Beam Action Spectroscopy via Inelastic Scattering

BASIS Technique

Bobby H. Layne and Liam M. DuffyDepartment of Chemistry & Biochemistry, the University of North

Carolina at Greensboro, Greensboro, North Carolina 27402

Hans A Bechtel, Adam H. Steeves and Robert W. FieldDepartment of Chemistry, Massachusetts Institute of Technology,

Cambridge, Massachusetts 02139

J. Phys. Chem. A (online)

Current Research

Using mm-Waves to probe:

• Photodissociation of atmospheric molecules:– characterizing quantum state distribution of products

– hyper-rovibronic detail

• Crossed molecular beams: reactive and inelastic scattering dynamics

Current Photodissociation Study

Chlorine Dioxide

h

Chlorine MonoxideOxygen

ClCl

OOO

O

Parent Molecule Products

OClO is an reservoir molecule for Cl radicals in the atmosphere

OClO Mode Specific UV Spectrum

0.0E+00

2.0E-18

4.0E-18

6.0E-18

8.0E-18

1.0E-17

1.2E-17

1.4E-17

1.6E-17

280 300 320 340 360 380 400 420 440

Wavelength (nm)

Ab

sorp

tion

Cro

ss S

ecti

on (

cm2 )

204 K

(14,

0,0)

(16,

0,0)

(18,

0,0)

(15,

0,0)

(17,

0,0)

(6,0

,0)

(5,0

,2)

(6,1

,0)

(1, 2, 3 )sym, bend, asym

(5,1

,2)

This Study

A. Wahner, G. S. Tyndall, and A. R. Ravishankara, J. Phys. Chem. 91, 2734 (1987).

mm-Wave: 1. Source Module2. Amplifier3. Multiplier4. Horn

Pulsed Slit Nozzle

Teflon Window & Lens

Top view of vacuum chamber with diffusion pump below

InSbHot Electron

Bolometer

ULN6pre

amp

Tunable UV from doubled OPO

4 3 2 1

CornerReflector MultipassCell

Photodissociation Setup

Current Available Range:50-330 GHz

Mm-WaveSource Module

“Armadillo”MicrowaveSynthesizer

Mm-WaveAmplifier &

Tripler

Frequency Resolution• 10 Hz at 100 GHz• or 1 part in 1010

Power > 1 mW

Parent Molecule

Photodissociationof Parent Molecule

Laser is fixed while mm-waves are stepped

UV Laser fixed toOClO (X 2B1 A 2A2 (15, 0, 0))

O35ClO hyperfine lines

Cl

OO

Products Probed in Hyper-rovibronic Detail

Cl

O

0

1

2

3

4

5

6

7

8

-500 -400 -300 -200 -100 0 100 200 300 400 500 600 700

Time (msec)

% A

bso

rpti

on

Hole

Millimeter-wave absorption time trace centered on a single hyperfine line of

O35ClO (268797.6550 MHz: N=65, J=6½5½, K-1=32, K+1=43, F=87)

Problem !

The “hole” shows a 50% depletion of the parent.

8.4% is expected from the product of laser fluence and UV cross-section.

“Hole burning” spectrum of O35ClO

30000 31000 32000 33000 34000 35000 36000

Frequency (cm-1)

Inte

nsi

ty (

arb

.)

. x 2.5

30000 31000 32000 33000 34000 35000 36000

Frequency (cm-1)

Inte

nsi

ty (

arb

.)

. x 2.5

(14,

0,0)

(15,

0,0)

(16,

0,0)

(17,

0,0)

?

BASIS spectrum of O35ClO and O37ClO

30650 30750 30850 30950 31050 31150 31250

Frequency (cm-1)

Inte

nsity

(arb

.)

y

35ClO

37ClO

O37ClO

O35ClOOCS

BASIS Signals

Products

Parent SignalBASIS Signal

SO2 Parent Hans Bechtel & Adam Steeves

(Field Group at MIT)

Low rotational transition313 - 202

High rotational transition817 - 808

artistic simulation

O C S

O C S

O C S

O C S

O C S

O C S O C S

O C S

O C S

O C S

O C S

O C S

O C S

Ar

Ar

Ar

Ar

Ar

Ar

ArAr

Ar Ar

Ar

Ar

Ar

ArAr

Ar

Ar

Ar

Ar

OCl

O

ArAr

UV BASIS Signals of OClO OCS reporter & fixed UV

J = 6

J = 25

Trot=22K

Raw time responses

Amplitudes from yellow cursors

Trot= 22 K

Trot= 25 K

Inte

nsit

yIn

tens

ity

diff

.

TransformedData (see settings)

Depletion

Buildup

Signals Inverted

J = 6

J = 25

Dynamic range 1:105

Observable rotational temperature shifts as small as 200 mK.

OCS acts as a “virtual bolometer”

Infrared BASIS Hans Bechtel & Adam Steeves

(Field Group at MIT)

H-C C-H + IR H-C C-H(vib. cold) (vib. hot)

Reporting Molecule

H-C C-H + OCS H-C C-H + OCS (vib. cold)(vib. hot) (rot. hot)(rot. cold)

collision

near nozzle

IR-BASIS Spectrum of Acetylenemonitored via OCS

BASIS Advantages

• Gain: – energy deposited in beam is large– e.g. 6x over traditional hole burning (f )– analogous to optothermal technique (but simpler)– may extend the sensitivity of direct-IR absorption

• General Method:– should work for any rotationally resolved

molecular beam spectroscopic technique

Best BASIS Conditions

• Cold rotational distribution

• High density region:– reporting molecule needs to stay in the beam– near nozzle for IR BASIS– down stream possible for slit jet photodissociation

• Reporting molecule chosen:– largest rotational line intensity – does not photodissociate– large RT collision cross section?

Possible BASIS Experiments• UV, Vis, IR – BASIS• Dark states• Surface BASIS:

– scattering off of optically excited SAMs

Side Implications• Slit-jet densities are so large that fragments are

entrained even 10 cm downstream• Pump-probe time delay important, particularly in CW

experiments– Lifetime broadening may be collisional broadening.

(e.g. OClO near Frank-Condon max)

Acknowledgements

UNCG Undergraduate:Bobby H. Layne

Hans A Bechtel, Adam H. Steeves and Robert W. Field

H.A.B. acknowledges the Donors of the AmericanChemical Society Petroleum Research Fund for support, andA.H.S. acknowledges the Army Research Office for a NationalDefense Science and Engineering Graduate Fellowship. The

work at MIT was supported by the Office of Basic EnergySciences of the U.S. Department of Energy

Helpful conversations & BASIS acronym:Prof. Robert M. Whitnell