PHOTOELECTRON

SPECTROSCOPY
COORDINATION CHEMISTRY-II

EBSIBA BEAULA.J
Ebsiba Beaula [Date] [Course title]
II M.Sc., CHEMISTRY
 UNIT-V
Photoelectron Spectroscopy

Theory, Types, origin of fine structures - shapes of vibrational fine

structures – adiabatic and vertical transitions, PES of homonuclear

diatomic molecules (N2, O2) and heteronuclear diatomic molecules

(CO, HCl) and polyatomic molecules (H2O, CO2, CH4, NH3).

Koopman’s theorem- applications and limitations. Shake-up and

Shake-off process. Optical Rotatory Dispersion – Principle of CD,

MCD and ORD; Δ and λ isomers in different Cobalt (III)

complexes, Assignment of absolute configuration using CD and

ORD techniques.
 PHOTOELECTRON SPECTROSCOPY (PES)

Basic principle:
During the absorption of electromagnetic radiation (EMR) if the energy of EMR is greater
than the difference between initial electronic energy level and any higher energy level, the
electron will leave the atom or molecule and travel in free space with the velocity determined
by the energy difference between the initial energy level and the energy of the EMR.

K.E of the electron = hυ – B.E. (Ionisation Potential)

Binding Energy = hυ – Kinetic energy

If the energy of EMR is large enough all the electrons in an atom or molecule can be ejected.
The ejection of electrons from atoms or molecule by EMR in this way is known as ‘photo
electric effect’ and the technique which measures the energy of the ejected electron is known
as ‘photo electron spectroscopy’. The final state of this situation is not quantized and thus no
selection rule that restrict the possible transitions.

(PES technique is based on Einstein’s idea about the photoelectric effect, developed around
1905 (Photoemission was first detected by Hertz in 1887 and explained By Einstein in 1905).
In 1957 Dr. Siegbahn and his research group developed the XPS technique. In 1981, Dr.
Siegbahn was awarded the Nobel Prize in Physics for the development of the XPS technique.
The phenomenon of ejection of electrons from a metal surface (K, Rb, Cs etc.) when a light of
certain frequency strikes on its surface is called photoelectric effect.)
 The inner shells remain as atomic in nature due to insignificant overlaps and are
known as core orbitals. The 2s orbitals overlap to form bonding and antibonding molecular
orbitals of σ character and 2p orbitals overlap to form bonding and antibonding M.Os of both
σ and π characters. These orbitals that are molecular in character are known as valence orbitals.

TYPES OF PES

If the EMR in the UV range (hυ = 5 to 100 eV), say He(I) resonance line at 58.4 nm (21.21
eV) is used all the valence orbital electrons could be ejected to give photo electrons. These
photo electrons ejected will have vibrational fine structure associated the electronic states
involved. This type of PES is called ‘UV PES’ or ‘molecular PES’. From an analysis of the
kinetic energy and angular distribution of the photoelectrons, information on the electronic
structure (band structure) of the material under investigation can be extracted with surface
sensitivity.

If the EMR in the X-ray range (MgKα photon with energy of 1253.6 eV or AlKα photon
with energy of 1486.6 eV) are used all the core and valence orbital electrons could be ejected
to give photo electrons. This type of PES is called as ‘X-ray PES’ or Electron Spectroscopy for
Chemical Analysis (ESCA). When the sample is bombarded with electrons and the energy of
the ejected (other) electron is monitored the PES is called Auger Electron Spectroscopy (AES).
 DIFFERENT TYPES OF PHOTOELECTRON SPECTROSCOPY

S.NO TECHNIQUE MODE OF ELECTRON

GENERATION FROM SAMPLE
1. ELECTRON SPECTROSCOPY FOR CHEMICAL X-ray photons
ANALYSIS (ESCA)
2. X-RAY PHOTOELECTRON X-ray photons
SPECTROSCOPY(XPS)
3. AUGER ELECTRON SPECTROSCOPY(AES) Bombardment with electrons

4. ELECTRON IMPACT SPECTROSCOPY(EIS) Monoenergetic electrons

5. ION NEUTRALIZATION SPECTROSCOPY(INS) Bombardment with low energy ions

6. PENNING IONIZATION SPECTROSCOPY(PIS) Collision of gas phase excited atom

7. ULTRAVIOLET PHOTOELECTRON Vacuum UV radiation

SPECTROSCOPY (UVPES)
8. LASER MICROPROBE MASS Photons
SPECTROMETRY(LMMS)
9. ELECTRON MICROPROBE(EM) X-ray photons

ORIGIN OF VIBRATIONAL FINE STRUCTURES:

The fine structure in the UV-PES spectrum arises because the ionised molecule is left in
a vibrationally excited state after it is ionised. Some of the energies of the incoming photon is
used to excite the vibration, so less energy is available for the kinetic energy of the ejected
electron. Each quantum of vibration that is excited results in a corresponding reduction in
kinetic energy of the photoelectron.

As a result, the photoelectrons appear at a series of kinetic energies separated by the energy
of each vibrational excitation that has occurred.

As per Frank-Condon principle, the internuclear distances should not change during an
electronic transition is so fast, molecules do not get time to realign.

Due to this restriction, there are two types of transitions

 i.) Vertical transition will correspond to the transition of greatest probability and thus
greatest intensity. The binding energy in the PES is called as vertical ionisation
potential.
ii.) Adiabatic transition the transition from the ground vibrational state in the neutral
molecule the ground vibrational state of molecular ion is called as adiabatic
ionization potential.

SHAPES OF VIBRATIONAL FINE STRUCTURES

The vibrational fine structure observed in UV PES can give valuable information about the
type of energy level from which the electron is removed.

Nature of the molecular orbitals:

The internuclear distance does not change if the electron is ejected from nonbonding
orbital. The removal of bonding electron will weaken the bonding in the ion, and the
internuclear distance will increase (bond order decrease). The removal of antibonding electron
will strengthen the bonding in the ion and the internuclear distance will decrease (bond order
increase).

• Ionization from a non-bonding M.O. does not change bond length, so the vertical transition
is the adiabatic transition, M (v = 0) - M+ (v' = 0). Ionization from a non-bonding M.O. gives
a peak with little or no fine structure.
• Removal of moderately strongly bonding or antibonding electron results in a vertical
transition which intersects the PE curve for the ion at a point of only slightly changing slope,
and hence appreciable vibrational fine structure results.

• If the electron is removed from more strongly bonding or antibonding, the PE curve for the
ion moved more with respect to the neutral molecule. The vertical transition then intersects
the PE curve for the ion at points corresponding to increasingly large vibrational excitation in
the ion, since larger the vibrational excitation the closer packed are the vibrational energy
level and the smaller the change in the slope of the lines.

Pre-dissociation:

In UV PES, it is possible that the molecular ion produced

by photo ejection process pre dissociates. If the pre
dissociation of molecular ion occurs, part of the vibrational
fine structure of the photo electron spectrum will be lost.
Jahn-Teller Distortion:

If an electron is removed from a degenerate level, the resulting ion may initially also be
in degenerate state, subsequent distortion in the ion may lead to a removal of the degeneracy
and hence the splitting of the energy level. This is known as Renner-Teller effect for linear
molecule and Jahn-Teller effect for non-linear molecule.

The UV PE spectrum of non-linear molecules with degenerate

molecular orbitals shows irregular vibrational structure on bands
(vibrational levels that are irregularly spaced). In extreme
circumstances may exhibit double or even triple maxima. For e.g. if
an electron is removed from one of the 4 equivalent hybridized orbital
of methane, the resulting CH4+ is not tetrahedral. The remaining 3
orbitals are no longer degenerate and so each contributes separately to the spectrum producing
3 maxima.

UV-PES

In UV-PES electrons are ejected from the orbitals they occupy in molecules and their
energies determined. In this technique, a sample is irradiated with hard (high frequency) UV
radiation (typically 21.2 eV radiations from excited He atoms) and the kinetic energies of the
photoelectrons (the ejected electrons) are measured. Because a photon of frequency υ has
energy hυ, if it expels an electron with ionization energy I from the molecule, the kinetic energy
EK, of the photoelectron will be EK = hυ - l. The lower in energy the electron lies initially (that
is the more tightly it is bound in the molecule) the greater its ionization energy and hence the
lower its kinetic energy after it is ejected. Because the peaks in a PES correspond to the various
kinetic energies of photoelectron ejected from different orbitals of the molecule, the spectrum
gives a vivid portrayal of the molecular orbital energy level of a molecule.
UV-PES SPECTRUM OF HOMONUCLEAR DIATOMIC MOLECULES

1. N2 molecule

The UV-PES spectrum of N2 molecule irradiated with He (I) resonance line along with MO
diagram is given below
From the figure it is seen that the photoelectrons have a series of discrete ionization energies
close 15.6 eV, 16.7 eV, and 18.8 eV.

This pattern of energies strongly suggests a shell structure for the arrangement of electrons
in a molecule. All the values are close to, but greater than, the ionization energy of the atom
(14.5 eV). Because the ionization energy corresponds to the removal of a valence electron, the
shell structure suggests that, when a molecule forms, the valence electrons arrange themselves
into the molecular analogous of atomic shells. The electrons in these shells differ only slightly
in the strengths with which they are bound to the molecule. The lines correspond to the lowest
ionization energy in N2 (close to 15.6 eV) are due to photoelectrons ejected from the occupied
molecular orbital 3σg with the highest energy in the molecule (the orbital in which an electron
is bound most weakly).

The higher ionization energies of 16.7 eV (1πu) and 18.8 eV (2σu) must indicate the
presence of molecular orbital with successively lower energies (in which the electrons are
bound more strongly), and indicate the existence of a ladder-like (shell structure) array of
orbital energies in the molecule. There may also be more deeply lying orbitals, but the 21.2 eV
UV photons cannot eject electrons from them, so they are not observed in the photoelectron
spectrum.

The fine structure of each group of lines in the PES is a result of the ionized molecule
being left in a vibrationally excited state after it is ionized. Some of the energies of the incoming
photon is used to excite the vibration, so less energy is available for the kinetic energy of the
ejected electron. Each quantum of vibration that is excited results in a corresponding reduction
in kinetic energy of the photoelectron. As a result, the photoelectrons appear at a series of
kinetic energies separated by the energy of each vibrational excitation that has occurred.
 (i) For atomic no.8 or more than 8, e.g., O2, F2, etc.
σ1s < σ*1s < σ2s < σ*2s < σ2pz < (π2px = π2py) < (π*2px = π*2py) < σ*2pz
(ii) (ii) For atomic no. 7 or less than 7, e.g., B2, C2, N2, etc.
σ1s < σ*1s < σ2s < σ*2s < (π2px =π2py) < σ2pz < (π*2px = π*2py) < σ*2pz

The inner shells remain as atomic in nature due to insignificant overlaps and are known as
core orbitals. The 2s orbitals overlap to form bonding and antibonding molecular orbitals of σ
character and 2p orbitals overlap to form bonding and antibonding M.Os of both σ and π
characters. These orbitals that are molecular in character are known as valence orbitals. If the
M.O. diagram of simple molecules like N2, CO, O2 and NO are concerned, UV PES must give
3 peaks for N2 and CO and four peaks for O2 and NO (because the 2sσg and K shells are below
-21.21 eV).

The K.E. (binding energy) of these ejected electrons from various orbitals reflects, the
energy of various molecular orbitals. Thus, the order of various M.O. can be obtained from
PES. There may also be more deeply lying orbitals, but the 21.21 eV UV photons cannot eject
electrons from them, so they are not observed in the photoelectron spectrum. The detailed
structure of each group of lines in the PES is a result of the ionised molecule being left in a
vibrationally excited state after it is ionised.
 2. O2 molecule

The He(I) Resonance line PES of O2 molecule show 4 peaks corresponding to 2sσu, 2pσg,
2pᴨu, 2pᴨg molecular orbitals (or electrons). The energy of He (I) line is 21.21 eV, hence the
electrons in 2sϭg and K shell orbitals are not ejected. These 4 peaks (or bands) further show
vibrational fine structure because the ionization leaves the molecular ion (O 2 +) in different
vibrational levels (or states).

As per Frank-Condon principle, during electronic transition there should not be any change
in bond length (because the electronic transition is so fast the molecules do not get time to
realign themselves). Due to this restriction, there are two transitions possible.

i.) One corresponds to transition of molecule from ground vibrational level υo (of the
neutral molecule) to the ground vibrational level υo' of the molecular ion. This is
 called as adiabatic ionization and the potential is called adiabatic ionization
potential.
ii.) ii.) The other transition is from ground vibrational level to most probable vibrational
level of molecular ion, this is called vertical transition and the potential is vertical
ionization potential. The difference between the intensities of these two potentials
is the measure of Frank-Condon factors.

Show the PES of O2 molecule. How would you calculate the vibrational energy spacing of the
molecule from the spectrum? (Or) Discuss how PES is used to find the vibrational frequency
of diatomic molecule?

The He(I) Resonance line PES of O2 molecule show 4 peaks corresponding to 2sσu, 2pσg,
2pᴨu, 2pᴨg molecular orbitals (or electrons). The energy of He (I) line is 21.21 eV, hence the
electrons in 2sϭg and K shell orbitals are not ejected. These 4 peaks (or bands) further show
vibrational fine structure because the ionization leaves the molecular ion (O 2 + ) in different
vibrational levels (or states). As per Frank-Condon principle, during electronic transition there
should not be any change in bond length (because the electronic transition is so fast the
molecules do not get time to realign themselves). Due to this restriction, there are two
transitions possible. One corresponds to transition of molecule from ground vibrational level
υo (of the neutral molecule) to the ground vibrational level υo' of the molecular ion. This is
called as adiabatic ionization and the potential is called adiabatic ionization potential. The other
transition is from ground vibrational level to most probable vibrational level of molecular ion,
this is called vertical transition and the potential is vertical ionization potential. The difference
between the intensities of these two potentials is the measure of Frank-Condon factors.

At the temperature of experiment, none of the excited vibrational states of the molecule
are highly populated (5.74 ×10-4 molecule in for O2 molecule at N.T.P). Now let us calculate
the vibrational spacing of O2 molecule through PES.
 The spacing of the vibrational fine structure lines reflects the separations between the
vibrational level of the ion rather than those of the neutral molecule. Thus, observation of fine
structure leads to an approximate value for the vibrational frequency of the ion concerned.
Consider the electron which is ejected from outer orbital (i.e. band No I), the binding energy
of the line at highest intensity is 12.54 eV. The next line appears at 12.32 eV. The difference
between these 2 lines corresponds to vibration spacing of the oxygen molecule, which is, 0.22
eV. If it is converted to wave number 0.22 × 8065.5 cm-1 = 1774.41 cm-1 (1eV = 8065.5cm-1).
This is vibrational energy spacing in O2 molecule.
The second band in UPS spectrum of nitrogen shows vibrational progression with an interval
of 0.225 and the first band of oxygen has vibrational progression with an interval of 0.22 eV.
Deduce the nature of the molecular orbitals. (Given: N2 = 2345 cm-1 ; O2 = 1568 cm-1 ). The
vibration spacing of the second band of N2 molecule, 0.225 eV can be converted to wave
number, 0.225 × 8065.5 cm-1 = 1814.74 cm-1 (1 eV = 8065.5 cm-1). This is much lower than
the vibrational frequency of free N2 molecule (υN2 = 2345 cm-1 ). Hence the electrons are
ejected from bonding molecular orbitals. The vibration spacing of the first band of O2
molecule, 0.22 eV can be converted to wave number, 0.22 × 8065.5 cm-1 = 1774.41 cm-1 (1
eV = 8065.5 cm-1). This is much higher than the vibrational frequency of free O2 molecule,
i.e., υO2 = 1568 cm-1. Hence the electrons are ejected from antibonding molecular orbitals.

UV-PES SPECTRUM OF HETERONUCLEAR DIATOMICMOLECULES

i. CO molecule

The UPES spectrum of CO is shown below. CO molecule has a total of 10 valence electrons -
4 from the carbon and 6 from the oxygen – the outer eight of which occupy the molecular
orbitals 2σ2 2∏2 n2. Where, n2 represents a pair of electrons in a nonbonding orbital. This
structure is confirmed by PES as given below.

Three main bands appear in the spectrum at 14.0, 16.5 and 19.7 eV. The 14.0 eV band
has virtually no fine structure –only two peaks appear, one of them very weak. This peak arises
from removal of an electron from the nonbonding orbital. The band at 16.5eV with its
associated fine structure arises from the 2p∏ bonding orbital which is higher in energy (i.e.,
has a weaker binding energy) than the 2sϭ, the latter gives rise to the 19.7 eV peak.
Confirmation of this assignment can be made by measuring the vibrational spacing. The
vibrational spacing is 1549 cm-1 for the 16.5 eV band and 1706 cm-1 for the 19.7 eV band,
both of these values are significantly lower than that of the fundamental frequency of the free
molecule (CO, 2143 cm-1) indicating removal of an electron from bonding molecular orbital.
The spacing of the two lines for the 14.0 eV band is about 2200 cm-1, which is reasonably close
to the neutral molecule and confirms a nonbonding orbital.

ii. HCl molecule

UV-PES SPECTRUM OF POLYATOMIC MOLECULES

i. H2O molecule:

The appropriate combinations of atomic orbitals to give the most important molecular
orbitals of water are shown in figure.

• The oxygen 1s orbital is of too low in energy and of too small a radius to be involved in
molecular bonding to any extent. The deepest filled orbital in water is therefore basically pure
O 1s, containing two electrons.

The "O 1s" and the “O 2s/H 1s-bonding" bands would not be expected to be observed in the
UV-excited spectrum. Both the X-ray and UV-excited spectra are shown in Figures; fine
structure is clearly seen in the UV- excited spectrum. Analysis of the fine structure supports the
MO interpretation.

The IPs and eigenvalues (eV), calculated are illustrated in Table

• The first band, 1b1 (vertical and adiabatic IP = 12.61 eV) by its sharpness indicates ionization
from a weakly bonding orbital. The irregularity of the peak intensities suggests that at least two
vibrational series are present. Two series can be assigned with vibrational frequencies of 3200
± 50 and 1380 ± 50 cm-1 which compare with the symmetric stretching ʋ1 and the bending ʋ2,
modes of water, 3652 and 1595 cm-1. This suggests a basically non-bonding orbital, i.e. the
oxygen "lone pair".

• The second band 3a1 (vertical IP 14.7 eV adiabatic IP 13.7 eV) has pronounced fine structure.
The spacing between the peaks are regular (0.120 eV) and correspond to a vibrational
frequency of 975 ± 50 cm-1. This must be associated with the bending mode and indicates a
strongly bonding orbital. This orbital contains a high proportion of H-H bonding. Removal of
an electron from it would therefore be expected to modify the H-H separation, i.e. cause the
molecule to bend.

• The third band, 1b2 (vertical IP 18.55 eV, adiabatic IP 17.22 eV) has fine structure but it is
not so well resolved. There are clearly two vibrational series, in that each peak has the
appearance of a doublet and close analysis assigns values of 2990 ± 100 and 1610 ± 100 cm-1
to these. The first of these frequencies is much lower than that of ʋ1 in the molecular ground
state suggesting an orbital associated with strong O-H bonding. Ionization from this orbital
would also be expected to excite the bending mode and it is seen to do so. The orbital has H-H
antibonding character so that removal of an electron from it would be expected to reduce the
H-O-H bond angle and increase the vibrational frequency of the bending mode. This increase
would be opposed by the increase in the O-H separation resulting from removal of an electron
from an O-H bonding orbital. Since the frequency of the bending mode is almost the same in
molecule and ion, it is inferred that these opposing effects are almost equal.
 ii. CO2 molecule:

The photoelectron spectrum excited by helium resonance radiation (21.21 eV) shows
four bands for carbon dioxide. Photoelectrons resulting from ionization of the neutral
molecule, with electronic configuration (3σg) 2 (3σu) 2 (1πu) 4 (1πg) 4 , to one of the four
lowest electronic states of CO2 + can be clearly observed. The peaks appears at 13.778 eV,

17.314 eV, 18.077 eV, and 19.394 eV corresponding to the adiabatic ionization energy to the
2σg, 2σu, 2∑u + , and 2∑g + states.

There is little vibrational structure associated with the first band system, indicating that the
electron removed from an orbital, which is mainly non-bonding character (weakly bonding) in
the neutral molecule. The adiabatic first ionisation potential (13.778 eV) is only a little less
than the vertical ionisation potential of 13.85 eV. This is consistent with a very small change in
molecular dimensions on ionisation. The most prominent vibrational feature in the first
photoelectron band system is 0.157 eV (∼1270 cm-1 ) above the adiabatic ionization energy.

The harmonic frequency for the ground state of the neutral molecule is reported to be 1388
cm-1 . The 18.08 eV level also appears to be non-, or only weakly bonding in agreement with
spectroscopic data for the 2∑u + state of CO2 + which has an internuclear separation only
0.018 Å greater than that in the ground state of CO2. On the basis of the photoelectron
spectrum, the 19.29 eV level is also non-bonding so that these last two which are undoubtedly
σ-orbitals may better be ascribed to the oxygen “lone pair” electron than to C-O bond orbitals.
This would be confirmed if the two deepest levels proved to be more strongly C-O bonding.
(σg)2C-O (σu) 2 C-O (σg) 2 O (σu) 2 O (πu) 4 (πg) 4.
 iii. Methane

The structure of methane molecule is a tetrahedron, with the C atom in the centre and a H
atom at each corner. In this structure all the orbitals are degenerate. Therefore, the valence
orbitals should give rise to a single line in the PE spectrum. However, it is energetically
favourable to lift the degeneracy of the orbitals by a Jahn-Teller distortion of the molecule
which is responsible to the doublet structure in the spectrum. In CH4, if an electron is removed
from one of the 4 equivalent hybridised orbital of methane, the resulting CH4+ is not
tetrahedral.

CH4 (1a1) 2 (2a1) 2 (1t2) 6 , possesses a triply degenerate outer orbital, and the removal of
a t2 electron removes the degeneracy. The remaining 3 orbitals are no longer degenerate and
so each contributes separately to the spectrum producing 3 maxima. Three overlapping peaks
can be discerned, at approximately 13.6, 14.4 and 15.0 eV as indicated by arrows.

iv. NH3 molecule

The UV-PES spectrum and the MO diagram of NH3 molecule is given below
The photoelectron spectrum of NH3 using helium 21.1 eV radiation and the molecular
orbital energy level diagram of NH3 when the molecule has its observed bond angle
(10700) and bond lengths.

In the 21 eV radiation source NH3 shows 2 peaks one at 11 eV and the other at 16 eV. (The
third occupied orbital is out of the range of 21 eV radiation used to obtain the spectrum). The
PE spectrum is consistent with the need to accommodate 8 electrons (5 from N 2s 2 2p3 and 3
from 3 1s orbitals of H) in the orbitals. The electrons enter the molecular orbitals in increasing
order of energy starting with the orbitals of lowest energy. The overall ground state electronic
configuration is 1a2 1e4 2a2 . No antibonding orbitals are occupied, so the molecule has lower
energy than separated atoms. The HOMO is 2a1, which is largely confined to the N atom and
makes only a small contribution to the bonding. The lone pair plays a considerable role in
determining shapes of the molecules. Photo ejection of 2a1 electron removes the effectiveness
of the lone pair and the shape of the ionised molecule is considerably different from that of the
NH3 itself. Photoionization therefore results in extensive vibrational structure in the spectrum.
The structure indicates that the orbital from which the electron is ejected plays a considerable
role in the determination of the molecule’s shape. The broad band at 16eV arises from electrons
that are bound more tightly.
KOOPMAN’S THEOREM

Koopman’s theorem states that the vertical ionisation energy for removal of an electron
from a molecular orbital is equal to minus the corresponding Eigen value obtained from a
Hartree-Fock Self Consistent field molecular orbital calculation. (A stable orbital has a negative
Eigen value).

Koopman’s Theorem can be explained by taking PES spectrum of a gaseous sample of

N2. When He (I) resonance line is used as a source (ionizing limit is 21.21 eV) 3 vibrationally
structured photo ionizations are seen at ̴ 15.6, 17.0 and 18. 8 eV. These can be assigned to
ionization from the three highest energy filled M.O of N2, the 2pσg, 2p∏u and 2sσu orbitals.

The peaks have been assigned on the basis of the observed vibrational structure. (XPS
spectrum also has the same 3 peaks but vibrational structure not seen because of low resolution.
In addition to these 3 peaks XPS of N2 has a peak at 37.3 eV for ionization from the 2sσg level
as well as a single peak at 409.9 eV for both the 1sσg and 1sσu levels).
 M.O diagram of N2 molecule (1s σg 2 σu 2 2s σg 2 2s σu 2 2p∏u 4 2p σg 2 2p∏g 2pσu)

Photo electron data for gaseous nitrogen (energies in eV) vertical ionization energies

Reasonable agreement is seen between XPS and PES values; however, the Eigen values
from the Hartree - Fock Self Consistent field molecular orbital calculations do not agree with
the observed peaks. In fact, the order of σg 2p and ᴨu 2p is reversed. This could be indicative
of a difference in relaxation energies. Even when the calculations are modified by calculating
+
the energies for N2 and N2 in different states and taking a difference in total electronic
energies, only a fair agreement is found. Again, there is a reversal in ϭg 2p and ᴨu 2p. For this
example, Koopman’s Theorem gives only qualitative agreement.

Drawbacks of Koopman’s theorem:

(i) The basic assumption behind this theorem is that the molecular orbitals appropriate for
the parent molecule will be the same as those for the ionised molecule. If there is any
electronic relaxation (that is a change in the M. Os of an ionised molecule because of a
change in electronic repulsion) or if there is an appreciable change in correlation
energies then the Koopman’s theorem break down.
(ii) The Koopman's theorem approximation for valence orbitals will give the wrong
prediction of the ordering of the molecular orbitals when there are large differences in
relaxation energy between orbitals.
(iii) For core orbitals it will give chemical shifts that are difficult to compare when there
are large differences in relaxation energies for different compounds.

XPS or ESCA

If the EMR in the X-ray range are used all the core and valence orbital electrons could
be ejected to give photo electrons. This type of PES is called as ‘X-ray PES’ or Electron
Spectroscopy for Chemical Analysis (ESCA). EMR in the X-ray range

Mg Kα1 α2 = 0.99 nm (1253.6 eV)

Mo Kα1 α2 = 0.071 nm (17479 eV)

= 0.073 nm (17374 eV)

XPS is primarily used for chemical analysis as determining thicknesses and empirical
formulas of different elements and, binding energies, densities of electronic states. XPS detects
all elements with (Z) >3. It cannot detect H (Z = 1) or He (Z = 2) because the diameter of these
orbitals is so small (smaller orbital’s radius), reducing the catch probability to almost zero.
Analysis of organic compounds cannot be done with XPS because of degradation with
radiation.

XPS is used also determination of metals, ceramics, semiconductors, papers, etc. Detection
unit: ppt and some conditions ppm
ESCA Satellite peaks

A single ESCA peak would be expected for each electron level from which an electron is
ejected. In addition to these, additional peaks are observed in some spectra on the high binding
energy side of the core photo lines. They are called as satellite peaks. The study of these
satellites is of great importance in the elucidation of the nature of chemical bonding. Several
mechanisms can lead to satellite peaks.

Shake up process:

After an inner shell electron is ejected, the outer shell electrons are suddenly exposed to a
greater nuclear charge, because the amount of electron shielding has been decreased. The
sudden change in nuclear charge makes the ion to undergo further ionisation. If an outer shell
electron is excited to a higher orbital, a satellite peak is observed. This process is called as
electron shake up or monopole excitation.

Definition:

Simultaneous excitation of a valence shell electron to an unoccupied bound level and ejection
of an inner shell electron [Ne+ * (1s1 2s2 2p5 3p1)] is called as electron shake up process.

Figure illustrates the photoelectron spectrum of the 1s core level in neon gas with associated
shake-up and shake-off peaks. The two most intense peaks are for example due to shake-up
processes where a 2p electron has been 'shaken-up' to a 3p orbital . Satellite peaks may have
intensities of 50 to 25 % of main peak. During shake up process only the principal quantum
number changes, all others remain same. Shake-up processes result from transitions of valence
electrons to discrete and/or nondiscrete levels located close to EF. The resulting satellite peak
is observed at a K.E. which is less than that of the peak due to the ejection of inner shell
electron. K.E. of ejected electron affected by shake up or shake off process = hν – B – ΔE. ΔE
is the energy used in exciting or ionizing valence electrons. These processes result in the
introduction of either or both satellites that can be of measurable intensity asymmetries on the
main photoelectron peak.

Shake-up features can be useful in providing information on an element’s speciation. An

example of this is illustrated in Figure, in which spectra from CuO exhibits appreciable shake-
up satellites, whereas spectra from copper metal and Cu 2O do not.

This Figure shows the easy distinction between the two cobalt oxides by the
existence of satellites only on CoO peaks.
Shake off process

Shake off process is one in which a second electron is simultaneously ejected from the
valence shell of atom with the inner shell electron. Ne2+* [(1s1 2s2 2p5)]. (Shake-off processes
result when electrons are excited from valence levels into unbound continuum states located
above Evac). This process is similar to the Auger effect, but the kinetic energy of the ejected
inner shell electron is monitored rather than that of second electron. As a result, these
contribute to the broad background at higher B.E values with respect to the main photo-
electron peak (discrete shake-off peaks are rarely observed). Shake off peaks are
characterised by a broad continuum.

Figure illustrates the photoelectron spectrum of the 1s core level in neon gas with associated
shake-up and shake-off peaks. A broad continuum represents the shake off peaks. Satellite
peaks may have intensities of 50 to 25 % of main peak. Apart from being interesting in their
own right, the presence of shake-up and shake-off features may provide valuable chemical
information and information about relative energetic levels within the sample.
 OPTICAL ROTATORY DISPERSION

The rate of change of specific rotation with respect to wavelength is known as optical
rotatory dispersion (ORD). It is known that since the speed of light in a medium is manifested
in the refractive index of the medium, essential property of an optically active substance is that
it has different refractive indices for the left and right circularly polarized light, nL and nR,
respectively. This is also the reason why an optically active substance is said to be "circularly
birefringent." For ORD we commonly we use molar rotation [M] (unit: cm2 / dmol) which
is defined as:

[𝑀] =  𝑀 / 100 𝑙 𝑐

Where  is the observed rotation at wavelength  in degrees, l is the light path in decimeters,
c is the concentration of the optically active substance in grams per ml, and M is the molecular
weight in grams per mol.

Circular Birefringence

The difference in indices of refraction for right circularly polarized light (RCP) and left
circularly polarized light (LCP) is known as circular birefringence. Thus, on passing plane
polarized light (PPL) through optically active compound results in an unequal rate of
propagation of right and left circularly polarized rays due to circular birefringence. This
unequal rate of propagation for both right and left circularly polarized light deviate the PPL
from its original direction and it is called optical rotation. Upon emerging from the medium,
the resultant beam is still linearly polarized, with its plane of polarization having been rotated
by an amount α.

Optical rotation (α) =  𝑙 (𝑛𝐿−𝑛𝑅) / λ𝑣𝑎𝑐.

 Optical Rotatory Dispersion (ORD) and Circular Dichroism (CD) are very useful in
determining the absolute configuration of any complex. CD and ORD spectra can be used to
identify two enantiomers of a chiral complex. Compounds with similar structures give similar
ORD/CD spectra provided that they have the same absolute configuration. Molecules of
identical configuration should have the same Cotton effect for electronic transitions of the same
origin, whereas enantiomers will have mirror image Cotton effects.

Plain Curve : These curves don’t contain any peak or inflections (maximum or minimum) and
these don’t cross the zero rotation line.

Plain positive ORD : Plain positive ORD curves are obtained when specific rotation increases
with decreasing wavelength. In other words, clockwise rotation is plotted positively.

Plain negative ORD : Plain negative ORD curves are obtained when specific rotation decreases
with decreasing wavelength. In other words, counter clockwise rotation is plotted negatively.
 The Cotton effect consists of two extremes, a geometric maximum called a "peak" and a
geometric minimum called a "trough". The Cotton effect is called positive if the optical rotation
first increases as the wavelength decreases that means positive Cotton effect curve has its peak
in the longer wavelength region, while it is called negative if optical rotation first decreases as
the wavelength decreases. Negative Cotton effect curve is defined as having its trough
appearing at the longer wavelength. Optically pure enantiomers always display opposite Cotton
effect ORD curves of identical magnitude.

These are again divided into two categories;

(i) Single Cotton effect curves
(ii) Multiple Cotton effect curves
Single Cotton effect curves:
These are anomalous dispersion curves which show maximum and minimum both
occurring in the region of maximum absorption. So there appears only one peak and one
trough in the Single Cotton effect curves. The vertical distance between peak and trough is
called amplitude “a”. Amplitude is measured in degrees.

Molecular amplitude, 𝒂 = 𝟏−𝟐 /𝟏𝟎𝟎

Where 1 is molar rotation of trough or peak from shorter wavelength and 2 is the molar
rotation of extreme peak or trough from higher wavelength.

Multiple Cotton effect curves: These are a little different from the single Cotton effect
curves. They contain more than one peak and one trough.

CIRCULAR DICHROISM

If the absorption is different for right and left-handed circular -polarized light then the
linearly polarized light will become elliptically polarized. The elliptical polarized light
(Brown) is shown in figure, which is composed of unequal contributions of left (green) and
right (Red ) circular polarized light . The ellipticity (θ) of light is defined by the arc tangent of
the ratio of major axis to the minor axis of transmitted light.
 the major axis of the ellipse is parallel to the polarization plane of the original light wave.
This is always the case, regardless of which circular component is absorbed to greater extent
by the medium. But the direction of rotation of elliptically polarized light (or, more exactly, its
field vector) is determined by the circular component that remains stronger after traversing the
material. In this case, the field vector of elliptically polarized light rotates in the same direction
as that of the component shown in red colour, which traverses through the medium with
comparatively higher intensity.

The quantities in CD are Δɛ (the difference in molar absorptivity)

∆𝜀 = 𝜀𝐿 − 𝜀𝑅 ≠ 0

The angle of ellipticity (θ) is defined as: 𝜃 = 𝜋 (κ𝐿 − κ𝑅) / λ

And 𝑡𝑎𝑛𝜃 = 𝐸𝑅− 𝐸𝐿 / 𝐸𝑅+ 𝐸𝐿

Where, 𝜅 (kappa) is the absorption index, it is related to absorption coefficient K by equation

𝐾 = 4𝜋/ λ 𝜅

θ has the units in radians/cm and can be converted from radians to degree by multiplying by
180/.

𝜃 = 180(𝐾𝐿 − 𝐾𝑅) /4 𝜋

∆ɛ = Circular Dichroism

All optically active compounds exhibit CD in the region of appropriate absorption band. CD is
plotted as 𝜀𝐿−𝜀𝑅 vs λ. CD is measured as a quantity called mean residue ellipticity [], whose
units are degrees -cm2/dmol.
 MAGNETIC CIRCULAR DICHROISM (MCD)

Magnetic circular dichroism (MCD) is a phenomenon, where a difference in the absorption of

left- and right-circular polarized light (LCP & RCP) in a sample is occurring. This only happens
when a strong magnetic field is induced parallel to the direction of light wave. This effect
allows to observe very minor electron transitions of matter, which would go unnoticed with
traditional optical absorption measurements. This allows to observe and study materials
undergoing transitions between ground state and excited states, especially transition metals.

Theory of MCD

The fundamentals of MCD lie within Faraday-effect. The main principle of MCD is that
induced magnetic field affects the energy levels and thus electronic states in sample matter.
Ground state of matter is split by Zeeman-effect due to magnetic field. As the circularly
polarized light interacts with said split energy levels, it promotes an electronic transition
between these energy levels. The transition to excited state leads to an energy difference
between initial and final state in the material. This energy level difference affects the
wavelength of light than can be absorbed by the material. The electronic spin’s orientation
relative to the magnetic field determines which electron transitions are energetically favoured
and which transitions are unfavoured. This leads to difference in the absorption of LCP and
RCP.

This difference

is known as MCD-signal. It is often expressed as a function of wavenumber l, magnetic field

H, or temperature T. The peaks in MCD-spectrum usually correspond to spin-orbit coupling or
electronic transitions, explained by excited state of atoms.
ASSIGNMENT OF THE ABSOLUTE CONFIGURATION OF CHELATE (CHIRAL)
COMPLEXES USING ORD AND CD TECHNIQUES

Optical Rotatory Dispersion (ORD) and Circular Dichroism (CD) are very useful in
determining the absolute configuration of any complex. CD and ORD spectra can be used to
identify two enantiomers of a chiral complex. Compounds with similar structures give similar
ORD/CD spectra provided that they have the same absolute configuration. Molecules of
identical configuration should have the same Cotton effect for electronic transitions of the same
origin, whereas enantiomers will have mirror image Cotton effects.

Assignment of the Absolute Configuration using ORD

Optical Rotatory Dispersion (ORD) involves measuring the variation of optical rotation
with wavelength. There is abrupt reversal of rotation in the vicinity of absorption band. If the
complex is laevorotatory, the ORD curve falls to a minimum, rises rapidly to a maximum and
then slowly falls (positive Cotton effects). If the complex is dextrorotatory, the effect is
reversed for the ORD curve, rising first to a maximum, then falls rapidly to a minimum and
then slowly rise (negative Cotton effect)

ORD curves are useful in the assignment of the absolute configurations. For example,
configurations of the enantiomers of tris(ethylenediamine) Cobalt (III), tris(alaninato) Cobalt
(III) and bis(ethylenediamine)glutamate Cobalt (III) are known from X-ray diffraction
technique. It is found that three - (+)-enantiomers or -(D)-enantiomers of these complexes
have similar ORD spectra. On the basis of -configuration, these complexes can be assigned
to any known configuration in the absence of X-ray data simply on the basis of the similarity
of ORD spectra.
For ORD spectrum to be instantly recognizable, no other absorption must be nearby. The ORD
spectra of -[Co(en)3] 3+
, -[Co(S-ala)3] 3+, and -[Co(en)2(S-glu)] 3+
are shown. All these
complexes represent positive Cotton effect. (Where en refers to ethylenediamine, S-ala refers
to the anion of S-(L)-alanine and S-glu refers to the dianion of S-(L)-glutamic acid. All of these
complexes have the  or D configuration.)

Although ORD was used extensively at one time because of simpler instrumentation, circular
dichroism (CD) is currently much more useful.

Assignment of the Absolute Configuration using CD

The circular dichroism (CD) refers to differential absorption of left and right circularly
polarized light, εL-εR. Complexes having same sign of CD for a given absorption band will
3+
have the same absolute configuration. The absolute configuration of [Co(en)3] had to be
determined first for assignment of the absolute configuration of other related complexes. The
absorption and CD spectrum of [Co(en)3] 3+ is shown The CD spectrum of [Co(en) 3] 3+ suggest
that - [Co(en)3] 3+ isomer shows positive deflections, however -[Co(en)3] 3+ isomer shows
negative deflections in CD spectrum which also corresponds to the sign of Cotton effect. The
CD spectrum of [Co(en)3] 3+ also shows the presence of another absorption band at ~24×10-3
cm-1 that is not obvious from the absorption spectrum.
∆ &  ISOMERS OF Co(III) COMPLEXES.

Tris(aminoacido)cobalt complexes were prepared using amino acids. CD spectra of

these complexes illustrate the application of circular dichroism for the determination of
configuration. If three glycinate ligands are bound to a metal ion, two diastereomers: fac and
mer are possible and each of these will have an enantiomer.

if a substituted amino acid such as alanine (R-alanine or S-alanine) is used, complex can
still exist as two geometrical isomers both of which are chiral, i.e. facial and meridonial. The
arrangement of ligands about metal ion can result in  or  chirality.
However, right-handed facial isomer is not enantiomeric with the left-handed facial isomer. For
example, the use of S-alanine can give the facial  has isomer shown. A mirror image of this
compound cannot be generated with S-alanine because the enantiomer necessarily contains R-
alanine. Therefore, facial ∆ isomer is a diastereomer of fac . This is the same case with mer
isomers.

The net result is that, if glycine is used while fac and mer isomers have an enantiomeric
partner, however, an optically active amino acid will give a diastereoisomer for each chirality
of each fac or mer isomer. All the four isomers are diastereomers, easily separable from one
another under achiral conditions. Each one of them is having different physical properties.

The fac and mer isomers show distinct spectroscopic properties from one another. The fac
isomers give rise to similar UV-visible spectra. similarly, mer isomers also give similar UV-
visible spectra. The absolute configuration of the optically pure alanine complexes can be
assigned by comparing their CD spectra to those for optically pure [Co(en)3] 3+. The absorption
and CD spectra for the four diastereomers of [Co(S-alaninate)3] are shown in figure. The sign
of the Cotton effect for the lowest energy CD band of the fac-(+) and mer-(+) isomers is positive
just as it is for -(+)-[Co(en)3 ] 3+. This result strongly suggests that these complexes have the
same absolute configuration, .

The negative sign of the Cotton effect for the lowest energy CD band of the fac-(-) and
mer-(-) isomers suggests that they have an absolute configuration opposite to that for the (+)-
isomers. This result suggests that these complexes have the same absolute configuration, . It
should be noted that CD curves for the (+) and (-) isomers are not exactly mirror images of one
another. This is due to the fact that, while complexes have opposite absolute configurations
based on the positioning of chelate rings, they are in fact diastereomers as mentioned above,
this arises because only S-alaninate was used as a ligand.

REFERENCES:

1. Asim K Das and Mahua Das, Fundamental concepts of inorganic chemistry,1st eBook
edition, Volume 4, CBS publishers and distributors PVT Ltd,2019.

2. B.R.Puri,L.R.Sharma and K.C.Kalia,Principles of inorganic chemistry,Vishal

publications,33rd edition,2016.

3.http://epgp.inflibnet.ac.in

4. James E.Huheey, Inorganic Chemistry- Principles of Structure and Reactivity, Vth Edition,
Harper Collins College Publishers, 1993.

5. Nagao Kobayashi, Atsuya Muranaka, John Mack, Circular Dichroism and Magnetic Circular
Dichroism Spectroscopy for Organic Chemists - Royal Society of Chemistry, 2011.

6. Johanna M Wagner, X-ray photoelectron spectroscopy, Nova Science Publishers, Inc.,2011.

7. Straughen and Walker, photoelectron spectroscopy.

8. https://youtu.be/6QfGNGXLs6Y?feature=shared
 QUESTION BANK

OBJECTIVE TYPE QUESTIONS:

1. In the ESCA process, which of the following photon is ejected? APR-2019

1P e-
2S e-
1S e-
2P e-
2. In PES of CO2, the vibrational spacing of 13.78eV is 1270cm-1. What is the nature of
molecular orbital? (In free CO2 molecule, VCO2 =1388 cm-1.) APR-2019
Bonding molecular orbital
Antibonding molecular orbitals
Non-bonding molecular orbitals
None of the above
3. The three maxima observed in the first band of photoelectron spectroscopy spectra of
CH4 at 13.6, 14.4, 15 eV are due to APR-2019
Pre dissociation
Jahn-teller distortion
Ejection of e- from Bonding molecular orbitals
None of the above
4. The energy of He(I) is NOV-2022
21.21eV
21.21eV
40.2 eV
1457 eV
5. ESCA gives sufficient chemical information upto a depth about____ Å in metals.
5-20
15-40
40-100
100-200
 6. ESCA is concerned with the measurement of _______ energy APR-2023
5-20
15-40
40-100
100-200
7. Energy required to remove an electron an electron from the highest occupied atomic
orbital is
Ionization potential
Kinetic energy
Binding energy
Vibrational energy
8. The no. of. P.E.S peaks given by NH3
1
2
3
4
9. The P.E.S of H2O ____ peaks
3
2
1
4

4 & 8 MRK QUESTIONS:

1. Discuss the principle of photoelectron spectroscopy. Explain with an example APR-

2023

2. What is Koopmans’ theorem? what are its drawbacks? Explain with an example. (NOV-
2019, 2023; APR-2019, 2023)

3. Elucidate the absolute configuration of chelate complexes with the help of chelate
complexes with the help of ORD & CD
4. Discuss the UV-PES OF N2 molecule.

5. Discuss the UV-PES OF NH3 molecule and CO2 .

6. Show the PES of O2 molecule. How would you calculate the vibrational energy spacing
of the molecule from the spectrum? (Or) Discuss how PES is used to find the vibrational
frequency of diatomic molecule? The He(I) Resonance line PES of O2 molecule show
4 peaks corresponding to 2sσu, 2pσg, 2pᴨu, 2pᴨg molecular orbitals (or electrons). The
energy of He (I) line is 21.21 eV, hence the electrons in 2sϭg and K shell orbitals are
not ejected.
7. The second band in UPS spectrum of nitrogen shows vibrational progression with an
interval of 0.225 and the first band of oxygen has vibrational progression with an
interval of 0.22 eV. Deduce the nature of the molecular orbitals. (Given: N2 = 2345 cm-
1 ; O2 = 1568 cm-1 ).