Lecture 7 Photoionization and photoelectron spectroscopy Photochemistry Lecture 7 Photoionization and photoelectron spectroscopy

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

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

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

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 that KE(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 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

PES - experimental

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

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

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

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

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)

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 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.

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

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

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 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

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 HNCO physchem.ox.ac.uk/~jhde IE MASS