Presentation on theme: "Photochemistry Lecture 7 Photoionization and photoelectron spectroscopy."— Presentation transcript:
Photochemistry Lecture 7 Photoionization and photoelectron spectroscopy
Hierarchy of molecular electronic states Neutral Ground state Excited states (S 1 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 AB e - A + + B + AB + h (AB + )* + e - (1) AB 2+ +e - (2) A + + B + Rule of thumb: 2 nd IP 2.6 x 1 st IP Vacuum ultraviolet 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 N 2 + h N e - N O N + NO + …. NO + + e- N* + O (dissociative recombination) In interstellar gas clouds H H 2 H H H C CH + + H 2 CH + + H 2 CH 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 N 2 N 2 ( u 2 u 4 g 2 ) N 2 + ( u 2 u 4 g 1 ) + e - 2 g + N 2 ( u 2 u 4 g 2 ) N 2 + ( u 2 u 3 g 2 ) + e - 2 u N 2 ( u 2 u 4 g 2 ) N 2 + ( u 1 u 4 g 2 ) + 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 + E ion + KE(e - ) + KE(AB + ) I = adiabatic ionization energy (energy required to produce ion with no internal energy and an electron with zero kinetic energy) E ion 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 - E ion
AB + h AB + + e - KE(e - ) h - I - E ion 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 – E ion 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 )
Photoelectron spectroscopy Measuring the “spectrum” of photoelectron energies provides a map of the quantised energy states of the molecular ion KE(e - ) h - I - E ion h I KE(e - ) E ion
PES - experimental
PES of H 2 molecule H 2 + has only one accessible electronic state H 2 ( g 2 ) + h H 2 + ( 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 H 2 + 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+2g+ 2u2u 2u+2u+ N 2 ( u 2 u 4 g 2 ) N 2 + ( u 2 u 4 g 1 ) + e- 2 g + N 2 ( u 2 u 4 g 2 ) N 2 + ( u 2 u 3 g 2 ) + e- 2 u N 2 ( u 2 u 4 g 2 ) N 2 + ( u 1 u 4 g 2 ) + e- 2 u +
Koopman’s Theorem Recognise that each major feature in PES of N 2 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 + E ion = - (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 u 4 g 2 configuration leads to two possible electronic states u 3 g 2 :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 O 2 (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 - E ion 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 + E ion 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+E ion KE(e - ) h - I - E ion h I KE(e - ) E ion Zero KE electron
Resolved rotational structure in ZEKE PES of N 2
ZEKE spectrum of N 2 – 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., CH 4 +, O 3 + CH 2 +, C 6 H 6 +, NH 4 + (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., S 1 state, and one to induce ionization from that state. The photoelectron spectrum thus recorded reflects orbital configuration of S 1 state.
Time resolved photoelectron spectroscopy If ISC takes place from intermediate then photoelectron spectrum may show excitation from both initially excited (“bright”) S 1 and T 1 (“dark”) state. Pump-probe photoelectron experiment (cf flash photolysis) on fluorene – delay ionizing light pulse with respect to excitation S1S1 Dark state
Preparing molecular ions in known energy states – photoelectron-ion coincidence KE(e - ) h - I - E ion 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.