1. SAMRAT PRITHVIRAJ CHAUHAN
GOVT. COLLEGE ,AJMER
MSc. CHEMISTRY SEMINAR PRESENTATION SEMESTER 2
X ray Photoemission spectroscopy
Presented by :-
Kanika Khandelwal
MSc. Chemistry Semester 2
2. contents
1. Introduction
2. Basic Principle Of Photoemission
spectroscopy(P.E.S)
3. Photoelectric Effect & P. E. S.
4. P.E.S & Koopmann’s Theorem
5. Principle Of XPS /ESCA
6. Applications Of XPS
3. INTRODUCTION
•Photoelectron spectroscopy (PES) is an experimental
technique used to determine the relative energies of electrons
in atoms and molecules.
•Photoelectron spectrometers work by ionizing samples using
high-energy radiation (such as UV or x-rays) and then
measuring the kinetic energies (KE) of the ejected electrons.
•Each electron is held in place by nucleus with characteristic
binding energy.
• A PES spectrum is obtained which is graph of photoelectron
count vs. binding energy.
4. BASIC PRINCIPLE OF P.E.S
Photoelectron spectroscopy is based on the phenomenon of Photoelectric effect ,which was first
characterized by Albert Einstein.
“When electrons in a metal are exposed to light of sufficient radiation, the electrons are ejected
from the metal surface. If we know the kinetic energy of the ejected electrons (known
as photoelectrons) and the energy of the incident radiation, we can calculate the energy of the
electrons in the solid metal”
5. When an atom/ molecule is subjected to high energy radiations , photons
present in the radiations collide with &eject electrons from atoms , leaving ions
behind .
These departed electrons have different velocities & PES help in determining
these velocity distribution of released electrons.
Each electron is held to nucleus with characterisitic binding energy.
If the energy of photon imparted to electron is greater then binding energy , the
electron will leave the atom with the excess energy
hv = binding energy + kinetic energy
Depending on the energy of exciting radiations , electron may be ejected from
the core or the valence shell of atom.
6. KOOPMANN’S THEOREM & P.E.S
According to PES equation i.e, “hv =1/2 mv2 + I” , each photoelectron originate from one
/different orbitals with different ionisation energy (I) therefore series of different kinetic energy of
photoelectron obtained each one satisfying equation
“hv =1/2 mv2 + Ii”
Here Ii =Ionisation energy for ejection of electron from orbital I
According to Koopmann’s theorem we can calculate ionisation energy with energy of orbital from
which it is ejected as it assumes that ionisation energy of each orbital is negative of the orbital
energy of ejected electron.
“IE = -Ei”
This theorem is but just an assumption & it does not comsider the fact that electron adjust there
distribution when ionisation occurs
7. PRINCIPLE OF XPS
X Ray Photoelectron spectroscopy (XPS) , here electron is ejected from an atom /molecule
following irradiations of x rays.
The ejection of electrons takes place from core energy levels.
XPS probes the binding energy of core electrons in an atom , though the core electron does not
play any significant role in chemical bonding however different chemical environment can
induce small changes in their binding energy because of the fact that formation of bond
changes distribution of electrons in the system and hence by modifying nuclear shielding ,
produces changes in the effective nuclear charge of the bound atoms.
XPS spectrum is always plotted between electron count and binding energy.
8. Here x ray beam of energy hv displaces an electron
from k orbital (Eb).
This process can be written as
A +hv = A+* + electron
Eb ,Eb’ ,Eb’’-energy of inner shell K &l
electrons of atom.
Ev, Ev’ , Ev’’ – energy of valence shell
electrons of atom
Incident beam :- X Rays photons
Emitted beam :-Electrons
Here A= atom , A* = Excited ion
Energy relation in photoelectron by x rays can be
expressed as
Eb=hv – (Ek + C)
here hv =Energy of X ray photon
Ek = kinetic energy
Eb = binding energy of an electron
c = Work function
Eb is specific for an electron in a given element hence
can act for identification of that element.
Eb significantly affects oxidation state since it changes
effective force field of nucleus.
9. XPS SPECTRUM
This spectrum is plotted between counting rate &
binding energy.
This curve shows that the analyte consist of
organic compound of 6 elements & each element
has its well specified peak except Hydrogen.
A peak for O is also present indicates oxidation on
surface of compound .
Eb for 1s orbital increase with atomic number due
to increase positive charge of nucleus.
More than one peak for an element can also be
observed Eg :- for both 2s / 2p electrons for S,P .
Hence XPS provides an excellent means of
qualitative identification of elements present on
surface of solid .
10. XPS SPECTRUM OF NO2 MOLECULE
This NO2 molecule is linear ,N-N-O & so
central nitrogen experiences great reduction
of electron density.
Hence more strongly bond core electrons
,from the electronegative Oxygen atom than
does the end one and hence core N
experience increased nuclear force &
consequently have higher binding energy.
This process is known as Chemical shift.
Thus central atom is assigned to 412.6 Ev
peak & the end atom to the 408.7 Ev peak.
11. INSTRUMENTATION OF XPS
The electron spectrometer for XPS / ESCA must contain the following components :-
Source Of Radiation:-
1.X ray tubes equipped with Mg or Al target &
suitable filters , here these light metals are taken as
they produce narrow wavelength bands of the light
(0.8 -0.9 eV) and hence an enhanced resolution.
2. Crystal monochromator :-They provide X ray
beam having bandwidth of about 0.3 eV. These
monochromators eliminate brehmsstrahlung
background & improve signal / noise ratio.
12. 2. A sample holder:- They are mounted in fixed position as close to photon or electron source &
entrance slit of spectrometer as possible.
To prevent attenuation of electron beam , the sample must be evacuated to pressure of 10-5
Torr or lesser .
Vaccums are required to prevent contamination of sample.
Gas samples are leaked into sample areas to slit of size as to provide a pressure of 1/100 torr .
If sample pressure is too low weak signals are obtained.
3. Energy analyzer (monochromator):- This is kept between sample & the detector & has more
stringent efficiency requirements then filters because the electron beams is order of magnitude
less intense.
Retarding field analyzer : used but geometric restriction are required so that electrons moves
normal to the grid.
Magnetic deflection analyzer : effective but less convenient then electrostatic analyzer.
Most monochromators use either cylindrical or spherical electrostatic field or hemispherical
type where electron beams deflect by electrostatic magnetic field in curve manner
13. Electron beam excitation can also happen by heated
cathode , accelerated by an electric field , they are
fairly homogeneous but have some spread because
they are emitted from cathode ,with various kinetic
energies .
To obtain greater degree of homogeneity , filters must
be applied like as shown in the figure,
A) Electrons subjected to retarding field between 2 grids
& only those electrons with sufficient energy could
overcome the field.
B) Here the electron beam enters at 45 degree angle &
only those electrons with specific narrow energy band
will emerge through 2nd slit .
C) Here the electrons enters the filter at slightly divergent
angles will be focused on an exit slid.
D) This use 180 degree hemisphere as electrodes &hence
large fraction of electron beam can be accepted.
14. 4.Detector :- Electron multiplier are mostly use as a detector .
There are various varieties of electron multipliers such as
Channel electron multiplier :- constructed from small curved glass tube , Its inner surface
is coated with high conductive material connected to a source of 2 -3 kV that act as a
combination of diode & resistive voltage divider .
This channel electron multiplier accepts electrons at one end & emits more at the other
thus acting as a current amplifier.
Nowadays these multipliers are doped with vanadium or lead .
When potential of several kilo volts of applied across material a pulse of several
thousands of electrons.
15. 5. High Vaccum System :-
Electron spectrometers require elaborate vaccum
system to reduce pressure in all components.
This has either oil diffusion pump or ion pump backed
up by force pump .
In few instruments specially for gaseous samples , 2
sets of pumps are required so pressure in the analyzer
can be held as low as possible.
The electron spectrometer must be magnetically
isolated since it distorts the electron trajectories.
This can be done by enclosing the sensitive region in
shigh permeablity ferromagnetic material or by using
Helmontz coil
16. APPLICATIONS OF XPS
-
A)Determination of elemental composition :-
The elemental composition of a compound can be determined by X Ray spectrum.
With Mg & Al source ,all elements (except H & He ) emit core electrons having characteristic binding
energy.
This spectrum encompasses the kinetic energy range of 250 – 1500 eV corresponds to binding energy
of about 0 to 1250 eV.
Every element has 1/more energy level peaks in this spectrum.
The peaks are unambiguous for each element and hence leads to unambiguous identification.
Sometimes peaks might overlap but such peaks could be identified by comparing spectra of 2 X ray
sources.
17. B) Chemical shift and oxidation state :
when x ray spectrum peaks are observed under high resolution , it has been found that
variations in number of valence electrons & the types of bonds formed by them influence binding
energy of core electrons.
As oxidation state becomes more positive , binding energy increases.
.
The chemical shift can be explained as the attraction of nucleus for core electron decreases
by the presence of outer electrons.
As a result of removal of these electrons ,effective charge increases and hence the binding
energy increases
18. C) Chemical shifts and structure:-
In this spectrum , each peak
corresponds to 1s electron of C atom
located just above its in the structural
formula.
Here F atom is most electronegative
and thus withdraw maximum electrons
from C atom towards itself.
Thus effective nuclear charge
experience by C atom increase , &
hence the binding energy.
19. This spectrum not only provide qualitative information
about types of atoms present in compounds but also
give relative number of each.
Eg :- NaN3 (sodium azide)
This curve has 2 peaks having relative area in the
ratio 2:1 , corresponds to 2 end nitrogen & 1 central
one .
It is true that core I.E are not affected by the type of
bond formation but here although the spectrum lies in
region of 400 e V which is unlike 1s electron of N
atom.
The presence of –ve charge on the terminal atom
decrease the core I.E but positive charge on central
atom increases it.
Thus results in 2 peaks of spectrum of ratio of area
2:1 .
