1. Lecture 7 Photoionization and photoelectron spectroscopy
Photochemistry
Lecture 7
Photoionization and photoelectron spectroscopy

2. Hierarchy of molecular electronic states
Ionic excited states
Ionic ground state (ionization limit)
Neutral Rydberg states
Excited states (S1 etc)
Neutral Ground state

3. Photoionization processes
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 IP
Vacuum ultraviolet  < 190 nm or E > 6 eV

4. 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

5. Ion density in the ionosphere (E,F regions)

6. 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(u2u4g2)  N2+(u2u4g1) + e- 2g+
N2(u2u4g2)  N2+(u2u3g2) + e u
N2(u2u4g2)  N2+(u1u4g2) + e- 2u+
There is no resonant condition for h because the energy of the outgoing electron is not quantised (free electron)

7. 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

8. 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

9. 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 that
KE(e-)  h - I – Eion
Typically use h = eV (He I line – discharge lamp)
or h = eV (He II)
For most molecules I  10 eV (1 eV = 8065 cm-1)

10. Photoelectron spectroscopy
KE(e-)  h - I - Eion
KE(e-)
Eion h
Measuring the “spectrum” of photoelectron energies provides a map of the quantised energy states of the molecular ion I

11. PES - experimental

12. PES of H2 molecule
H2+ has only one accessible electronic state H2(g2) + h  H2+(g) + e- 2g+
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

13. PES of H2

14. 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.

15. 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

16. N2(u2u4g2)  N2+(u2u4g1) + e- 2g+
N2(u2u4g2)  N2+(u2u3g2) + e u
N2(u2u4g2)  N2+(u1u4g2) + e- 2u+
2g+
2u
2u+

17. Koopman’s Theorem I + Eion = - (orbital energy)
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

18. PES of oxygen
Removal of electron from u orbital of u4g2 configuration leads to two possible electronic states
u3g2: three unpaired electrons give either 2u or 4u states
Breakdown of Koopman’s theorem (no one-to-one correspondence between orbitals and PES bands)

19. PES of O2 (First band not shown)

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

21. 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

22. High resolution photoelectron spectroscopy – ZEKE spectroscopy
KE(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.

23. ZEKE Photoelectron spectroscopy
KE(e-)  h - I - Eion
KE(e-)
Zero KE electron
Eion h
Measuring the production of zero KE electrons (only) versus photon wavelength
h = I+Eion I

24. Resolved rotational structure in ZEKE PES of N2

25. 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

26. 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).

27. Vibrational structure in H bonded complex of phenol and methanol

28. 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.

29. Time resolved photoelectron spectroscopy
Dark state S1
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

30. 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.

31. PEPICO (photoelectron-photoion coincidence apparatus)

32. PEPICO spectrum of HNCO physchem.ox.ac.uk/~jhdeIE
MASS