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X-Ray Photoelectron Spectroscopy (XPS)
Bushra Naveed MS Physics Page 1
X-RAY PHOTOELECTRON
SPECTROSCOPY (XPS)
Submitted To: Submitted By:
Dr. M. Tanvir Hussain
Assistant Professor
School of Science
Bushra Naveed
MS Physics
Section (A)
School of Science
University of Management and Technology
Lahore

ABSTRACT:
X ray photoelectron spectroscopy is derived from three words which are x ray, photo
electron and spectroscopy.
X ray means to excite a particular material then photoelectron means to utilize the effect
of photoelectron from the surface and then we achieve a spectrum to eventually overall
achieve the spectrum. With this technique it is possible to analyze the chemical
composition of the particular material surfaces up to few nm depths. More importantly,
we have explained the importance of the using complimentary surface analysis
techniques to investigate the valence state of ceria nanoparticles.
Keywords: X-ray photoelectron spectroscopy; photoelectric effect

INTRODUCTION:
X-ray photoelectron spectroscopy (XPS or ESCA) of course owes its quantiﬁcation to
Einstein’s explanation of the photoelectric effect in 1905 and the technique in fact has a
long history and that can be traced to contemporary measurements in which either X-
rays or gamma rays were used to excite the photoelectrons from solids. In the period
since the late 1950s, the photoelectric effect has been developed into one of our most
powerful tools for studying the composition and electronic structure of matter, with Kai
Siegbahn receiving the Nobel Prize in 1981 for the development of high- resolution XPS.
Einstein recognized that when light is incident on a sample, an electron can absorb a
photon and escape from the material with a maximum kinetic energy
Ek=hν-EB –eФ
Where ν is the photon frequency, EB electron binding energy and Ф work function,
which gives the minimum energy through photon or x rays required to remove an
electron from the surface of the metal. The work function is a measure of the potential
barrier at the surface that prevents the valence electrons from escaping. Here we will
show how the photoelectric effect can be used to explore chemical composition and
quantum states of material surface which we use.
His group’s early pioneering work is documented in the two well-known ESCA books
with many other reviews and overviews appearing later [e.g.]There has been much
progress in the intervening decades, and new modes of measurement and more precise
theoretical interpretation methodologies continue to be developed, with many of these
being discussed in the other articles in this issue.

Figure 1: An example of a photoelectron emitted due an incident photon.
in this newsletter, i'm able to make quick connection with the records of diverse
measurement modes and consequences, but awareness in general on a number of the
maximum recent trends, pointing to greater targeted discussions somewhere else as
appropriate, and trying in some instances to invest on destiny interesting instructions that
have yet to be exploited. i can additionally recognition on measurements of condensed
count phases (solids, surfaces, interfaces, and to some degree liquids the maximum
thrilling area for basic and applied scientists the usage of xps. as a handy operational
deﬁnition of xps, i will recall excitation energies above some hundred ev and going into
the hard x-ray regime up to fifteen kev. for that reason, each middle degrees and valence
degrees are easily observable in spectra. the subjects considered will reﬂect to a sure
degree my own personal biases, however, together with the other articles on this
difficulty, i believe that the reader can have get admission to to evaluation of the cutting-
edge repute of xps, in addition to of a number of the most thrilling guidelines of xps for
its future.

PRINCIPLE OF XPS
Surface of the specimen is irradiated by X-ray with the energy of hν. Mono-energetic
photon knocks out the electron from atoms in the surface region. Photons with higher
energy hν penetrate deeper into the samples surface. Electrons emitted by photons from
lower energy X-rays originates from inner atomic energy levels and were bound to
atomic nuclei with the binding energy Eb. Electrons emitted in this manner are referred to
as photoelectrons. The photoelectric effect generates free electrons with a certain kinetic
energy Ek.
As we will see in detail throughout the paper, due to the complexity of the photoemission
process in solids the quantitative analysis of the experimental data is often performed
under the assumption of the independent particle picture and of the instantaneous more or
less (i.e., disregarding the many-body interactions as well as the moderation of the system
during the photoemission itself). The problem is further simplified within the so-called
three-step model, in which the photoemission event is decomposed in three independent
steps: photo excitation of an electron in the solid, propagation of the excited electron to
the surface, and escape of the photoelectron from the solid into vacuum after transmission
through the surface potential barrier. However, from the quantum mechanical point of
view photoemission should not be explained in terms of several independent events but
rather as a one-step process. The photoemission process should be expressed in terms of
an optical transition between initial and final states consisting of many- body wave
functions that obey appropriate boundary conditions at the surface of the solid.

Figure 2: Photoelectron spectrum of lead showing the manner in whic
h electron escaping for the solid can contribute to discrete peaks or suffer energy loss and
contribute to background; the spectrum is superimposed on a schematic of an electronic
structure of lead.
A general xps spectrum is a plot of the variety of electrons detected versus their kinetic
strength. this spectrum offers us the records approximately electron electricity
distribution in a fabric. in real data accumulation, the reference factor for solids is taken
to be fermi power as “natural” zero. each element produces a feature set of xps peaks on
the binding power values that immediately perceive every detail that exist in or at the
surface of the material is being analyzed. those feature peaks correspond to the electron
configuration of the electrons inside the atoms, e.g., 1s, 2s, 2p, 3s, etc. (discern 2).
the measured spectrum of electron kinetic energies ek is a superposition of the primary
electrons with a binding energy eb. within the non-interacting electron photo, it's miles
specially straightforward to take benefit of the electricity conservation law and relate the
kinetic electricity of the photoelectron to the binding electricity eb of the electronic-
nation inside the strong:
Ek =h ν− Eb −e Φ0.
Measurement of core-level binding energies for gas phase molecules is generally
accomplished by measuring the kinetic energy of photoelectron relative to a standard
species known ionization potential. Unfortunately, no such standards are available for the
solids. A conventional approach for metal is to reference EB to its Fermi level. If the
metal sample and spectrometer are in electrical contact, the Fermi levels must be at equal
energies. The potential energies that the photoelectron experiences, depend on the

difference between sample work function Фs and the work function of the spectrometer
energy analyzer Ф0. The Ф0 - Фs term is often referred as “contact potential” between
sample and analyzer. The measured EB referenced to Fermi level depends only on Ф0. If
EB were reported with respect to vacuum level, then both Фs and Ф0 must be known.
Since Фs can vary by several eV, depending on the presence of impurities, EB referenced
to Fermi level is clearly better choice.
The intensity of electrons Id emitted from all depths greater than d in a direction normal
to the surface is then given by the Beer-Lambert relationship.
I d= I ∞ exp (−d / λ sinθ)
where I∞ is the intensity from an infinitely thick material
Sample. Only those electrons that originate within tens of Angstrom’s below the solid
surfaces can leave the surface without energy loss and contribute to the peaks in the
spectra. The remaining electrons that undergo inelastic processes suffer energy loss and
increase the spectrum background or contribute to secondary emission. This is the main
cause for high surface sensitivity of X ray photoelectron spectroscopy.
1. SOME BASIC CONSIDERATIONS:
Fig. 1 explains in a schematic way some of the most important aspects of the XPS
experiment, including some new aspects of development. These will be discussed in
subsequent paragraphs.
As an additional important starting point for quantiﬁcation, the fundamental energy
conservation equation in photoemission is the following [5–8]:
Hv = E Vacuum + E∗ kinetic + V charge + V bias= E binding + ϕ spectrometer + E
kinetic + V charge + V bias

In which h is Planck’s constant; v is the photon frequency; E Vacuum is the binding
electricity of a given electron relative to the vacuum level of the pattern; e kinetic is the
kinetic electricity of the emitted electron simply because it leaves the sample; e kinetic is
the kinetic power as measured finally within the spectrometer, which may be special from
e kinetic by using a small touch potential difference if the pattern is a solid; e fermi is
the binding strength relative to the fermi level or electron chemical ability; ϕ
spectrometer is the work function of the spectrometer used to degree kinetic energy, v
price is a probable charging ability on the sample that can increase if the emitted
photoelectron and secondary electron present day is not fully compensated through flow
from the sample ground, and v bias is a time-based bias capability that can be located
between the sample and the spectrometer, right here with sign such that a fantastic bias
acts to lower the speed of the photoelectrons.
Fig.3. Illustration of a typical experimental configuration for X-ray photoelectron
spectroscopy experiments, together with the various types of measurements possible,

including (a) simple spectra or energy distribution curves, (b) core-level photoelectron
diffraction, (c) valence-band mapping or binding energy vs. k˙ plots, (d) spin-resolved
spectra, (e) exciting with incident X-rays such that there is total reﬂection and/or a
standing wave in the sample, (f) using much higher photon energies than have been
typical in the past, (g) taking advantage of space and/or time resolution, and (h)
surrounding the sample with high ambient sample pressures of several torr
XPS SPECTROMETER
as cited before the whole instrumentation must be in ultra-excessive vacuum (10−9 mbar)
chamber. the pattern is determined thru a prechamber that is in contact with the outside of
the environment. this prechamber is closed and pumped to low vacuum. after it's miles at
low vacuum the pattern is further delivered into the main chamber in which a uhv
(extremely-high vacuum) environment exists .the surface of the sample is irradiated with
the beam of x-rays.
The source of X-rays photons used for irradiation of solid samples is standard X-ray tube
.A heated filament provides electrons witch are accelerated to potential of between
Figure 4: The picture of the XPS spectrometer: (1) X-ray source, (2) monocromator,

(3) UHV chamber, (4) hemispherical mirror analyzer.
10 and 20 keV toward a water cooled solid anode. The electrons create core holes in the
anode atoms, which are filled by relaxing electrons from higher levels. The relaxation
process is then accompanied by X-ray fluorescence. The anode material defines the
energy of radiation. In XPS instruments normally Al and Mg anodes are used because of
a dominant, strong resonance in the X-ray spectrum. For the Al X-ray, a doublet arises
from the 2p1/2, 2p3/2 →1s electronic relaxation. These are so called Kα1,2 lines. The
energies of X-rays produced by few metals are tabulated in Table 1. In the Table 1 we
notice that the energy and beam natural width (FWHM) of the lines increase with atomic
number.
Table 1: A few common anode materials used for X-ray source
x-rays can be productively monochomatized through mirrored image from bent quartz
crystal to generate, so called, monochromatic x-ray resources. one of the advantages in
using monochromatic x-rays is that the beam natural width is slim as compared to the
unfiltered x-ray line and therefore improves the resolution of the photoelectric peaks in
the xps spectrum. a further final results of filtering the x-rays previous to irradiating the
sample is that minor resonance traces inside the x-ray spectrum are removed from the
excitation method. if unfiltered, those minor x-ray traces generate additional satellite

peaks so referred to as "ghost" peaks in x ray photoelectron spectrum and those appear at
kinetic energies characteristic of the energy detachment between the primary X-ray lines.
The photoelectrons from material excited by X-ray beam travel through electrostatic
transfer lens to electrostatic hemispherical mirror analyzer .A standard hemispherical
analyzer consists of a multi-element electrostatic input lens, a hemispherical deflector
with approach and exit slits, and an electron detector (i.e., a channeltron or a multi-
channel detector)
Figure 6: Hemispherical mirror analyzer
Alternatively, an electron energy distribution may be obtained by variation of the pass
energy, keeping the ratio between the pass energy and the retardation voltage constant.
This is done in the constant retard ratio mode. Since the spatial divergence of the electron
trajectories in the analyzer increases with decreasing pass energy, the energy resolution in
the constant retard ratio mode is proportional to the detected energy, whereas in the
constanta nalyzer energy mode the energy resolution is constant over the entire spectrum.
INTERPRETATION OF XPS SPECTRUM
xps counts electrons depicted from a pattern floor while irradiated with the aid of x-rays.
if solid sample is analyzed then the zero of the spectrum is on the fermi strength. a

spectrum displaying the number of electrons recorded at a series of energies involves
both a contribution from a history sign and additionally resonance peaks assets of the
bound states of the electrons within the floor atoms. we notice that the history depth rises
with the lowering kinetic electricity. that is due to the secondary electrons that variably
lost their electricity on their manner into the vacuum. the resonance peaks above the
heritage are the consequential features in an xps spectrum. two sorts of spectra, examine
and excessive decision spectrum
Figure 7: Survey and high resolution spectra of Ni
are measured. to start with, the survey spectrum this is sampled with lower strength
decision is measured on scale of binding energies generally between 1-1000 ev. on this
region the plot has feature peaks for each detail found at the surface of the sample. the
height intensities degree how a lot of a cloth is at the floor, whilst the height positions
suggest the elemental and chemical composition. other values, along with the full width
at half most (fwhm) are useful signs of chemical country adjustments and bodily
influences. if the chemical bonds or digital states want to be analysed, the high resolution
spectrum needs to be measured in a slender energy range around each man or woman
peak that we are interested in. photoelectric peaks shift in eb with difference of x-ray
photon power hν, whilst a one of a kind excitation supply is used. the dependence of the

energy spectrum at the anode used to provide the x-rays way that the photoelectric peaks
move in position when the same sample is analysed but using a different anode to
produce the X-rays.
INITIAL STATE EFFECTS
An electron spectrum is essentially obtained by monitoring a signal representing the
number of electrons emitted from a sample over a range of kinetic energies. The energy
for these electrons, when excited using a given photon energy, depends on the difference
between the initial state for the electronic system
Figure 8: Schematic drawing of the electron configuration of Li metal and Li2O, and the
corresponding XPS spectra and the final state. If both initial and final states of the
electronic system are explained well, a single peak materializes in the spectrum. Position
of orbitals in atom is diplomatic to chemical environment of atom. For different chemical
bonds of the element, different peaks of the components can appear. Sometimes there is
no additional peak but the energy shift of the main element peak is observed. These
energy shifts called also chemical shifts are used to interpret the bonding of the elements.

By using a much clarified model we discuss how this chemical shifts appear. As an
example we choose the lithium 1s levels in lithium metal and lithium oxide. Figure gives
a simplified representation of the electronic structure of these two systems.
In lithium metal the lithium 2s electrons form a band, and their wave function is therefore
only partly at the site of a particular lithium atom. However, in lithium oxide each lithium
atom donates its 2s electron totally in 2p shell of oxygen such that a closed 2p6
configuration is obtained.
therefore in lithium oxide the li 2s electron has no part of this wave characteristic close to
the lithium atom. the 1s electron in lithium oxide consequently feels a really more potent
coulomb interplay than in li steel, wherein the lithium nucleus samples like organic
macromolecules, containing numerous extraordinary chemical bonds then multiple peaks
overlap precise qualitative and quantitative interpretation is difficult in this situation. is
screened from the 1s2 shell trough the 2s valence electron. therefore the binding strength
of the li 1s degree is larger in li20 than in li metallic and a chemical shift between
compounds are located. if coping with complex.
Figure 9: Spectrum of C1s carbon peak is used from a sample PET (polyethylene

terephthalate). The spectrum is fitted with the peak that belongs to the different carbon
bonds.
FINAL STATE EFFECTS
Multiple splitting arises when an atom contains unpaired valence electrons. Some of the
transition metal electronic states give rise to significant intensity components in their 2p
spectra due to multiple splitting .When a core electron vacancy is formed by
photoionization there can be coupling between the unpaired electrons in the core with the
unpaired outer shell electron.
The 2p hole, created by photoelectron emission, and the 3d hole of the missing valence
electron, has radial wave functions that overlap significantly. This wave function
overspread is an atomic consequence that can be very large. Coupling of electrons from e
2p and 3d orbitals can create a number of final states, which will be seen in the spectrum
as a multi-peak envelope. There is a finite probability that an ion after photoionization
will be left in a specific excited energy state a few eV above the ground state through
excitation of the ion by the outgoing photoelectron. Such features are denoted satellite
peaks.
To valence electrons associated with an atom the loss of a core electron by photoemission
appears to increase nuclear charge. This major perturbation gives rise to substantial
reorganization of valence electrons witch may involve excitations to a higher unfilled
level.
The power required for this transition isn't always to be had to the primary electron and
accordingly the 2 electron process ends in discreet shape at the low kinetic strength facet
of the principle height. those peaks are known as shake-up satellites. shake off satellites

are comparable in beginning handiest in place of the valence electron being excited to an
unoccupied energy level; it's far lost to the continuum, resulting in a doubly ionized very
last country. for solids, the shake off satellites does not seem in maximum of the
instances in the spectrum as wonderful peaks because they commonly fall into the
strength location of the inelastic secondary electrons.
Occasionally satellites are also determined on the excessive kinetic strength side of the
primary top but his effect could be very rare. those are the shake-down satellites. some of
these satellites are very last nation results that get up at some point of atom relaxation and
creation of photoelectron following center-hole creation in evaluation to chemical shifts
which ends from initial country consequences.
Figure 10: Multiple splitting of Cr 2p3/2 peak for Cr2O3 specimen, Cr metal 2p3/2 peak is
positioned on 574.2 eV When a photoelectron is emitted from a solid it is a final
probability that it may excite a collective oscillation in the conduction electron gas. This
excitation called pasmon is a coherent superposition of electron-hole pairs that represents
a wave-like disturbance of the charge density. The photoelectrons that are responsible for
Plasmon excitations lose an amount of energy that correspond to an integer times the

Plasmon energy wp, and therefore appear to the high binding energy side of the main
photoemission peaks. Plasmon’s are rather wide features in photoemission spectra and
are seldom mistaken to be direct photoemission peaks. This effect occurs during transport
of electron to surface and is treated as final effect.
Figure 11: Plasmon loss structure in a spectrum of aluminum metal

CONCLUSION
X-ray photoelectron spectroscopy (XPS) is a strapping technique mostly used for the
surface analysis of materials. It does not just provide qualitative and quantitative
information on the elements present on the surface but it also gives information on
electronic structure of material on the surface, chemical state and bonding of those
elements.
XPS is one of a number of surface analytical working of method that bombard the
sample with photons, electrons or ions in order to excite the emission of photons,
electrons or the ions. In XPS, the sample is throw light on with low-energy (~1.5 keV) X-
rays, in order to give rise to the photoelectric effect. Surface atom core electrons are
excited. The energy spectrum of the emitted photoelectrons is determined by means of a
high-resolution electron spectrometer witch sees the surface in the area of a circle with 40
μm of diameter.
Photoemission affords facts on electron binding energies, chemical states, electronic
states and quantitative elemental composition. counting and measuring of kinetic
electricity of the emitted photoelectrons allows dedication in their binding strength for the
reason that strength of x-rays photons is known. the measured spectrum consists of
feature peaks which correspond to electronic strength degrees of the investigated fabric.
because every element has a unique set of binding energies, xps may be used to pick out
and determine the attention of the elements at the surface.
Further advantage of XPS is the ability to determine the chemical state of the investigated
material. Chemical state is determined from the chemical shift (slight shift of the
elemental binding energy) which is due to different chemical environment of the

elements in compounds. From this information oxidation state of metals, bonding in
polymers or even proteins can be analysed.
this technique is sensitive to nearly all components and was 1st named as lepton
qualitative analysis for analysis (ESCA) however this word form is no longer used to any
extent further. XPS terribly is extremely is incredibly} surface sensitive technique that is
thanks to very short distance that photoelectrons will travel within the solid material.
solely photoelectrons made within the surface region with a thickness of regarding few
nanometers will break loose the fabric. Since XPS is surface sensitive technique, it may
also be used for depth identification together with particle etching (sputtering) or with
variation of the specimen incoming X-ray activity angle, the depth of the knowledge
gathered may be varied by 1-10 nm. this will be seen from Photoionization has the
advantage over lepton impact ionization in this it's a lot of doubtless to eject inner shell
leptons at an equivalent likelihood as outer shell electrons which it systematically ejects
solely one electron. Another good thing about XPS is that incident gauge boson beams
area unit less harmful than lepton bombardment of the sample, significantly once coping
with organic materials.
Table 2: Table of available surface analytical techniques depending on excitation and
emission particles

References
1. Fadley, C. S. (2010). X-ray photoelectron spectroscopy: Progress and
perspectives. Journal of Electron Spectroscopy and Related Phenomena, 178, 2-
32.
2. Frost, D. C., Ishitani, A., & McDowell, C. A. (1972). X-ray photoelectron
spectroscopy of copper compounds. Molecular Physics, 24(4), 861-877.
3. Uwamino, Y., Ishizuka, T., & Yamatera, H. (1984). X-ray photoelectron
spectroscopy of rare-earth compounds. Journal of Electron Spectroscopy and
Related Phenomena, 34(1), 67-78.
4. Franinović, M. (2012). X-ray photoelectron spectroscopy. University of
Ljubljana, Faculty of Mathematics and Physics, Ljubljana.
5. Zhang, F., Wang, P., Koberstein, J., Khalid, S., & Chan, S. W. (2004). Cerium
oxidation state in ceria nanoparticles studied with X-ray photoelectron
spectroscopy and absorption near edge spectroscopy. Surface Science, 563(1-3),
74-82.

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