.

U) 60 C U) 50

Cu 40 C

30 0 a. 20

10

vi

0 L o0_' S02X('A1)

so SO + Q(3P)

Fig. 7.1.: Schematic representation of the excitation scheme used (a) in the OODR experiments of the

SO ion-pair state via the coupled B( 3 ) state and (b) the OODR reverse resonances observed as

accidental overlapping peaks in Fig. 7.2.

Within the S ion-channel, several reverse resonances can be observed where

the functions of the pump and probe lasers are reversed, as shown in Fig. 7.1 (b). The

pump laser, at 274.4 nm, which excites the (1,5) B +- X transition, is also able to

probe the (9,2) and (11,3) i.p. f- B bands. Therefore, when the probe laser is scanned

Chapter 7. Observations of an ion-pair state ofSO using OODR via the coupled B (3r) state 192

the (2,n) and (3,n) bands, of the B 4— X transition, are also observed. The width of

these bands are the same for a particular B state vibrational level and are determined

by the number of rotational lines that the fixed (pump) wavelength laser is able to

probe. The bands observed via v 3 of the SO B( 3 ) state are much narrower than

those observed via v =2, although this may be due in part to there being only a

limited number of rotational levels of v = 3 that are not predissociated. No (I ,n) or

(0,n) B 4-X transitions are observed, since 274.4 nm radiation does not excite any

(n, 1) or (n,O) i.p. +— B bands. Similarly, the spectrum recorded via v =2 of the

SO B( 3 ) state, pumping at 269.72m n, probes the (9,1) and (10,2) i.p. 4—B bands,

thereby the (1 ,n) and (2,n) B 'c—X bands are observed.

7.2. Experimental.

The two-colour OODR experimental were performed upon SO2 seeded in He

(5% SO2, 95% He). Pump photons used to excite (1,5) and (2,5) B — X transitions

were generated from frequency doubled C 153 and probe photons used to excite the

one- and two-photon a 4— B transitions were generated from the frequency doubled

and fundamental outputs of C102 and C153 laser dyes, respectively. A 500/o/50%

beam splitter was incorporated into the laser set-up and the pump and probe laser

beams were both fully focussed to allow an intense ion signal to be observed.

The spectra have been fully calibrated by simultaneously monitoring the

known 12 fluorescence excitation spectrum above 500 rim (250 rim, frequency

Chapter 7. Observations of an ion-pair state ofSO using OODR via the coupled B () state 193

doubled) and from 1 +1' atomic S lines, observed in the S spectra, below this

wavelength.

7.3. Results and discussion.

7.3.1. The SO a (5fl) ion-pair state vibrational structure.

The OODR spectra of v = 0 - 30 of the a (511) ion-pair state of SO are shown

in Fig. 7.2. In the spectrum shown in Fig 7.2(a), ion-pair state resonances are

observed via optical pumping from v = 1 of the SO B( 3 ) state which in turn was

pumped from v = 5 of the SO X(3 E ) ground state. The assignment for the v = 0 of

the ion-pair state, observed via the v = I of the coupled SO B(3 ) state, has been

made based upon the fact that one vibrational quanta below the assigned origin, no S

ion-signal is observed, even though at this energy the laser dye power is at a

maximum. Experimentally, it was found to be difficult to observe vibrational levels

higher than v = 30 of the ion-pair state due to a one-colour signal which is in

competition with the ion-pair resonances. A smaller range of a (5fl) ion-pair state

vibrational levels have also been excited via v =2 of the SO B(3 ) state, as shown

in Fig. 7.2(b). It was not possible to observe transitions to the a (5fl) ion-pair state via

v = 0 of the SOB ( 3 ) state, a phenomenon which will be discussed in Section 7.3.4.

Chapter 7. Observations of an ion-pair state of SO using OODR via the coupled B (SE) state 194

01.11111 I liii 111111111 I I 1111111 0 5 10 15 20 25 30

(a)

MW

5 B-X (2,n)

B-X(3,n)

33400 34400 35400 36400 37400 38400 39400

ion-pair I I I I I I I I I I I i I I I I TI in ic

BX(2n) 6 5 4 3

7 16 15 14 Ia I I I I I B-X(1,n)

33000 34000 35000 36000 37000 38000 39000 40000 41000 42000

Probe energy I cm

Fig. 7.2.: The SO a ([i) ion-pair state *- coupled B( 3 ) state vibrational transitions observed in the

S ion-channel for v =0 to 30. (a) Recorded via v = 1 of the coupled B( 3 ) state using a pump

wavelength of= 274.4 nm and (b) Recorded via u = 2 of the coupled B( 3 ) state using a pump

wavelength of -. 269.72 am. Reverse resonances are also observed as indicated.

The observed a - B transition energies, measured from the band maxima, for

excitation via v 1 of the coupled SO B( 3 ) state are presented in Table 7.1. The

rotational contours for the (0, 1), (1, 1), (2,1) and (3,1) i.p. '—B bands are red-

degraded and the contours are similar for any particular pump wavelength. Therefore,

these levels will be described as unperturbed and denoted by asterisks (*),in Table

7.1. The contours of the other i.p. - B bands are both red- and blue-degraded and

very irregular. Bands marked with daggers (t) are overlapped by resonances

(U

0) Cl) C 0

+ (1)

40400 41400 42400

attributed to the reverse pump and probe scheme as shown in Fig. 7.1(b). The data in

Chapter 7. Observations of an ion-pair state of SO using OODR via the coupled B (3E) state 195

Ion-pair state vibrational level

a 4- B transition energy

Vibrational energy Vibrational spacings Term value

/cm Icm /cm- ' cm'

0 34403.5* 0 -- 76976.5 1 34712.2* 308.7 308.7 77285.2

2 35018.5* 615 306.3 77591.5

3 35322.1* 918.6 303.6 77895.1 4 35623 1219.5 300.9 78196 5 2

-- -- --

6 36175 1771.5 -- 78748 7 36478 2074.5 303 79051 8 36776* 2372.5 298 79349 9 37047t 2643.5 271 79620 10 37335 2931.5 288 79908 11 -- -- -- --

12 37891 3487.5 -- 80464 13 38172' 3768.5 281 80745 14 38442! 4038.5 270 81015 15 38709* 4305.5 267 81282 16 38970* 4566.5 261 81543 17 4819.5 253 81796 18 39492 5088.5 269 82065 19 39760 5356.5 268 82333 20 40029 5625.5 269 82602 21 40244t 5840.5 215 82817 22 40510 6106.5 266 83083 23 40730 6326.5 220 83303 24 40977 6573.5 247 83550 25 41220 6816.5 243 83793 26 41465 7061.5 245 84038 27 41698 7294.5 233 84271 28 41925 7521.5 227 84498 29 42161 7757.5 236 84734 30 42392 7988.5 231 84965

Table 7.1.: The one-photon a *- B transition energies, vibrational energies, vibrational spacings and

vibrational term values, all in cm', of the a (5H) ion-pair state of SO pumped via the

(1,5) B X transition at 36427.5 cm'. The bands marked with asterisks are "unperturbed" (see text)

and those marked with daggers (t) are overlapped by a reverse resonance.

Table 7.1 shows that the ion-pair state possesses a v = 0 - 1 separation of 308.7 ± 0.3

cm-1 . The vibrational spacing decreases from - 309 cm -1 to -j 250 cm-1 in the spectral

region covered. Calculating the exact term energies, from Fig. 7.2, for the ion-pair

state vibrational levels is difficult since the rotational levels of both the coupled


SO B( 3 ) state and the SO X( 3 ) ground state used are unknown. The

determination of the term value for v= 0 of the ion-pair state will be discussed below

in Section 7.3.2.

The intensity distribution of the observed ion-pair vibrational progression

appears to follow a smooth oscillation. Minima are observed in the oscillation at v =

4, 11 and 23 and increase in width as v increases. The same oscillation is observed in

both spectra in Fig. 7.2, recorded via different vibrational levels of the coupled

B( 3 ) state, and a particular ion-pair state vibrational level appears to possess the

same intensity irrespective of whether it is excited from v = 1 or 2 of the intermediate

state. Therefore the oscillation cannot to be due to F.C. factors between the

intermediate coupled B( 3 ) state and the ion-pair state. The observed features are

characteristic of an interaction with a repulsive state or dark state possessing a slightly

different co,to that of the ion-pair state.

7.3.2. The rotational structure for v = 0 of the a (511) ion-pair state.

A rotational analysis for v = 0 of the ion-pair state has been carried out. The

excitation scheme used to study the rotational structure of the ion-pair state was

similar to that used to study the vibrational structure as shown in Fig. 7. 1(a). The

coupled SO B( 3 ) state was first pumped using a fixed wavelength laser producing

fixed but unknown rotational levels. Depending upon the laser wavelength used,

between one and four rotational levels could be pumped simultaneously. The

rotational levels pumped were then probed using two-photon excitation to the a (5H)


ion-pair state. For each rotational level populated in the coupled SO B( 3 ) state

three rotational branches, the 0-, Q- and S branches, were observed at the ion-pair

state level. Comparing the Q-O and S-Q splitting allowed the B0 and r0 values of the

ion-pair state to be calculated shown by equations (7.1) to (7.23) where; h = Planck's

constant = 6.626E 34 Js, It = reduced mass = (m m0 / m + m0) = 1.78453E -21 kg, c =

speed of light = 2.99E 10 cm s' and r0 = bond length in cm. Since small errors in the

calibration process and peak energy determination produced large errors in the B 0

value the experiment was repeated several times and an average B,, value determined

from data recorded for several intermediate J levels.

Using T=T.,,o +B01 f(J'+ 1) (7.1)

For the 0-branch.

Where J'= J"-2 (7.2)

T 0 =T0o +B0'(J"-2)((J"-2)+ 1) (73)

T=T 0 + B0'(J"- 2) ((J"- 1)) (7.4)

= T0 + B 0' f" 2 - 3B 0 ' J" + 2B 0 ' (7.5)

For the Q-branch

Where X = J" (7.6)

T= T0,0 + B0t(J") ((J') + 1) (7.7)

Tj = T0 + B0'J"2 - B01 J" (7.8)


For the S-branch

Where X = J" +2 (7.9)

Tj=To+B'(J"+2)((J"+2)+ 1) (710)

Tj = T0 + B' (J" + 2) ((J" + 3)) (7.11)

T= T + B'J"2 + 5B'J"± 6B' (7.12)

(Q-O) splitting.

TVf=TL, o±Bv'J"2 -B'J" (7.13)

- (Tj = T0 ± B'J"2 - 3B'J"± 2B) (7.14)

= 4B' J" - 2B' (7.15)

(S-Q) splitting.

Tj= T0 + B'J"2 + 5B'J"+ 6B' (7.16)

(7.17)

= 4B' J" + 6B' (7.18)

(S-Q) - (Q-O) difference.

(S-Q) = 4B'J" + 6B' (7.19)

- [(Q-O) = 4B' J" - 2B1 (7.20)

= 8B' (7.21)

Using B' = h I ( 8it2j.trv2c) (7.22)

V 112 ry = [h / ( 81t21.LBv c)] (7.23)

Chapter 7. Observations of an ion-pair state of SO using OODR via the coupled B (3) state 199

An example of the rotational branches observed from two-photon excitation of

the ion-pair states is shown in Fig. 7.3. Here, excitation via a single unknown

rotational level of the intermediate state, later determined to be J = 14, is shown. The

three rotational branches 0, Q and S are clearly observed.

A second experimental scheme was used to determine the rotational levels of

the intermediate state used to study the ion-pair state, similar to that shown in Fig.

7.1(b). This scheme involved first fixing the probe photon energy such that it excites a

single rotational transition of the (0,!) a 4— B transition. The fixed frequency

rotational transition chosen was the Q-branch (J' = 14) observed in Fig. 7.3. The

pump energy exciting the (1,5) B —X transition was then scanned to determine the

various pump energies that excite the particular rotational level, J=r 14, of the

Q branch

0 branch S branch

I • I I •

34360 34370 34380 34390 34400 34410

Two-photon probe energy I cm 1

(5 C 0)

(1) C 0

+ C,)

Fig. 7.3.: Ion-pair state rotational structure as observed via two-photon excitation from v = I, J = 14 of

the coupled B() state. The three rotational branches 0, Q and S are clearly observed.

Chapter 7. Observations of an ion-pair state of SO using OODR via the coupled B () state 200

coupled B( 3 ) state which is being probed. The resulting spectrum is shown in Fig.

7.4. The spectrum shows many peaks and pump energies corresponding to observed

peaks were then investigated to determine if they truly populated the probed rotational

level of the coupled B( 3 ) state. This was achieved by fixing the pump energy to

correspond to a peak in Fig. 7.4 and scanning the probe energy. If the resulting

spectrum was identical that shown in Fig. 7.3 then it was concluded that the pump

energy used truly excites the same rotational level of the coupled B(3 ) state. Only

the six peaks, marked with asterisks (*), produced a spectrum identical to that of Fig.

*1

-J

Co

D) U) C 0

+ Cl)

18160 18170 18180 18190 18200 18210 18220

Pump energy I cm"

Fig. 7.4.: (1,5) B - X excitation spectrum for J= 14 of the coupled B( 3 ) state. The peaks marked

with asterisks (*) can be used as pump lines to populate J = 14 of the coupled B( 3 ) state (see text).

The probe laser, 17194 cm', excites J= 14 of the coupled B( 3 ) state to J = 14 of v = 0 of the ix ion-

pair state by a two-photon coherent process.


Since the six pump energies correspond to excitation to a fixed rotational level

of the intermediate state, the difference in these energies will correspond to the

rotational spacings of v =5 of the SO X( 3 ) ground state. The calculated rotational

energies of v =5 of the SO X(3 ) ground state, relative to the N= 0, F2 rotational

level of the X( 3 E)ground state, are shown in Table 7.2. The observed pump energies

were therefore compared with the calculated rotational spacing for the ground state,

remembering that AJ can only change by ± 1 in a parallel one-photon transition. Table

7.3 shows the comparison between the separation between the observed pump

energies and the calculated rotational spacings for the N = 16; F3, N= 15; F2, N 14;

Fl, N= 14;F3, IV= 13;F2 and N=12;F1 levels ofv=Softhe

N Fl Energy I cm F2 Energy / cm' F3 Energy /cm'

0 -8.52 0 0 T -6.462 1.379 0.006 2 1 -3.238 4.137 1.775 T 1.125 8.273 5.232 4 9.976 13.783 10.282 T 14.045 20.684 16.85 6 22.453 28.957 24.889 7 32.21 38.609 34.366 8 43.325 49.641 45.263 9 55.804 62.051 57.568 10 69.649 75.84 71.273 11 84.864 91.008 86.373 12 101.54 107.555 102.864 13 119.409 125.48 120.743 14 138.742 144.785 140.008 15 159.45 165.469 160.658 16 181.534 187.531 182.692 17 204.994 210.973 206.109 18 229.83 235.793 230.908 19 256.043 261.992 257.089 20 283.633 289.57 284.651

Table 7.2.: Calculated rotational energies, in cm', for u = 5 of the SO X( 3 ) ground state relative to

the N= 0; F2 level.


Observed pump Energies I cm- '

Separation I cm

u = 5 of the SO x(3 )

ground state rotational level

Ground state Rotational energy

,

Separation I cm

36314.6 - N16:F3:J15 182.692 -

36331.8 17.2 N15:F2:J15 165.469 17.223 36357 25.2 N=14:F3:J13 140.008 25.461

36358.2 1.2 N14: Fl: Jl5 138.742 1.266 36371.6 13.4 N13:F2:J13 125.48 13.262 36395.2 23.6 N12: Fl: Jl3 101.45 24.03

Table 7.3.: The experimental energies, in cm -1, used to pump J = 14 of the coupled intermediate v = 1:

SO B( 3 ) state compared with calculated rotational energies of the v 5 of the SO X( 3E) ground

state.

SOX(T)ground state. The-rotational levels of the ground state chosen fit the

observed data well ± 0.4 cm 1 , which is within the experimental error of± 0.5 cm t .

The effect of altering the N assignment of the ground state rotational structure on the

fit is shown in Table 7.4. In fits for AN= +2, +2, 0, -1 and -2 as shown in Table 7.4, it

can be seen that as AN increases or decreases from zero the fit deteriorates The

assignment of the lower ground state rotational levels allows both the intermediate

and ion-pair state rotational structure to be assigned. Therefore, the spectra in both

Figs. 7.3 and 7.4 were assigned to spectra recorded via J= 14 of the intermediate

state. Spectra similar to that shown in Fig. 7.4 have been recorded via J = 12, 15 and

16 of the intermediate state. Each of the spectra also shows six pump lines, which

could be assigned within ± 0.5 cm-1 , to the equivalent six rotational levels of the

ground state. The six rotational levels being N= (J+ 2); F3, N= (J+ 1); F2, N= J;

Fl,N=J; F3,N= (J- 1); F2 and N=(J-2); Fl of the ground state where Jequals

the rotational level of the intermediate state which is being pumped. A schematic

representation of the rotational transitions are shown in Fig. 7.5 for J = 13 - 15.

Chapter 7. Observations of an ion-pair stale of SO using OODR via the coupled B (3) slate 203

Experimental 1 36$I.

57 80.6

2 .4 $')S 634

- 1 2 14 h 38.2

-4$(4

.7 - , 8 -î I 26

-($4 .1

AN = +2 Observed - Calculated EJ\EJ 230.908 210.973 182.692 181.534 165.469 138.534

230.908 0 19.935 48.216 49.374 65.439 92.374 0 -2.735 -5.816 -5.774 -8.439 -11.774

210.973 -19.935 0 28.281 29.439 45.504 72.439 2.735 0 -3.081 -3.039 -5.704 -9.039

182.692 -48.216 -28.281 0 1.158 17.223 44.158 5.816 3.081 0 0.042 -2.623 -5.958

181.534 -49374 -29.439 -1.158 0 16.065 43 5.774 3.039 -0.042 0 -2.665 -6

165.469 -65.439 45.504 -17.223 -16.065 0 26.935 8.439 5.704 2.623 2.665 0 -3335

138.534 -92374 -72.439 -44.158 -43 -26.935 0 11.774 9.039 5.958 6 3.335 0

AN = +1 Observed - Calculated FJ\EJ 206.109 187.531 160.658 159.45 144.785 119.409

206.109 0 18.578 45.451 46.659 61.324 86.7 0 -1.378 -3.051 -3.059 4.324 -6.1

187.531 -18.578 0 26.873 28.081 42.746 68.122 1.378 0 -1.673 -1.681 -2.946 4.722

160.658 45.451 -26.873 0 1.208 15.873 41.249 3.051 1.673 0 -0.008 -1.273 -3.049

159.45 46.659 -28.081 -1.208 0 14.665 40.041 3.059 1.681 0.008 0 -1.265 -3.041

144.785 -61.324 42.746 -15.873 -14.665 0 25.376 4.324 2.946 1.273 1.265 0 -1.776

119.409 -86.7 -68.122 -41.249 40.041 -25.376 0 6.11 4.722 3.049 3.041 1.776 0

AN = 0 Observed - Calculated EJ\EJ 182.692 165.469 140.008 138.742 125.48 101.45

182.692 0 17.223 42.684 43.95 57.212 81.242 0 -0.023 -0.284 -0.35 -0.212 -0.642

165469 -17.223 0 25.461 26.727 39.989 64.019 0.023 0 -0.261 -0.327 -0,189 -0.619

140.008 42.684 -25.461 0 1.266 4.528 38.558 0.284 0.261 0 -0.066 0.072 -0.358

138.742 43.95 -26.727 -1.266 0 13.262 37.292 0.35 0.327 0.066 0 0.138 -0.292

125.48 -57.212 -39.989 -14.528 -13.262 0 24.03 0.212 0.189 -0.072 -0.138 0 -0.43

101.45 -81.242 -64.019 -38.558 -37.292 -24.03 0 0.642 0.619 0.358 0.292 0.43 0

AN = -1 Observed - Calculated RI \ FJ 160.658 144.785 120.743 119.409 107.555 84.864

160.658 0 15.873 39.915 41.249 53.103 75.794 0 1.327 2.485 2.351 3.897 4.806

144.785 -15.873 0 24.042 25.376 37.23 59.921 -1.327 0 1.158 1.024 2.57 3.479

120.743 -39.915 -24.042 0 1.334 13.188 35.879 -2.485 -1.158 0 -0.134 1.412 2.321

119.409 41.249 -25.376 - 1.3341 0 11.854 34.545 -2.351 -1.024 0.134 0 1.546 2.455

107.555 -53.103 -37.23 -13.188 -11.854 01 22.691 -3.897 -2.57 -1.412 -1.546 0 0.909

84.864 -75.794 -59.92 I -35.879 -34.545 -22.691 1 0 4.806 -3.479 -2.321 -2.455 -0.909 0

AN = -2 Observed - Calculated EJ\EJ 140.008 125.48 102.864 101.54 91.008 69.649

140.008 0 14.528 37.144 38.468 49 70.359 0 2.672 5.256 5.132 8 10.241

125.48 -14.528 0 22.616 23.94 34.472 55.831 -2.672 0 2.584 2.46 5328 7.569

102.864 -37.144 -22.616 ' 0 1.324 11.856 33.215 -5.256 -2.584 0 -0.124 2.744 4.985

101.54 -38.468 -23.94 -1.324 0 10.532 31.891 -5.132 -146 0.124 0 2.868 5.109

91.008 -49 -34.472 -11.856 -10.532 0 21.359 -8 -5.328 -2.744 -2.868 0 2.241

69,649 -70.359 -55.831 -33.215 -31.891 -21.359 0 -10.241 -7.569 -4.985 -5.109 -2.241 0

Table 7.4.: Comparison of the fit for the observed rotational spacing, in cm - 1 , of v = 5 of the ground

state for AN=±O, 1 and 2.

J 18

J 42660 -N

18

- 17

16

15

14

13

12

11

150 > 0) a) w

250

200 F

11

We

16 42650

15 42640

14

1342630

12 42620

42610

42600 FV

Chapter 7. Observations of an ion-pair stale of SO using OODR via the coupled B (3r) stale 204

I ground state intermediate state I 50 L -j 42590

Energy I CM-1

Fig. 7.5.: A schematic diagram of the coupled B +- X rotational transitions for = 13, 14 and 15 of the

coupled B( 3 ) state. The six experimentally observed rotational transitions are shown (see text).

The determination of the pump energies and the rotational numbering of both

the ground and intermediate state allowed both the term values to be calculated and

rotational structure of the ion-pair state to be assigned. Spectra similar to that shown

in Fig. 7.3 have been recorded via J= 11 - 16 of the intermediate state and are shown

in Fig. 7.6. Here, the rotational structure relating to v = 0 of the ion-pair state can be

observed and assigned and the rotational energies are tabulated in Table 7.5. It can

clearly be observed that excitation via J = 11 also excites a second transition via a

different rotational level of the intermediate state due to the accidental overlap of

pump transitions. The observed rotational levels of the ion-pair state were calculated

using equation (7.24) and the spectroscopic constants T0,0 = 76976.5 ± 0.5 cm -1 and B0

Chapter 7 Observations of an ion-pair state ofSO using OODR via the coupled B (3r) state 205

= 0.217 ± 0.001 cm-1 which reproduce to ± 0.2 cm -1 the observed rotational energies

in Table 7.5.

E=T,0 +BJ(J+1)

(7.24)

I I I I I I I I I I 9 10 11 12 13 14 15 16 17 18

AJL,k_cwviaJ=11

via J=12

20

Co via J=13

0) C,)

0 __________________ via J14 + C/)

via J=15 *J

via J=16 WL 76990 77000 77010 77020 77030 77040 77050 77060

Energy I cm

Fig. 7.6.: OODR spectra showing v =0 of the a (11) ion-pair state rotational levels observed via J = 11

- 16 of the coupled B( 3 ) state. The energies are relative to the lowest rotational level of v = 0 of the

SOX( 3 )ground state.


Ion-pair u = 0 J level Observed via Observed energy Calculated energy Obs.-cal. / cm' / cm' / cm- '

9 11 76996.3 76996.0 0.3

10 12 77000.5 77000.4 0.1

11 11 77005.3 77005.1 0.2

11 13 77005.2 77005.1 0.1

12 12 77010.4 77010.4 0.0 12 14 77010.4 77010.4 0.0

13 11 77015.8 77016.0 -0.2

13 13 77015.8 77016.0 -0.2

13 15 77015.8 77016.0 -0.2

14 12 77022 77022.1 -0.1

14 14 77022.1 77022.1 0.0

14 16 77021.9 77022.1 -0.2

15 13 77028.5 77028.6 -0.1

15 15 77028.4 77028.6 -0.2

16 14 77035.5 77035.5 0.0 - 16 16 77035.5 77035.5 0.0

17 15 77042.9 77042.9 0.0

18 1 16 1 77050.8 1 77050.7 1 0.1

Table 7.5.: The observed and calculated rotational term values, in cnf', of u =0 of the ion-pair state

observed in Fig. 7.6 via various rotational levels of the intermediate state.

Calculation of the B0 value can also be achieved using equation (7.21). Firstly,

the difference between the (Q-O) and (S-Q) splitting is independent ofJand equals

8B0. Twelve spectra have been averaged to give a difference of 1.8 cm 1 giving B0

0.23 ± 0.02 cm!. Secondly, the individual (O-Q) and (S-Q) spacings are equal to 4B0

(J + 2) and 4B0 (J - 6) respectively, where Jrefers to the rotational level of the

intermediate state. Twenty-four spacing have been averaged to give B0 = 0.217 ±

0.002 cm- .

Chapter 7. Observations of an ion-pair state of SO using OODR via the coupled B (3r) state 207

7.3.3. The SO a (H) ion-pair state rotational structure for v> 0 of

the ion-pair state.

Rotational data have also been recorded for v = 1, 14 and 15 of the ion-pair

state. As previously stated, these levels are relatively unperturbed and possess no

accidental overlapping reverse resonance structure.

Ion-pair v = 1 J level Observed via Observed energy Calculated energy Obs.-cal. / cni' I cm' / cm

10 12 77309.2 77309.2 0.1

12 12 77319 77319.0 0.0

12 14 77319.2 77319.0 0.2

14 12 77330.6 77330.7 0.0

14 14 77330.8 77330.7 0.2

14 16 77330.6 77330.7 0.0

16 14 77344.4 77344.0 0.4

16 16 77344 77344.0 0.0

18 16 77359 77359.0 0.0

Table 7.6.: The observed and calculated rotational term values, in cm', of v = I of the ion-pair state

observed via various rotational levels of the intermediate state.

The v = I level of the ion-pair state has been studied using an OODR

excitation scheme similar to that shown in Fig. 7.1(a). Three different rotational levels

of the intermediate state J= 12, 14 and 16 have been used to access the ion pair state.

Although the spectra are not shown, the resulting ion-pair term values are tabulated in

Table 7.6. The rotational energies were calculated using equation (7.24) and the

spectroscopic constants T1,0 = 77285.5 ± 0.5 cm- ' and B 1 = 0.215 ± 0.001 cm', which

reproduce to ± 0.2 cm the observed term values. The individual (O-Q) and (S-Q)

Chapter 7. Observations of an ion-pair state ofSO using OODR via the coupled B (r) state 208

spacings were also investigated and six spacings have been averaged to give B 1 =

0.214 ± 0.002 cm'

It is now possible to calculate the v = 0 - 1 vibrational spacing of 309.0 cm

from the term values for v =0 and v = 1 of the ion-pair state. This value compares

favourably with that of 308.7 cm -1 obtained from Table 7.1 relating to vibrational

energy levels observed in Fig. 7.2.

Ion-pair J level

Observed via Observed energy for

v=14.

Calculated energy for

u=14.

Obs.-cal.

____

Observed energy for

v= 15.

Calculated energy for

v=15.

Obs.-cal.

______ /cm' /cm' 1c__m4 Icm' 1cm' I cm'

11 12 81042.8 81043.0 -0.2 81309.3 81309.2 0.1

13 12 81053 81052.8 0.2 81318.8 81319.0 -0.2

13 14 81052.8 81052.8 0.0 81318.8 81319.0 -0.2

15 12 81064 81064.1 -0.1 81330.2 81330.3 -0.1

15 16 81064.1 81064.1 0.0 81330.4 1 81330.3 0.1

17 1 16 81077 1 81077.1 1 -0.1 1 81343 1 81343.2 1 -0.2

Table 7.7.: The observed and calculated rotational term values, in cm4, of v = 14 and v= 15 of the ion-

pair state observed via various rotational levels of the intermediate state.

Rotational data has also been obtained for v = 14 and v = 15 of the ion-pair

state. Three rotational levels of the intermediate state J = 12, 14, 16 were pumped then

probed using single-photon excitation yielding six rotational levels for each of the two

vibrational levels, as tabulated in Table 7.7. It is also possible to calculate the

observed rotational levels using equation (7.24) and the spectroscopic constants T14,0

= 81017.1 ± 0.5 cm-'and B14 = 0.196 ± 0.002 cm-1 and T 15,0 = 81283.5 ± 0.5 cm and

B 15 = 0.195 ± 0.002 cm' for the v = 14 and v = 15 level respectively.

The v = 14 - 15 vibrational spacing of 81283.5.5 - 81017.1 = 266.4 cm

compares well with that tabulated in Table 7.1 of 267 cm' taken from Fig. 7.2. The


fact that both the v = 0 - 1, v = 14 - 15 vibrational spacings are essentially the same in

the vibrational spectra, Fig. 7.2, and the rotational analysis work indicates that the

vibrational spacing tabulated in Table 7.1 can be taken to be correct. Therefore the

term value for v = 0 can be added to the vibrational energies for v =0- 30, in order

that the term values for these vibrational levels can be calculated. The term values for

the a (I1) ion-pair state vibrational levels are tabulated in Table 7.1.

The term values can be fitted using the standard equation,

= Te + co,,. (v + 0.5) - WeXe (v + 0.5)2 ... (7.25)

where T. = 76823.1 cm', We = 308.7 cm' and WeXe = 1.375 cm -1 . The calculated co, is

consistent with the observed v =0 - 1 vibrational spacings of 308.7 cm -' and 309.0

cm obtained from Fig. 7.2 and the rotationless term values for v =0 and 1,

respectively.

A second observation concerning the rotational analysis work can be made.

The B values can be used along with equation (2.24) to calculate the bond length, rv,

for a given vibrational level. The rv values for v = 0, 1, 14 and 15 can therefore be

calculated to equal 2.69 A, 2.702 A, 2.83 A and 2.838 A, respectively.


7.3.4. Determination of the nature of the intermediate state.

It is possible to obtain an estimate of the B value of the intermediate state

since an assignment of the rotational energies of the intermediate state has been made

based upon the ground state rotational assignment. Experiments (such as that used in

Fig. 7.4) where a single rotational level of the ion-pair state was probed by fixing the

probe laser frequency and scanning the pump laser allow the rotational transition

energies of the (1,5) B +— X transition to be calculated. It was then a simple matter,

knowing the rotational level pumped, to add the correct rotational term energies of v

= 5 of the SO X( 3 ) ground state to calculate the term energy for the intermediate

state. The term energies of the intermediate coupled SO B( 3 ) state for the rotational

levels J= 11, 12, 13, 14, 15 and 16 have therefore been measured and are tabulated in

Table 7.8. The rotational energies can be calculate using equation (7.24) and the

spectroscopic constants T,0 = 42572.5 ± 0.2 cm-1 and B,, = 0.295 ± 0.001 cm-'. The J

dependence of the fit has been investigated by comparing standard deviations for J =

±0, 1, 2 and 3; as shown in Table 7.9.

Intermediate J level Observed energy Calculated energy Obs.-cal. /cm' 1cm' 1cm'

11 42611.4 42611.4 0.0 12 42618.3 42618.5 -0.2 13 42626.2 426262 0.0 14 42634.4 42634.5 0.0 15 42643.2 42643.3 -0.1 16 1 42652.6 1 42652.7 1 -0.1

Table 7.8.: The observed and calculated rotational term values, in cm, of an unknown vibrational level

of the intermediate Q( H) state which is coupled to v = I of the SO B(3E) state.

Chapter 7. Observations of an ion-pair state ofSO using OODR via the coupled B (3E) state 211

.j J levels observed To ± B ± S.D. of fit cm' cm cm' cm 1

-3 8-13 42584.7 0.4 0.375 0.0030 0.28141 -2 1 9-14 42580.6 0.3 1 0.344 0.0019 0.19324 -1 10-15 42576.5 0.2 0.317 0.0011 0.12685 0 11-16 42572.5 0.2 0.158 7.80E-04 0.09071 1 12-17 42568.4 0.2 0.275 7.60E-04 0.09568 2 13-18 1 42564.3 0.3 0.258 9.40E-04 0.12596 3 14-19 42560.2 0.3 0.243 0.0011 0.16239

Table 7.9.: The rotational assignments and standard deviations for the intermediate Q( 5 11) state of SO.

The A J column refers to the difference in rotational numbering from the preferred fit of J = 11-16.

Once the & value of the intermediate state is known it is possible, using

equation (2.24), to calculate an ru value of 2.31 A. The nature of the intermediate state

can now be investigated. Work carried out by Clerbaux and Colin 4 investigating

the -X transitions derived both a T 1 ,0 = 42567.0 1(2) cm- ' and a B 1 = 0.49337(4)

cm-1 value for the v = 1 vibrational level. The B1 value of 0.49337(4) cm -1 equates to

an r 1 value of 1.784 A. The term value for v= 1 of the B( 3 ) state lies 5.5 cm-1

above the term value observed for the intermediate state. The B 1 and r 1 values for v =

I of the B( 3 ) state are significantly different to those obtained for the intermediate

state, indicating that the B( 3 ) state can not truly be the intermediate state.

It has been known for some time that the B( 3 ) state is perturbed '. The

rotational perturbation of the B( 3 ) state has been studied by Martin , Abadie 6 and

Clerbaux and Cohn 'l Martin proposed that in order to rotationally perturb

the B( 3 ) state below the dissociation limit and to predissociate it above the

dissociation limit the perturbing state must be shallow-bound. The onset of

predissociation corresponds to the 3Pj + 3P threshold. Martin comments that ' (x2),

1 fJ (x2), ' is, (x2), , ii (x2) , , 5n (x2) and 5 A correlate with the

Chapter 7. Observations of an ion-pair state ofSO using OODR via the coupled B (3E) state 212

3Pj + 3P threshold. He further proposed that only the and 3u states can interact

with the B( 3 ) state and that it must be one of the 3 ii states, which he labelled

the C state. This interpretation was also used by Abadie 6 pertaining to the rotational

analysis of the (1,14) band of the B 4—Xtransition and by Clerbaux and Cohn, 4

although neither could derive any spectroscopic information about the perturbing

state. Ab initio calculations on some of the states which correlate with the 3P + 3P and

+ 'D dissociation products have been carried out by Ornellas and Bonn 6 They

predicted that, of the two 3r, states that correlate with the 3P + 3Pj products, one is

bound A( 3 [11) and the other is repulsive C'(3 III). The C'(3 LI) state undergoes an

avoided crossing with the bound C(3 n) state, which correlates with 3Pj + 'D products.

The results do not predict the existence of the shallow-bound state that perturbs

the B( 3 ) state. These conclusions have been further supported by the results of

further calculations by Archer et al. 3 The same group also produced calculations on

the quintet manifold and predicted that only one state of any multiplicity, a 5fl, is

bound. It is predicted to have aT e around 41000 cm, i.e. it is bound by 3000 cm,

and an re of 2.3 A. It was proposed 3 that the repulsive wall of this state might be

responsible for the predissociation of the B( 3 ) state. If this coupling is allowed,

then coupling of the bound levels is just as likely and appears to be the most likely

assignment for the perturbing intermediate state. Most of the arguments used in

interpreting the coupled B( 3 ) state in previous papers assuming a 31-1 perturber

probably hold true for a III perturber.

Therefore, the intermediate levels used in these experiments will be classified

as unknown vibrational levels of the Q( 5 LI) state which are coupled to v = 1, 2, and

Chapter 7. Observations of an ion-pair state of SO using OODR via the coupled B (3) state 213

low rotational levels of v =3 of the bound B( 3 ) state. A potential energy curve has

been generated for the Q( 5 H) state, shown in Fig. 7.7, using a Morse potential based

upon a curve generated by Archer et al.

It is apparent that both the B( 3 ) state and the Q( 5 11) state are required at the

intermediate level for a resonance to be observed at the ion-pair level for two reasons;

Firstly, the B(3 ) state is required to allow a degree of oscillator strength for the

pump transition from the ground state; the Q +— X transition strength is expected to be

weak. Furthermore, transitions from the B( 3 ) state to the ion-pair state are either

forbidden or extremely weak, as shown by the absence of ion-pair bands from v =0

of the B( 3 ) state, which is not coupled to the Q('171) state. Secondly, The Q( 5 H)

state has an r which is greater than that of the B( 3 ) state but very similar to that of

the ion-pair state resulting in a large Frank Condon overlap with the ion-pair state.

Thus transitions to the ion-pair state are observed strongly so long as the Q( 5 11) state

is coupled with the B( 3 ) state. This observation probably explains why excitation

via lower rotational levels of the intermediate state are weaker, since the lower

rotational levels appear to be less well coupled. 6

A summary of the spectroscopic constants determined for v = 1 of the

B( 3 ) state, v = n of the Q( 5 11) state and v =0, 1, 14 and 15 of the a(511) ion-pair

state is shown in Table 7.10.


State Tv By ru

cm cm cm' A

B(3E),v=1 421949 .8a 622 . 5a 0.49337 1.784

Q( 5 fl), v = n 42572.5 - 0.295 2.310

a(5171), v= 0 76975.6 309.0 0.217 2.690

a(fl), v = 1 77285.5 - 0.215 2.702

a(5[1), u= 14 81017.1 266.4 0.196 2.830

a(5fl), v = 15 81283.5 - 0.195 2.838

Table 7.10. A summary of the spectroscopic constants determined for u = I of the B( 3 ) state, v = n

of the Q( 5 11) state and u0, 1,14 and 15 of the a( 5H) ion-pair state

120 s+ +0- I

100 Zion-pair

C.)

C 40

54

B (3Z.) 54

S ( 1 D) + 0 (P)

(3P) + 0 (3P)

(ri)

EEl :1' Q(1,

a ( A) 10 tO 35 25 30 35

%/A

x ()

0 5 10 15 20

re /A

Fig. 7.7.: Potential energy curves for several states of SO relative to the SO ground state. The potential

energy curve for the Q(5 lIT) state has been generated using a Morse potential based upon a curve

generated by Archer et al. 3 The potential energy curve for the ion-pair state has been generated using a

Rittner potential generated as described in section 7.4. The insert shows the B( 3 - Q( 5 fl) state

crossing in greater detail.

20

Chapter 7 Observations of an ion-pair state of SO using OODR via the coupled B (3) state 215

7.3.5. Determination of the nature of the ion-pair state.

If the spectra in Figs. 7.3 and 7.6 showing the two-photon excitation of the i_p.

+— B transitions are reconsidered, three rotational branches are observed, the 0-, Q-

and S-branches. Of the three branches the Q-branch is observed with the greatest

intensity. The spectra are therefore typical examples of non-resonant two-photon

spectra recorded via a An =0 transition. It should also be noted that under circular

polarising conditions it was observed that the Q-branch diminished in intensity; an

observation that showed further evidence for a An =0 transition.

In previous studies of ion-pair states, particularly of the halogens, 92 has

proved to be a good quantum number. In the present study the perturbing state has

been assigned as a :51`1 state, which possesses Q = 3, 2, 1 and 0 components.

The B( 3 ) state, to which the Q ([I) state is coupled, possesses n = 1 and 0

components. Assuming that the coupling occurs between states with the same Q value

then only the Q = 1 and 0 components of the Q( 5fl) state and B( 3 )state can

couple. Since the transition from the coupled B( 3 ) state to the ion-pair state has

been shown to be a An =0 transition, then we can say that the ion-pair state has n =

1 or 0

Assuming that S and A is conserved for the i.p. — Q (Ll) transition, the ion-

pair state would be a (5J]) state. It can be shown that a (H) state can correlate with the

A 2 +4 S + i 5) +0- (P) asymptote. The S (S) ion possesses three half filled 3p orbitals,

which can be denoted by lI10i'il and the 0- (2P) ion possesses five 2p electrons of

configuration If li0OiiI. Combining the two ion-cores can lead to the molecular


orbital configuration (7I(2)2(3*)3(7o*)2 which possesses the requisite four

half filled shells needed to produce a 5r1 ion-pair state.

7.4. Rittner plot.

The Rittner equation (7.26)8 was first derived to calculate the binding energy

of alkali halide molecules. Alkali halide molecules are almost completely ionic;

therefore, their potential surfaces will be dependent upon the coulombic attractive

force that exists at large r. The Rittner equation can also be applied to ion-pair states

where the potential surface, at large r, is also dependent upon the coulombic attractive

force.

Ae —e 2 141rc0r (7.26)

If r = re then W= De

De = Ae —e 2 /4,'rcore (7.27)

d(W)/d(r)=O=—Abe +e2 14lreore2 (7.28)

d2 (W)/d(r) 2 = k = Ab2e - e 2 /22z ore3 (7.29)

Where: A = constant / J

b = constant / m- I

e = proton charge / C

CO = permittivity of free space / r'C 2m'

r = bond length / m


re = equilibrium bond length / m

De = ion-pair dissociation energy / J (1 cm t = 1.986E23 J)

k = Force constant / Jm -2

The ion-pair state dissociation energy can be calculated from simple energetic

considerations:

De [S + 0] relative to SO ground state = De [SO] + IP [Si + EA [0]

Where: De [SO] = 43680 CM-1 4

IP [Si = 83559.3 cm' 9

EA[O]-11824.l cm' 10

De = 115415.2 cm' relative to SO ground state

From equation 7.25, Te = 76823.1 cm for the ion-pair state

De [ion-pair] = 115415.2-76823.1 = 38592.1 cm 1

The equilibrium bond length for the a ion-pair state can be calculated from Equation

(7.30). To a first approximation, the rotational constant B e is given by

Bv Be O(V+0.5).... (7.30)

Empirically, it has been found that ae IBe is only slightly larger than WeXe /O)e, which

can be calculated for the ion-pair states using equation (7.25). B has been calculated

for v = 0, 1,14 and 15 for the ion-pair state.

re [ion-pair] = 2.687 A

Chapter 7. Observations of an ion-pair state of SO using OODR via the coupled B (3E) state 218

From the known re and De values for the ion pair state the constants A and b

can be found to equal 1.03 x 10' 5 J and 3.469 x 1010 nf', respectively, and the Rittner

Equation solved for all values of r. The Rittner potential generated is shown in Fig.

7.7.

Since co, [Hz]=1 / 2ir(k / p)1'2 (7.31)

An co,. value of 372.7 cm-1 can be calculated from the equation (7.31), which shows

reasonable agreement (since equation 7.26 is only a first approximation) with the

experimentally derived value of 308.7 cm -1 .

7.5. Conclusions.

v =0 to 30 of an ion-pair state of SO, labelled a, has been observed for the

first time for using OODR/REMPI pumping via both v = 1 and 2 of the

SO B(3 ) state, which are coupled to unknown vibrational levels of the Q( 5 fl) state.

The observed SO ion-pair state has been tentatively assigned as a (5 11) or

5 th +4 -2 ( ( Ll ) state, which can correlate with e S (5) +0 P) asymptote.

Experiments where single rotational levels of the intermediate coupled

SO B(' Z - ) state were pumped allowed both an accurate term value and B value to be

calculated for v = 0, 1, 14 and 15 of the ion-pair state. The ion-pair state was found to

possess a T0,0 value of 76976.5 ± 0.5 cm-1 and a B0 value equalling 0.217 ± 0.001 cm

',which equates to an r 0 value of 2.69 ± 0.006 A for v = 0 of the ion-pair state.


Excitation scans probing fixed rotational levels of the intermediate state

yielded a B value for the intermediate state of 0.295 ± 0.001 cm-1 . The SO B( 3

state possesses a B value of- 0.5 cm 1 , and hence cannot be the true intermediate

state used in these experiments unless it is perturbed. Upon comparison with

previously published ab initio calculations the perturbing state was proposed to be a

5171 state, which is the only bound state within the energy region pumped possessing a

B similar to that found experimentally.

The ion-pair nature of the new state is supported by Rittner potential

calculations assuming that the ion-pair state correlates with the S (4S) + 0- (2P)

asymptote and using 0e and re values derived from anharmonic oscillator calculations

and rotational analysis. The Rittner potential provides an toe value that shows

reasonable agreement with the experimental determined value.

Chapter 8. Analysis of the S ion-signal observedfrom the photodissociation ofSO 2. 221

Chapter 8.

Analysis of the S ion-signal observed from the

photodissociation of SO2.

8.1. Introduction.

In Chapter 5, it was shown that the one-colour REMPI spectrum of SO2, in the

245 - 295 urn wavelength region, recorded by collecting S was significantly different

from that recorded by collecting S0, as shown in Fig. 8.1. In particular, two very

strong bands, one broad around 262 rim and one sharp around 283 nm were observed.

It was shown in the previous Chapter that the a( 5H) ion-pair state of SO was detected

primarily by collecting S. It will be shown in this Chapter that a large percentage of

the S observed in the one-colour spectrum of SO2 is produced through excitation of

the ct(5fl) ion-pair state of SO via the X( 3 ) and coupled B(' Y- - ) states. This newly

identified source of S is compared with previous SO2 one-colour photodissociation

experiments where S has been observed and proposed as a primary product of SO2

dissociation.

5 8.2. S± production via the a( H) ion-pair state of SO.

One-colour REMPI spectra of SO2 at wavelengths corresponding to the 245-

295 rim region recorded by monitoring the S and SO ion-channels are shown in Fig.


8.1. The spectra were recorded under defocussed conditions, such that, the (2+1)

REMPI atomic sulfur resonances observed by Appling et al. 1 were minimised. The

observed dense structure in the SO ion-channel (Fig. 8.1(a)) can be assigned to

REMPI signals observed by Speth et al. 2 and reassigned by Bonn and Ornellas 3 and

Archer et al.to overlapping rotational structure of the (O,n) and (1 ,n) vibrational

bands of the SO e(' IT) — SO a(A)transition for n =6 - 11.

Co C D)

CI) C 0

i.p. -B(n,2)

25 20 15 10 5 0

I I I HII I II II I I ITT T] i.p. B

20 15 10 0

S,/0 2.

B-X(1,n) 2 3 4 5 6 7 I I I I

B-X 2 3 4 5 6 7 8 (2,n) ) • I . . 1 . •

240 245 250 255 260 265 270 275 280 285 290 295 300

Wavelength I nm

Fig. 8.1.: The 245-285 nm one-colour photodissociation spectra of SO 2 X('A1 ) ground state recorded

by collecting (a) the SO and (b) the S ion-channels using defocussed conditions. Calculated origin

bands for the SO B( 3 ) <— X(3 ) and i.p. — B(3 E- ) transitions are labelled.

The spectrum recorded by monitoring the S ion-channel (Fig 8.1(b)) contains

several strong one-colour resonances which are not observed in the spectrum shown

in Fig. 8.1(a). Using the data reported in Chapter 7 it is now possible to assign these

peaks to accidental resonances in SO. In these resonances, the incident radiation

Chapter 8. Analysis of the S ion-signal observedfrom the pholodissociation ofSO2. 223

firstly two-photon dissociates SO2, then excites certain rovibromc levels of the

perturbed B( 3 ) /Q() H) state of SO from the X(

3 y-) state, further excites the

perturbed levels to rovibronic levels of the a(R) ion-pair and finally ionises to S by

the absorption of at lease two more photons. Ladders showing the B( 3

X(3 ) and a(5fl) - B( 3 )/Q( 5 H)transitions are shown in Fig. 8.1. When the

steps on these ladders, which have a common value of B/Q coincide, an accidental

resonance occurs. The peak near 283 nm, indicated in Fig. 8.1(b) by an asterisk is an

example of an accidental resonance and arises from excitation of the (1,6)

B/Q - X band followed by excitation of the (3,1) i.p. - BIQ transition. It may be

of interest to note that this-one colour resonance, recorded in the S ion-channel, was

the first ion-pair resonance to be observed and investigation of this band led to the

discovery of the ion-pair state. The bandwidth of the 283 nm peak is determined by

the number of rotational levels that can be both pumped and probed using one-colour

excitation. Although the 283 nm peak appears to be a single narrow band in Fig.

8.1(b), in fact the peak is made up of several rotational transitions, as shown by Fig.

8.2.

A strong broad resonance can be observed at 262 nm, in Fig. 8.1(b), and

attributed to v = 15 of the SO ion-pair state observed via v =2 of the coupled B( 3 E)

state, which in turn, is pumped from v 4 of the X( 3 ) state. The band is broader

due to an increase in the number of rotational transitions that can be probed at these

wavelengths.

Chapter 8. Analysis of the S ion-signal observedfrom the photodissocialion ofSO 2. 224

-S

c 0)

Cl) C 0

+ Cl)

282.6 282.8 283.0 283.2 283.4 283.6

Wavelength I nm

Fig. 8.2.: The one-colour REMPI spectrum of v = 3 of the SO ion-pair state observed via v = I of the

coupled B( 3 ) state, which in turn, is pumped from v = 6 of the X(3 y - )state. Several branches

within the ion-pair state resonance can be observed.

Many other weaker accidental resonances make-up the dense structure

observed in the S ion-channel spectrum in Fig. 8.1(b). A further example of such a

peak is that at 255 nm corresponding to (2,3) B +— Xand (19,2) i.p. — B band

excitation.

A peak can also be observed at 248.4 nm, which could be due to (2+1) REMPI

excitation of ground state S (3Pj) atoms. However, the noted absence of S atomic

resonances observed by Appling et al. 1 , in the 252 - 263 rim wavelength regions

(removed using defocussed conditions whilst recording Fig. 8.1) appears to exclude

this assignment. Although, it should be noted that the (2+1) REMPI signal of S (3P3)

Chapter 8. Analysis of the S ion-signal observedfrom the pholodissociation ofSO2. 225

atoms at 248 mn is expected to be very intense due to the number of unresolvable J

levels of the upper (4S°) 6f3 F4,1,1 state which can be populated in the excitation

scheme. If the peak of 248.4 rim is not due to atomic sulfur detection then it could be

due to another accidental ion-pair state resonance, although no definite assignment

can be made at this time.

In summary, it can be said that under the experimental conditions used for the

present work (a defocussed beam with photons of 5 mJ/pulse) the main pathway to

the formation of S is via the a(511) ion-pair state of SO.

8.3. Primary production of sulfur atoms from the

dissociation of SO2 .

The photofraginents SO, 5, 02 and 0 can be theoretically formed at the

thresholds shown in Table 8. 1, for photodissociation of SO2 and SO. Several authors

1,1042 have proposed that ground state S atoms can be seen as a primary

photodissociation product of SO2 (channel (8.10)).

The exact excitation process occurring in SO2 is unknown, although, at these

excitation wavelengths a minimum of two photons are required to photolyse SO2. One

previously proposed 1,10,11 excitation scheme consists of sequential optical pumping of

the one-photon B('B 1 ) - X(A 1 ) and G - B('BI) transitions in SO2 followed by

dissociation, as shown in Fig. 8.2. Two-photon dissociation of S02 within the 245 -

285 rim region provides 80645.2 - 70175.4 cm-1 of energy, sufficient to allow

dissociation via channels (8.1) - (8.5) and (8.10) - (8.16), energetically allowing


production of SO (X( 3 ),

a('A) and b(' Y-' ) states), 0 (( 3P) , ('D) and ('S) states),

02 (X( 3 E),a('A) and b(' )

states) and S (( 3P) ,(D) and ('S) states).

Precursor Channel Thermodynamic Threshold / cm' (No.)

so2 X('A) -~ sO X( 3 )+O(3P2 ) 45725.3 (8.1)

-* SO a('A)+O(3P2 ) 51599.1 (8.2)

-,-so 56239.4 (8.3)

->so X( 3 )+0('D) 61593.0 (8.4)

-~ SO a('A)+ O('D) 67466.8 (8.5)

->so b('.)+o('D) 72107.1 (8.6)

-~ so X( 3 )+0('S) 79517.7 (8.7)

- -> SO a('A)+O('S) 85391.5 (8.8)

- so b(')+o('S) 90031.8 (8.9)

s02 X('A) -*02 X( 3 )+s(3P2 ) 44194.0 (8.10)

+02 a('A)+S( 3P2 ) 52083.4 (8.11)

->02 b(')+s(3P2 ) 57316.2 (8.12)

->02 X( 3 )+s('D) 53433.0 (8.13)

->02 a(1A)+S('D) 61322.4 (8.14)

-*02 b( 1 E)+s(1 D) 66555.2 (815)

-*02 X( 3 )+s('S) 66375.4 (8.16)

->02 a('A)+S('S) 74264.8 (8.17)

-+02 b( 1 E)+S('S) 79497.6 (8.18)

SO X( 3 Y- - ) ->o(3P2 )+s(3P2 ) 43793.0 (8.19)

-*0 ('P2)

+ S ('D) 53032.0 (8.20)

_-o( 3P2 )+S(S) 65974.4 (8.21)

-*0('D)+s(3P2 ) 59660.7 (8.22)

->0('S)+s(3P2 ) 77585.4 (8.23)

-*0(D)+s('D) 66899.7 (8.24)

-+ o ('S) + s ('D) 86824.4 (8.25)

-*o(D)+s('S) 81842.1 (8.26)

o('S) + S ('S) 99766.8 (8.27)

Table 8.1.: Thermodynamic thresholds for the formation of low-energy states of SO, 02, S and 0 via

2.5-9 the photodissociation of SO 2 and SO calculated from references

Chapter 8. Analysis of the S ion-signal observedfrom the photodissocialion ofSO 2. 227

hv

T3p,(S)

3F s+

S*

(d)

nhv.t IP(S)

- 3P, 3F ion-pair

.(c)

3 B state (g - continuum.

(b) hv - - 3

(e - (a)

B1B 1B

160

140

120

E 100

M

80 > D) a)

60

all

S+202(P)

EJE

x

201 S + 02 SO + 0( 3P)

SO + 0(3P)

Al X 1A1 802 so

Fig. 8.2.: A schematic diagram of various excitation and dissociation process occurring during 248 am

photolysis of SO2 . See Fig.4, Wilson et al. 10 Note pathways (a) through (g) are noted in the figure, see

text.

Wilson et al. 10 studied the photodissociation of SO2 using 248 nm radiation.

Three fluorescence bands were observed around 182 rim. The three bands were

reported to result from (3p 4) 3P2j,0 - (3p34s) 3S 1 transitions in atomic sulfur, where

ground state atomic sulfur, produced from the primary dissociation of SO2 via channel

(8.10), pathway (a), was first excited by the two-photon transitions (8.28) - (8.30),

Chapter 8. Analysis of the S ion-signal observedfrom the photodissociation ofSO2. 228

pathway (b), which relax to give S (3p34s) 3 S1 and finally fluoresce to produce the

observed atomic lines, pathway (c).

S (3p4 3P2) + 2hv2486 nm s (3p36f 3F) (8.28)

S (3p4 3P1) + 2hv248 i nm S (3p38p 3P) (8.29)

S (3p4 3P 1 ) + 2hv2486 —* S (3p3 8p 3P) (8.30)

The evidence for primary sulfur production from SO2 presented by Wilson et

al. 10 consisted of a power dependence study. The following observations were made;

the intensity dependence of the integrated 182 nm fluorescence exhibited a cubic

dependence (13) at low intensities and a quadratic dependence (1 2) at intensities higher

than approximately E 2 .sat

One-colour, two-photon, dissociation of SO2 at 248 and 308 nm, observed by

monitoring the SO and S(/02) ion-channels using photofragment transitional

spectroscopy (PTS) has been reported by Effenhauser et al. ' The S ion TOF

distributions were collected at both 308 and 248 nm reportedly via pathways (a), (b)

then (d). The distribution at 308 nm was noted to contain no obvious kinetic energy

thresholds pertaining to dissociation via channel (8.10) or similar channels. Whereas,

dissociation at 248 nm produced a TOF distribution where three thresholds

corresponding to channels (8.10, 8.13 and 8.14) were identified. It was noted that the

spectra were complicated by an overlap S_'_ ion-signal originating from SO which

contributed by way of its dissociation product. The spectra of Effenhauser et al. 1 1

should be viewed thus; sulfur atoms, in any ground state, cannot be resonantly

Chapter 8. Analysis of the S ion-signal observedfrom the photodissociation ofS02. 229

detected at 308 nm, therefore, spectra obtained at this wavelength primarily result

from S ion-signals originating from SO and SO. Using KrF excimer emission, 248

nm, ground state sulfur atoms can be resonantly detected via (8.28) - (8.30) as

discussed by Wilson et al. 10 TOF spectra, recorded at 248 nm, should therefore show

kinetic threshold information for the S (3P) ground states only, not the S ('D) and ('S)

states as assigned by Effenhauser et al. 11 The only threshold identified in the spectra

which is plausible is that pertaining to 02 X( 3 E ) + S ( 3P) formation, at 75 M.

Appling et al. 1 studied the one-colour, two-photon dissociation of SO2 in the

252 - 263 nm wavelength region. A REMPI spectrum monitoring the S ion channel

is reported between 254 -259 nm and resonances corresponding to various atomic

ground state sulfur resonances are observed. The resonances were assigned using

photoelectron spectroscopic analysis. Their discussion concentrated upon the atomic

sulfur resonances; the source of the ground state sulfur atoms was not discussed apart

from a brief summary of other people's work in their introduction.

8.4. Secondary production of sulfur atoms from the

dissociation of SO.

A second source of ground state sulfur atoms in the spectroscopy of SO2 is

that produced by secondary dissociation of SO, channels (8.19, 8.22 and 8.23), which

in turn is produced from primary dissociation of SO2, channels (8.1) - (8.9). This

secondary source of sulfur will now be discussed.

Chapter & Analysis of the S ion-signal observedfrom the photodissociation ofSO 2. 230

Fluorescence bands in the 250 - 400 nm ultraviolet region have also been

observed by Wilson et al. 10, whilst exciting SO2 at 248 nm. The fluorescence bands

were attributed to (2,v") of the SOB( 3 )4—SOX( 3 )transition. All but six

vibrational levels within the v" =0 -21 were identified. The excitation process

whereby SO B( 3 )molecules were generated was also proposed. Firstly, the 248 nm

radiation was able to pump the - B(B ) - X('A 1 )two-photon transitions in SO2.

Dissociation, of the G state, is energetic enough to allow SO to be produced in its

X , a and b state via reactions (8.1) - (8.3). The fixed wavelength 248 nm radiation

was found to be able to excite high rotational levels (J= 15 - 25) of the (2,2) band of

the SOB( 3 ) SOX(3)transition 12,13 via pathway (e): see Fig. 8.2.

Fluorescence from this band produced the observed (2,v') of the SO B( 3 )

4—SO X( 3 )transitions.

Direct dissociation of v =2, J :!~ 25 of the SO B( 3 E ) state is not possible,

since these levels are 400 cm - 1 below the first dissociation threshold of SO and

collisional dissociation cannot be induced 10 It was proposed 10 that ground state

sulfur atoms cannot be formed from dissociation of these levels at 248 rim. However

it is possible to dissociate SO via the SO B( 3 E ) state if excitation occurs from higher

vibrational levels of the SO X( 3 E ) ground state into the continuum of the

SO B( 3 )

state, pathway (f). It is energetically possible to produce ground state

sulfur atoms via pathway (f) at wavelengths lower that 346.7 nm. If ground state

sulfur atoms produced by pathway (f) were probed using a two-photon excitation

scheme, pathways (c) or (d), the power dependence of such a study would follow that

observed by Wilson et al. 10 since the one-photon transition in SO is easily saturated

Chapter & Analysis of the S ion-signal observedfrom the photodissociation ofSO 2. 231

and resonant at all wavelengths as a continuum is being excited. Therefore, the

arguments made by Wilson et al. 14 against sulfur atom production via the

SO B(' Y- - ) state are not valid.

Wilson et al. 10 also comment that, another possibility for the production of

sulfur atoms from SO is the subsequent excitation of the SO B(' Y, - ) state to high lying

states which could predissociate to form S ( 3P). If this premise is taken one step

further, resonant transitions can occur from the SO B( 3 ) state to the newly observed

ion-pair state, pathway (g). The ion-pair state resonances are primarily observed in the

S ion-channel formed from absorption of a minimum of two further photons.

Therefore, the ion-pair resonances can result in another source of S ions, which

should be considered in REMPI experiments.

8.5. Conclusions.

Sulfur atom production from SO2 has been investigated using knowledge of

the ion-pair state excitation scheme observed in Chapter 7. Ground state sulfur atom

production via primary dissociation of SO2 has been argued against and previous

studies supporting this pathway have been questioned. A second, more plausible,

source of sulfur atoms is the one-photon dissociation of the SO X(3 Z- ) state at

wavelengths :!~ 346.7 rum.

A new source of S ions has also been discovered relating to one-colour, two-

photon transitions in SO to the newly observed ion-pair state. These resonances are

Chapter 8. Analysis of the S ion-signal observedfrom the photodissocialion of SO 2. 232

observed when the one-colour radiation is able to pump and probe B(' E - ) —

X(3 )and i.p. +— B( 3 Y)transitions.

8.6. References.

J.R. Appling, M.R. Harbol, R.A. Edgington and A.C.Goren. J. Phys. Chem. 97

(1992) 4041.

R.S. Speth, C. Braatz, and E. Tiemann. J. Mo!. Spectrosc. 192 (1998) 69.

A.C. Bonn and F.R. Omellas. Chem. Phys. 96 (1992) 8054.


S. Becker, C. Braatz, J. Lindner and E. Tiemann. Chem. Phys. 196 (1995) 275.

C. Clerbaux and R. Cohn. J. Mol. Spectrosc. 165 (1994) 334.

S. Becker, C. Braatz, J. Lindner and E. Tiemann. Chem. Phys. Left. 208 (1993)

15.

C.E. Moore "Atomic energy levels", Circular of the National Bureau of Standards

467. vol. 1 (1949).

T.G. Slanger and P.C. Cosby. J. Phys. Chem. 92 (1988) 267.

M.W. Wilson, M. Rothchild, D.F. Muller and C.K. Rhodes. J. Chem. Phys. 77

(1982) 1837.

C.S. Effenhauser, P. Fielder and J.R. Huber. Chem. Phys. 142 (1990) 311.

T. Venkitachalam and R. Bersohn. J. Photochem. 26 (1984) 65.

C. Fotakis, A. Torre and R.J. Donovan. J. Photochem. 23 (1983) 97.

Appendix A. University regulations 233

Appendix A

University regulations.

A.!. Lecture courses attended.

Lasers I

10 lectures.

Laser II

10 lectures.

Atmospheric chemistry.

5 lectures.

Laser spectroscopy

5 Lectures

Physical group colloquia.

Academic years 1999-2000, 2000-2001, 2001-2002.

Physical chemistry evening seminars.

Academic years 1999-2000, 2000-2001, 2001-2002.


A.2. Conferences attended.

"Molecular Ionisation".

Faraday Discussion 115.

University of York.

3-5 April 2000.

Meeting attended.

"Physical Group meeting".

Firbush, University of Edinburgh.

August 2000.

Oral presentation.

"Stereochemistry and Energy Landscape".

Joint Meeting of the Molecular Beam and Dynamics Group and Theoretical

Chemistry Group of the Royal Society of Chemistry.

Warwick University.

26-27 March 2001.

Poster presentation.

"ANUMOCP".

Annual Northern Universities Meeting on Chemical Physics.

University of Leeds.

4 July 2001.

Oral presentation.

"Physical Group meeting".


August 2001.



"From Spectroscopy to Rates".

A Joint Conference of the High Resolution Spectroscopy and Gas Kinetics Discussion

Group of the Royal Society of Chemistry.

University of Birmingham.

18-20 December 2001.


"Time Resolved Chemistry: From Structure to Function".

University of Manchester.

24-26 June 2002.


"Physical Group meeting". - - -


August 2002.

Oral presentation.


A.3. List of Publications.

Trevor Ridley, Kenneth Lawley, Howard Sheard and Robert Donovan.

"An Optical-Optical Double Resonances Study of the d3s ( 1 11g) Rydberg State of

02 Using b ( 1 g) as the Resonant Intermediate State"

J. Chem. Phys. 116 (2002) 451.

P. O'Keeffe, T. Ridley, H.A. Sheard, K.P. Lawley, R.J. Donovan and B.R. Lewis

"The d 1 1-Ig (v = 1) Rydberg state of 02: Optical-Optical Double-Resonance and

Huggins-Band Ozone-Photolysis, Resonance-Enhanced Multiphoton-lonization

Studies with a b 1g (v = 0)-State Platform."

J. -Chem. Phys. 117 (2002)1705. -

H.A. Sheard, T. Ridley, K.P. Lawley and R.J. Donovan.

"An Optical-Optical Double Resonance Study of the Rydberg State of 02. 1. The ns

and nd (gerade) States Excited via Single Rotational Levels of the ( ' g) Valence

State"

J. Chem. Phys. 118 (2003) 8781.

H.A. Sheard, T. Ridley, K.P. Lawley and R.J. Donovan.

"An OODR Study of a Newly Observed Ion-Pair State of SO via the coupled B (3)

State."

In preparation.

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