photochemistry lecture 7 photoionization and photoelectron spectroscopy
TRANSCRIPT
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
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 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
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 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
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).
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.