1. PHOTOELECTRON SPECTROSCOPY

2. Spectroscopy uses interaction of electromagnetic radiation with matter to learn something about the matter.
Characteristics of absorbed, emitted or scattered electromegnetic radiation are measured
Photoelectron spectroscopy is entirely different!
Kinetic energies of the ionized electrons are monitored
It is an extension of PHOTOELECTRIC EFFECT

3. Photoelectric Effect
Ionization occurs when matter interacts with light of sufficient energy (Heinrich Hertz, 1886)
(Einstein, A. Ann. Phys. Leipzig 1905, 17, ) e- e- e- hn
Ehn = electron kinetic energy + electron binding energy
Photoelectron spectroscopy uses this phenomenon
to learn about the electronic structure of matter

4. Virtual orbitals (empty)
Ionization continuum
Virtual orbitals (empty) E N R G Y
hn = I.E + K.E
PHOTO
IONIZATION
PHOTO
EXCITATION
Valence level (partially filled)
Core level (completely filled)
Excitation and Photoionization Processes from Core & Valence level

5. Photoelectron spectroscopy is based on Einstein's photoelectric effect.
A photon can ionize an electron from a molecule if the photon has an energy greater than the energy holding the electron in the molecule.
Any photon energy in excess of that needed for ionization is carried by the outgoing electron in the form of kinetic energy.

6. OR OR

7. INSTRUMENTATION
UV P.E.S.
He(I) ev. Ar (I), He (II)
XPS Mg Ka – ev Al Ka – ev •

8. • Spectrum consists of bands, corresponding to ionizations
P. E. S. of Molecules
Electron signal
Counts/s
secsecec
• Spectrum consists of bands, corresponding to ionizations
from individual orbitals.
• Bands may have a single peak or consist of a series of peaks (fine structure), arising from excitations to the various vibrational-rotational states of the ion, M+. (Usually, only vibrational fine structure is resolved.)
• Spin-orbital coupling (producing two distinct J values and corresponding energies) or Jahn-Teller effects (causing splitting of degenerate states) can cause additional band splitting.

9. Molecular Ionization from a Non-Bonding MO
Ionization from a non-bonding M.O. does not change bond length, so the vertical transition is the adiabatic transition,
M(v = 0) to M+ (v' = 0).
P.E.S. bands for ionizations from non-bonding M.O.s tend to have little or no vibrational fine structure.

10. Molecular Ionization from a Bonding MO
Ionization from a bonding or anti-bonding M.O. causes a change in bond length in the ion M+, compared to the bond length in M.
• The vertical transition is to an M+ vibrational state v' > 0 with maximum wave-function overlap at the same inter-nuclear separation as the normal bond length of the neutral molecule,
M. (Not the adiabatic transition).
• Transitions to other v' states are possible with lesser probability, resulting in vibrational fine structure on the ionization band.
The frequency separations between the vibrational peaks can
be used to calculate the vibrational frequency of the ion, M+.
P.E.S. bands for ionizations from bonding or antibonding
M.O.s tend to have pronounced vibrational fine structure.

11. Ionizations as State-to-State Transitions
Possible ionizations may be viewed as state-to-state transitions.
• Adiabatic Transition - Transition from M in v = 0 to M+ in v' = 0.
• Vertical Transition - Transition without change in bond length (Franck-Condon Principle).

12. MO diagram & Photoelectron spectrum of H2
PES of H – atom
one peak at ev
H + hn H+ + e-
1S1 + hn S0 + e- 1S 1S
PES of H2
15.45 ev to 18 ev
H 2 (sgb)2 + hn H2+ (sgb)1 + e-
(sgb)2 electrons are more stable than 1S1 electron of H-atom
First Peak at ev
n=0 of H 2 to n’ = 0 of H2+ so Adiabetic Ionization Energy
Most Intense Peak
n=0 of H 2 to n’ = 2 of H2+ so Vertical Ionization Energy

13. MO diagram & Photo Electron Spectrum of O2.
One e- M.O
Ionic electronic state
PES peak (ev)
n’ = 0 to n’ = 1 of nO2+
(vibrational spacing, cm-1)
Nature of orbital
Pg2p
2Pg
12.10
1780
Antibonding
Pu2p
4Pu , 2Pu
16.26
17.7
1010
Bonding
sg2p
4Sg
2Sg
18.18
20.3
1090
1130
Vibrational frequency of O2 = 1568 cm-1

14. O2 ground state O2+ 3sg (p) quartet spin down e- ionized 4Sg
1Pu
1Pg
O ground state
O2+ 3sg (p) quartet
spin down e- ionized Sg
O2+ 3sg (p) doublet
spin up e- ionized Sg
3sg (p)
1Pu
1Pg
O ground state
O2+ 1Pu quartet
spin down e- ionized Pu
O2+ 1Pu doublet
spin up e- ionized Pu

15. s-p mixing MO diagram & Photoelectron spectrum of N2 Koopman’s Theoram
Quantum mechanical calculations –
The order of energies of orbitals
(2sg)2 (2su)2 (3sg)2 (1Pu)4
Koopman’s Theoram
Most widely used approximation
IEn = - En Valid for one e- atoms
From PES –
The lowest ionic state of N2+
(2sg)2 (2su)2 (1Pu)4(3sg)1
For multi electron atoms & molecules, “frozen orbital” approximation
Still valid provided the orbitals in the ionized molecule are unchanged
from those of the neutral molecules
The ordering of the orbitals in the ground state
Different from that of the molecule ion
Breakdown of Koopman’s theoram
s-p mixing

16. s-p mixing
Interactions or mixing between 2s and 2pz AOs cause a reordering of the MOs . This mixing results in a decrease in energy for MO1 and MO2, while MO3 and MO6 increase. Hence 2σu and 1πu switch positions (b). This switch is the order that is observed for Li2 … N2, while O2 and F2 show the order in (a).
Mixing between 2s and 2pz AOs leads to a new arrangement of MOs (b).

17. Vibrational frequency of N2 = 2345 cm-1
One e- M.O
Ionic electronic state
PES peak (ev)
n’ = 0 to n’ = 1 of N2+
nN2+
(vibrational spacing, cm-1)
Nature of orbital
sg2p
2Sg
15.57
2150
Weakly bonding
Pu2p
2Pu
16.96
1810
Strongly Bonding
su2s
2Su
18.75
2385
Weakly Antibonding
Vibrational frequency of N2 = 2345 cm-1

18. MO diagram & Photoelectron spectrum of F2
1pg
1pg
1pu
3sg
1pu
3sg
1pg band shows vibrational fine structure
Increase in n F antibonding
Same orbital energy sequence as that of O2

19. CO is isoelectronic with N2
MO diagram & Photoelectron spectrum of CO
CO is isoelectronic with N2
PES is similar
orbital
PES peak (ev)
Nature of orbital 3s
13.98
Weakly bonding 1p
16.58
Strongly Bonding 2s
19.67
Weakly Antibonding

20. MO diagram & Photoelectron spectrum of HF

21. MO diagram & Photoelectron spectrum of NH3
3a ev
1e ev

22. MO diagram & Photoelectron spectrum of water 
1b1
3a1
1b2
orbital
PES peak (ev)
Nature of orbital
1b1
12.61
non bonding
3a1
14.70
Bonding
1b2
18.60

23. XPS (ESCA)
BE of core electrons of atom in sample BE of core electrons of atom in standard environment

24. [Co(NH2CH2CH2NH2)2(NO2)2]NO3 1:3
CH3CH2CH2COONa
[Co(NH2CH2CH2NH2)2(NO2)2]NO3
1:3
NH2
4:2:1
NO2
CH3CH2COONa
NO3
1:2
CH3COONa
1:1
HCOONa
C 1S XPS
CF3COOCH2CH3
CO : CH 1:0 IP IP

25. N 1S XPS of Na2N2O3 _ O
_ _
O = N – O – N – O
_ _
O – N = N- O – O
O = N – N _ O
I II III
XPS structurally non-equivalent nitrogen atoms
MO calculations & detailed BE considerations structure II O
R – S – S – R
R – S – S – R
O O O
I II Two sulphur core lines ,
so structure II

26. Elemental Analysis: atoms have valence and core electrons: Core-level Binding energies provide unique signature of elements.
Quantitative analysis: measure intensities, use standards or tables of sensitivity factor

27. When atom loses valence charge (Si0 --> Si4+ ) BE increases.
• When atom gains valence charge (O --> O--) BE decreases.

28. Auger Electron Spectroscopy
KL1L2 Process

29. Summary PES is a fairly new technique, continuing to develop
PES has unique features compared to other spectroscopies
Valence spectroscopy: information on bonding
Core spectroscopy: qualitative and quantitative analysis, “chemical shift”

30. References
International series of Monographs, Vol. 53: Photoelectron Spectroscopy,
D. Beckerand D. Betteridge
Physical Methods for Chemists,
Russell S. Drago
Electronic Absorption Spectroscopy and Related Techniques
D.N. Satyanarayana

31. THANK YOU