Photochemistry

Lecture 7Photoionization and

photoelectron spectroscopy

Hierarchy of molecular electronic states

Neutral Ground state

Excited states (S1 etc)

Neutral Rydberg states

Ionic ground state (ionization limit)

Ionic excited states

Photoionization processes Photoionization

AB + h AB+ + e-

Dissociative photoionization AB + h A + B+ + e-

Autoionization AB + h AB* (E > I) AB+ + e-

Field ionization AB + h AB* (E < I) apply field AB+ + e-

Double ionization AB + h AB2+ + 2e- A+ + B+

AB + h (AB+)* + e-(1) AB2+ +e-(2) A+ + B+

Rule of thumb: 2nd IP 2.6 x 1st IPVacuum ultraviolet < 190 nm or E > 6 eV

Importance of molecular ion gas phase chemistry In Upper atmosphere and astrophysical environment,

molecules subject to short wavelength radiation from sun, gamma rays etc.

No protection from e.g., ozone layer Most species exist in the ionized state (ionosphere) e.g., in atmosphere

N2 + h N2+ + e-

N2+ + O N + NO+ ….

NO+ + e- N* + O (dissociative recombination)

In interstellar gas clouds H2

+ + H2 H3+ + H

H3+ + C CH+ + H2

CH+ + H2 CH2+ + H

Ion density in the ionosphere (E,F regions)

Selection rules (or propensity rules) for single photoionization Any electronic state of the cation can be produced in

principle if it can be accessed by removal of one electron from the neutral without further electron rearrangement

- at least, there is a strong propensity in favour of such transitions

e.g., for N2

N2(u2u

4g2) N2

+(u2u

4g1) + e-

2g

+

N2(u2u

4g2) N2

+(u2u

3g2) + e-

2u

N2(u2u

4g2) N2

+(u1u

4g2) + e-

2u

+

There is no resonant condition for h because the energy of the outgoing electron is not quantised (free electron)

Conservation of energy in photoionization

AB + h AB+ + e-

h = I + Eion + KE(e-) + KE(AB+)

I = adiabatic ionization energy (energy required to produce ion with no internal energy and an electron with zero kinetic energy)

Eion is the internal energy of the cation (electronic, vibrational, rotational…..)

KE(e-) is the kinetic energy of the free electron

KE(AB+) is the kinetic energy of the ion (usually assumed to be negligible)

Thus KE(e-) h - I - Eion

AB + h AB+ + e-

KE(e-) h - I - Eion

The greater the internal energy of the ion that is formed, the lower the kinetic energy of the photoelectron.

This simple law forms the basis of photoelectron spectroscopy

Photoelectron spectroscopy Ionization of a sample of molecules with h » I will

produce ions with a distribution of internal energies (no resonant condition)

Thus the electrons ejected will have a range of kinetic energies such thatKE(e-) h - I – Eion

Typically use h = 21.22 eV (He I line – discharge lamp)

or h = 40.81 eV (He II)For most molecules I 10 eV (1 eV = 8065 cm-1)

Photoelectron spectroscopy

Measuring the “spectrum” of photoelectron energies provides a map of the quantised energy states of the molecular ion

KE(e-) h - I - Eion

h

I

KE(e-)

Eion

PES - experimental

PES of H2 molecule

H2+ has only one accessible electronic state

H2(g2) + h H2

+(g) + e- 2g+

But for h = 21.2 eV, and I = 15.4 eV the ions could be produced with up to 5.8 eV of internal energy – in this case vibrational energy

Peaks map out the vibrational energy levels of H2

+ up to its dissociation limit

PES of H2

Franck Condon Principle Large change of bond length on reducing

bond order from 1 to 0.5.

Franck Condon overlap favours production of ions in excited vibrational levels.

PES of nitrogen

I = 15.6 eV, h = 21.2 eV Three main features represent different

electronic states of ion that are formed Sub structure of each band represents the

vibrational energy levels of each electronic state of the ion

2g+

2u

2u+

N2(u2u

4g2) N2

+(u2u

4g1) + e- 2g

+ N2(u

2u4g

2) N2+(u

2u3g

2) + e- 2u N2(u

2u4g

2) N2+(u1u

4g2) + e- 2u

+

Koopman’s Theorem Recognise that each major feature in PES of N2

results from removal of electron from a different orbital.

More energy required to remove electron from lower lying orbital (because this results in a higher energy molecular ion)

If the orbitals and their energies do not “relax” on photoionization then I + Eion = - (orbital energy)

But in practise remaining electrons reorganise to lower the energy of the molecular ion that is produced hence this relationship is approximate

PES of oxygen Removal of electron from u orbital of

u4g

2 configuration leads to two possible electronic states

u3g

2: three unpaired electrons give either 2u or 4u states

Breakdown of Koopman’s theorem (no one-to-one correspondence between orbitals and PES bands)

PES of O2 (First band not shown)

PES of HBr reveals spin-orbit coupling splitting as well as vibrational structure

PES of polyatomic molecules Vibrational structure –

depends on change of geometry between neutral and ion

e.g., ammonia; neutral is pyramidal, ion is planar

Long progression in umbrella bending mode

If many modes can be excited than spectrum may be too congested to resolve vibrational structure

High resolution photoelectron spectroscopy – ZEKE spectroscopyKE(e-) h - I - Eion

Instead of using fixed h and measuring variable KE(e-), use tuneable h and measure electrons with fixed (zero) kinetic energy

Each time h = I + Eion the “ZEKE” (zero kinetic energy) electrons are produced – this only occurs at certain resonant frequencies.

ZEKE Photoelectron spectroscopy

Measuring the production of zero KE electrons (only) versus photon wavelength

h = I+Eion

KE(e-) h - I - Eion

h

I

KE(e-)

Eion

Zero KE electron

Resolved rotational structure in ZEKE PES of N2

ZEKE spectrum of N2 – predominant J=2 Note that the outgoing electron can have

angular momentum even though it is a free electron

Thus change of rotational angular momentum of molecule on ionization may be greater than 1, providing

Note the above formula ignores electron spin

lJJ

ZEKE spectroscopy The best resolution for this method is far superior

to conventional PES (world record 0.01 meV versus typical 10 meV for conventional PES)

Thus resolution of rotational structure, or of congested vibrational structure in larger polyatomic molecules, is possible.

Gives rotational constants of cations hence structural information e.g., CH4

+, O3+ CH2

+, C6H6+,

NH4+ (direct spectroscopy on ions difficult)

In practise can only be applied in gas phase (unlike conventional PES- solids, liquids and surfaces).

Vibrational structure in H bonded complex of phenol and methanol

Time resolved photoelectron spectroscopy

Photoelectron spectrum of excited states –

Use two lasers one to excite molecule to e.g., S1 state, and one to induce ionization from that state.

The photoelectron spectrum thus recorded reflects orbital configuration of S1 state.

Time resolved photoelectron spectroscopy

If ISC takes place from intermediate then photoelectron spectrum may show excitation from both initially excited (“bright”) S1 and T1 (“dark”) state.

Pump-probe photoelectron experiment (cf flash photolysis) on fluorene – delay ionizing light pulse with respect to excitation

S1

Dark state

Preparing molecular ions in known energy states – photoelectron-ion coincidence

KE(e-) h - I - Eion

If the ionization events happen one at a time, we can determine internal energy of each ion that is produced by measuring the kinetic energy of the corresponding electron. If the ion subsequently fragments, we can investigate how fragmentation depends on initial state of the ion populated.

PEPICO (photoelectron-photoion coincidence apparatus)

PEPICO spectrum of HNCOphyschem.ox.ac.uk/~jhde

MASS

IE