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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

Photoionization


Dissociative photoionization


AB + h  AB* (E > I)  AB+ + e-
Field ionization


AB + h  A + B+ + e-
Autoionization


AB + h  AB+ + e-
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+ + eN2+ + 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
2 +
N2(u2u4g2)  N2+(u2u4g1) + eg
2
N2(u2u4g2)  N2+(u2u3g2) + eu
2 +
N2(u2u4g2)  N2+(u1u4g2) + eu

There is no resonant condition for h because the
energy of the outgoing electron is not quantised
(free electron)
Conservation of energy in
photoionization


h = I + Eion + KE(e-) + KE(AB+)





AB + h  AB+ + e-
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+ + eKE(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
I
Measuring the
“spectrum” of
photoelectron energies
provides a map of the
quantised energy
states of the molecular
ion
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) + eN2(u2u4g2)  N2+(u2u3g2) + eN2(u2u4g2)  N2+(u1u4g2) + e-
2
2 +
u
u
2 +
g
2
u
2 +
u
2 +
g
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
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-)
Eion
h
I
Zero KE
electron
Measuring the
production of zero KE
electrons (only) versus
photon wavelength
h = I+Eion
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
   
J   J l

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
Dark
S
spectroscopy
state
1
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

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