Presentation on theme: "Lecture 7 Photoionization and photoelectron spectroscopy"— Presentation transcript:
1 Lecture 7 Photoionization and photoelectron spectroscopy PhotochemistryLecture 7Photoionization and photoelectron spectroscopy
2 Hierarchy of molecular electronic states Ionic excited statesIonic ground state (ionization limit)Neutral Rydberg statesExcited states (S1 etc)Neutral Ground state
3 Photoionization processes AB + h AB+ + e-Dissociative photoionizationAB + h A + B+ + e-AutoionizationAB + h AB* (E > I) AB+ + e-Field ionizationAB + h AB* (E < I) apply field AB+ + e-Double ionizationAB + 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
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 layerMost species exist in the ionized state (ionosphere)e.g., in atmosphereN2 + h N2+ + e-N2+ + O N + NO+ ….NO+ + e- N* + O (dissociative recombination)In interstellar gas cloudsH2+ + H2 H3+ + HH3+ + C CH+ + H2CH+ + H2 CH2+ + H
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 transitionse.g., for N2N2(u2u4g2) N2+(u2u4g1) + e- 2g+N2(u2u4g2) N2+(u2u3g2) + e uN2(u2u4g2) N2+(u1u4g2) + e- 2u+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 electronKE(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 thatKE(e-) h - I – EionTypically 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 - EionKE(e-)EionhMeasuring the “spectrum” of photoelectron energies provides a map of the quantised energy states of the molecular ionI
12 PES of H2 moleculeH2+ 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 energyPeaks map out the vibrational energy levels of H2+ up to its dissociation limit
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 formedSub structure of each band represents the vibrational energy levels of each electronic state of the ion
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 thenI + 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 oxygenRemoval of electron from u orbital of u4g2 configuration leads to two possible electronic statesu3g2: three unpaired electrons give either 2u or 4u statesBreakdown of Koopman’s theorem (no one-to-one correspondence between orbitals and PES bands)
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 ione.g., ammonia; neutral is pyramidal, ion is planarLong progression in umbrella bending modeIf 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 - EionInstead of using fixed h and measuring variable KE(e-), use tuneable h and measure electrons with fixed (zero) kinetic energyEach 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 - EionKE(e-)Zero KE electronEionhMeasuring the production of zero KE electrons (only) versus photon wavelengthh = I+EionI
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 electronThus change of rotational angular momentum of molecule on ionization may be greater than 1, providingNote the above formula ignores electron spin
26 ZEKE spectroscopyThe 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 stateS1If 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 - EionIf 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.