University of LjubljanaFaculty of mathematics and Physics

SEMINAR

X-ray photelectron spectroscopy

Marko Franinović

Advisor :prof. dr. Dean Cvetko

Abstract

X-ray photelectron spectroscopy (XPS) is a photoemission experiment for spectroscopicpurposes. Photons from soft x-ray radiation are directed on a sample under ultra high vacuumand the photoelectrons emitted from the sample by the photoelectric effect, are analysed withrespect to kinetic energy by an electrostatic analyser. With this technique it is possible toanalyse the chemical composition of material surfaces up to few nm depth. Here the XPStechnique and XPS spectrum peak identification is discussed.

Ljubljana, April 2012

Table of ContentsIntroduction..........................................................................................................................................................................1

Principles of the XPS technique..........................................................................................................................................1

Photoemission theory...........................................................................................................................................................3

XPS spectrometer................................................................................................................................................................5

Interpretation of XPS spectrum ..........................................................................................................................................6

Initial state effects ...............................................................................................................................................................7

Final state effects.................................................................................................................................................................8

Conclusion...........................................................................................................................................................................9

Bibliography......................................................................................................................................................................10

IntroductionPhotoelectron spectroscopy is a general term that refers to all

those techniques based on the application of the photoelectriceffect originally observed by Hertz [1] and later explained as amanifestation of the quantum nature of light by Einstein [2].Einstein recognized that when light is incident on a sample, anelectron can absorb a photon and escape from the material with amaximum kinetic energy Ek=hν-EB -eФ where ν is the photonfrequency, EB electron binding energy and Ф work function,which gives the minimum energy required to remove adelocalised electron from the surface of the metal. The workfunction is a measure of the potential barrier at the surface thatprevents the valence electrons from escaping. Here we will showhow the photoelectric effect can be used to explore chemicalcomposition and quantum states of material surface.

The surface of a solid has different chemical composition andphysical properties from the interior of the solid. An intensebeam of ultraviolet or X-ray light ionizes the molecules or atoms.The light used must have an energy sufficient to ionize electrons

at least from the highest valence shell of atoms. If hν is larger,

electrons may be ejected also from deeper levels.

When photons with the wavelength in the lower energy¸ X-ray region are incident on the crystal surface then core electronsare knocked out of atoms (Figure 1). Spectrum is obtained bymeasuring the characteristics of electrons that escape from thesurface. This electron spectroscopic method is called X-rayphotoelectron spectroscopy (XPS). Here, details of this methodare explored.

Principles of the XPS techniqueSurface of the specimen is irradiated by X-ray with the

energy of hν. Mono-energetic photon knocks out the electronfrom atoms in the surface region. Photons with higher energy hνpenetrate deeper into the samples surface. Electrons emitted byphotons from lower energy X-rays originates from inner atomicenergy levels and were bound to atomic nuclei with the bindingenergy Eb. Electrons emitted in this manner are referred to asphotoelectrons.

The photoelectric effect generates free electrons with acertain kinetic energy Ek.

As we will see in detail throughout the paper, due to thecomplexity of the photoemission process in solids thequantitative analysis of the experimental data is often performedunder the assumption of the independent particle picture and ofthe sudden approximation (i.e., disregarding the many-bodyinteractions as well as the relaxation of the system during thephotoemission itself). The problem is further simplified withinthe so-called three-step model, in which the photoemissionevent is decomposed in three independent steps: photoexcitationof an electron in the solid, propagation of the excited electron tothe surface, and escape of the photoelectron from the solid intovacuum after transmission through the surface potential barrier.

1

Figure 1: An example of a photoelectron emitted due anincident photon.

However, from the quantum mechanical point of viewphotoemission should not be described in terms of severalindependent events but rather as a one-step process. Thephotoemission process should be expressed in terms of an opticaltransition between initial and final states consisting of many-body wave functions that obey appropriate boundary conditionsat the surface of the solid.

A typical XPS spectrum is a plot of the number of electronsdetected versus their kinetic energy. This spectrum provides usthe information about electron energy distribution in a material.In actual data accumulation, the reference point for solids istaken to be Fermi energy as “natural” zero. Each elementproduces a characteristic set of XPS peaks at characteristicbinding energy values that directly identify each element that

exist in or on the surface of the material being analysed. Thesecharacteristic peaks correspond to the electron configuration ofthe electrons within the atoms, e.g., 1s, 2s, 2p, 3s, etc. (Figure2).

The measured spectrum of electron kinetic energies Ek is asuperposition of the primary electrons with a binding energy Eb.Within the non-interacting electron picture, it is particularlystraightforward to take advantage of the energy conservationlaw and relate, as pictorially described in Figure 3, the kineticenergy of the photoelectron to the binding energy Eb of the electronic-state inside the solid:

Ek=h ν−Eb−eΦ0 (1)Measurement of core-level binding energies for gas phasemolecules is generally accomplished by measuring the kineticenergy of photoelectron relative to a standard species knownionization potential. Unfortunately, no such standards areavailable for the solids. A conventional approach for metal is toreference EB to its Fermi level [4]. If the metal sample andspectrometer are in electrical contact, the Fermi levels must beat equal energies. The potential energies that the photoelectronexperiences, depend on the difference between sample workfunction Фs and the work function of the spectrometer energyanalyser Ф0. The Ф0 - Фs term is often referred as “contactpotential” between sample and analyser. The measured EB

referenced to Fermi level depends only on Ф0. If EB werereported with respect to vacuum level, then both Фs and Ф0

must be known. Since Фs can vary by several eV, depending onthe presence of impurities, EB referenced to Fermi level isclearly better choice

In each orbital an atom, the electrons have a characteristicbinding energy EB, the minimum energy needed to eject them toinfinity.

The fastest electrons of this spectrum are primary electronsemitted directly from the Fermi edge EF, having Ek=hν-eФ. Theslowest electrons are secondary electrons that just barely made itout of the sample after having lost energy in scatteringprocesses, before arriving at the sample surface. These electronshave a kinetic energy of Ek≈0eV. The spectra are usuallycalibrated using reference photoemission peaks from thesubstrate. For example, carbon peak C1s is present in everyorganic sample and it is known to have the binding energy equalto Eb=285 eV.

All elements with order number 3 and above can bemeasured, but hydrogen or helium cannot be detected becausethe diameter of these orbitals is so small, reducing the electronscross section for photoemission to almost zero. For the energiesof X-rays, it turns out the inner electrons are the most likely tobe knocked out.

To ensure the longest possible mean path of thephotoelectrons and to avoid the contamination of the samplesurface, ultra high vacuum (10−9 mbar) is required. A drawbackof ultra high vacuum is that fluids or other outgassing materialscannot be analysed under low pressure and therefore they arenot appropriate for XPS characterisation.

It is important to note that XPS detects only those electronsthat have actually escaped into the vacuum of the instrument.The photo-emitted electrons that have escaped into the vacuumof the instrument are those that originated from within the top

2

Figure 2: Photoelectron spectrum of lead showing the mannerin which electron escaping for the solid can contribute to discretepeaks or suffer energy loss and contribute to background; thespectrum is superimposed on a schematic of a electronic structureof lead.

Figure 3: Schematic view of the photoemission process .Electrons with binding energy EB can be excited above the vacuumlevel Evac by photons with energy hν > EB + Ф . The photoelectrondistribution I(Ekin) can be measured by the analyser and is animage of the occupied density of electronic states N(EB) in thesample [3] EB -binding energy, EF -Fermi energy, Ф0 - analyserwork function.

few nanometres of the material. All of the deeper photo-emittedelectrons, which were generated as the X-rays penetrated 1–5micrometers of the material, are inelastically lost prior to escape.Probabilities of electron interaction with matter exceed those ofthe photons. Path length of the photons is of the order ofmicrometers, while the path length of the electrons is of the orderof tens of Ångströms. Consider a volume element of the sampleof thickness dz at a depth z beneath the sample surface. Thephotoelectrons emitted at an angle θ with respect to the normal tothe sample surface enter the detector and contribute to thespectrum. The probability that a photoelectron will escape fromthe sample into semi infinity space without losing energy is:

P ( z)=exp(−z /λ sin θ) (2)

where λ is the photoelectron inelastic mean free path. Supposethat one layer of thickness dz, by photoionization produces theintensity of photoectrons dI and assuming that the thickness ofthe sample is much larger than few Ångströms then we cancalculate the intensity of the electrons emitted from the depth dby following integral:

∫0

d

dI=∫0

d

αexp (−z /λ sin θ)dz (3)

where α is a coefficient depending on photoemission cross-section incident X-ray flux, angle between photoelectron pathand analyser sample axis and others.

The intensity of electrons Id emitted from all depths greaterthan d in a direction normal to the surface is then given by theBeer-Lambert relationship [5].

I d= I∞ exp(−d /λ sinθ) (4)

where I∞ is the intensity from an infinitely thick materialsample. Only those electrons that originate within tens ofÅngströms below the solid surfaces can leave the surface without

energy loss and contribute to the peaks in the spectra. Theremaining electrons that undergo inelastic processes, sufferenergy loss and increase the spectrum background or contributeto secondary emission. This is the main reason for high surfacesensitivity of XPS. From Figure 4 we can see that this surfacesensitivity is best in the region from 10 to 1000 eV.

Photoemission theoryIn a photoemission process a photon with energy hν liberates

an electron from system. We consider the interaction of theelectrons of the atom with the radiation.

The sample represents always a many-body system that isinvolved as a whole in the photoemission process. Here we firsttreat the electron in the atom quantum mechanically and the X-ray radiation field classically. This is called the semiclassicaldescription.

Electrons are excited from an initial bound energy level intofree electronic states above the Fermi level. The simplifiedsingle-particle picture is a good starting point for theunderstanding of many photoemission applications, as long asthe spectrum is not significantly influenced by electroniccorrelation effects. This is called a sudden approximation, ordiabatic process witch means that a quantum mechanical systemsubjected to rapidly changing conditions prevent the systemfrom adapting its configuration during the process. The electronsstill find themselves, immediately after the perturbation, in thesame sate as before it [6]. This approximation is used for high-energy photoelectron limit.

To develop a formal description of the photoemissionprocess, one has to calculate the transition probability w for anoptical excitation between the N-electron ground state ψi withenergy Ei and one of the possible final states ψf with energy Ef.,with ψi and ψf being eigenfunctions of the unperturbedHamiltonian H0. This can be approximated by Fermi’s goldenrule [6]. For small perturbations H' the transition probability perunit time w is given by:

w∝2πℏ∣⟨ψ f∣H '∣ψi⟩∣

2δ(E f−E i−h ν) (5)

Fermi's golden rule is based on first order perturbationtheory. So, it assumes that the perturbation is weak.

In most general form the Hamiltonian of an electron in anelectromagnetic field disregarding the spin has the form:

H= 12m[ p̂− e

cA]

2

+eV (r )+ϕ (6)

Where A is electromagnetic vector potential of the light andp=iћ is electronic momentum operator. The scalar potential can always be chosen to be zero [7]. Scalar potential shouldnot be confused with work function in equation (1). This yieldsthe unperturbed Hamiltonian H0 without electromagneticinteraction:

H 0=p̂2

2mc+eV (r) (7)

The perturbation H' of the system caused by the incidentradiation is found by replacing the momentum operator p̂ inH by p̂−e A :

H=p̂2

2m e

+e

2me

(A⋅p̂+ p̂⋅A)

+e 2

2me

A2+eV (r )

(8)

If we rewrite the upper equation in the form of H=H0+H' we

3

Figure 4: Measured electron mean free path as a function oftheir kinetic energy for various metals are represented with blackdots. The universal curve of inelastic mean free λ of the electron isa best fit trough the measured points [4].

find that the photoemission perturbation operator H' is of theform:

H '= e2me

(A⋅p̂+ p̂⋅A)+ e2

2me

A⋅A (9)

In this part of semi classical description we treat vectorpotential A as classical field.

If we use the gauge =0 then we can neglect the quadraticterm in A because in the linear optic regime it is typicallynegligible [8]. This term represents the two photon process andbecomes relevant only for high photon intensities, that areusually not produced by standard light sources and can thereforebe omitted Error: Reference source not found. The first term canbe rewritten using commutation relation:

A⋅ p̂+ p̂⋅A=2 A⋅p̂+i ℏ(∇⋅A) (10)

H '= em c

A⋅p̂+i e2mc

∇⋅A (11)

Assuming that A is constant over atomic dimensions say thatA=0. Although this is a routinely used approximation, itshould be noted that A might become important at the surfacewhere the electromagnetic fields may have a strong spatialdependence. This surface photoemission contribution, which isproportional to (ε-1) where ε is the medium dielectric function,can interfere with the bulk contribution resulting in asymmetricline shapes for the bulk direct-transition peaks Error: Referencesource not found.

H '= emc

A⋅ p̂ (12)

The photocurrent produced by photoemission is a result fromthe excitation of electrons from the initial state i with the wavefunction ψi to the final state f with the wave function ψf by thephoton field having the vector potential A. Thus, applying theGolden Rule to calculate the photocurrent, we obtain:

I=∣M if∣2=∣⟨ ψ f∣H '∣ψi ⟩∣

2 (13)

The perturbation operator H' describes the interaction of an

electron in the system with the incident radiation. The matrixelement H' from equation (3) is now:

⟨ψ f∣H '∣ψi⟩=

−e

2mc∫ ψ f

A⋅p̂ψi d r(14)

This is an appropriate basis for the theoretical description ofmost photoemission studies.

Now we take A to be associated with the wave function of aphoton with an energy E=ћω. The theoretical description of theinteraction between the electromagnetic field and the system isusually simplified by means of the so-called dipoleapproximation. The dipole approximation assumes that thevariation of the external field A is small in the spatial region inwhich the matrix element Mif is not negligible. When thewavelength of the radiation is much larger than the scale ofelectronic orbitals in the atom we can approximate the electricfield amplitude at position of atoms electron by the fieldamplitude at the atom centre off mass position. X-ray withenergy around 100 eV has the wavelength λ≈102 Å. Knowing that

wave vector |k|=2π/λ and r is the estimation of the k-shelldiameter, we can assume that wavelength is large compare to theatomic distance or kr<<1.

When the external electromagnetic field is periodic in space,it can be expressed as:

A(r )=A0 ϵ ei( k r )=A0 ϵ(1+i k r+…) , (15)

where k is vector pointing in the propagation direction and εis a unitary vector in the direction of the light polarization andA0 is complex amplitude of the field in direction of ε witchsquare is correlated to the photon flux. This approximationcalled dipole approximation. The dipole approximation consistsin keeping only the first term of this expansion in the calculationof the photocurrent via Equation . Furthermore, thecommutation relations lead to the following equivalence:

⟨ψ f∣A(r )⋅p̂∣ψi ⟩∝

⟨ψ f∣A(r )⋅∇V∣ψi ⟩∝

⟨ ψ f∣A(r )⋅r∣ψi ⟩ .(16)

By using the Taylor expansion of A to first order in r, thematrix element Mif from Equation 11can be calculated as:

M if=−ieℏ c

A0(E i−E f ) ⟨ψ f∣ϵ⋅r∣ψi⟩ (17)

Here we can introduce the dipole operator μ= er. For asingle atom, the dipole approximation leads to certain selectionrules in the symmetry of the photoemitted electron wavefunction. The transition probability can be rewritten as:

w∝∣⟨ψ f∣ϵ⋅μ∣ψi⟩∣2∝∣ϵD fi∣

2 , (18)

where Dfi stands for the dipole matrix element. It determineshow the system will interact with an electromagnetic wave of agiven polarization, while the square of the magnitude gives thestrength of the interaction due to the distribution of chargewithin the system.

In order to establish which transitions are allowed, it isnecessary to examine the dipole matrix element and determinateunder what condition this matrix element is non vanishing.

The selection rules in the dipole approximation are strictlyvalid only for atomic systems. Nevertheless, in the case ofmolecules, clusters or solids, the scattering theory provides asimplified picture in which the selection rules fit as well. Thephotoemission process from a core level can be described as thephotoexcitation from a single atom, followed by the transport ofthe photoelectron on its way to the detector. In this picture, theselection rules remain valid for the first step of the process (thephotoexcitation from the single atom). The subsequentscattering of the outgoing electron by the surrounding atomswill add other partial wave contributions to the finalphotoelectron pattern. The method describe above is called theon-step model approach.

However, due to the complexity of the one-step model,photoemission data are usually discussed within the three-stepmodel, which although purely phenomenological has proven tobe rather successful. Within this approach, the photoemissionprocess is subdivided into three independent and sequentialsteps:

(i) Optical excitation of the electron with the probability w

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(ii) Travel of the excited electron to the surface that can bedescribed in terms of an effective mean free path, proportional tothe probability that the excited electron will reach the surfacewithout scattering.

(iii) Escape of the photoelectron into vacuum that can isdescribed by a transmission probability through the surface,which depends on the energy of the excited electron and thematerial work function Φ.

While the three-step model makes the photoemission processquite easy to discuss, it is no more than a useful approximation.

XPS spectrometer As mentioned before the whole instrumentation has to be in

ultra high vacuum (10−9 mbar) chamber. The sample isintroduced through a prechamber that is in contact with theoutside environment. This prechamber is closed and pumped tolow vacuum. After it is at low vacuum the sample is furtherintroduced into the main chamber in which a UHV (ultra highvacuum) environment exists (Figure 5, 3). The surface of thesample is irradiated with the beam of X-rays.

The source of X-rays photons used for irradiation of solidsamples is standard X-ray tube (Figure 7, 2). A heated filamentprovides electrons witch are accelerated to potential of between

10 and 20 keV toward a water cooled solid anode. The electronscreate core holes in the anode atoms, witch are filled by relaxingelectrons from higher levels. The relaxation process is thenaccompanied by X-ray fluorescence. The anode material definesthe energy of radiation. In XPS instruments normally Al andMg anodes are used because of a dominant, strong resonance inthe X-ray spectrum. For the Al X-ray, a doublet arises from the2p1/2, 2p3/2 →1s electronic relaxation. These are so called Kα1,2

lines. The energies of X-rays produced by few metals aretabulated in Table 1. In the Table 1 we notice that the energy andbeam natural width (FWHM) of the lines increase with atomicnumber.

X-rays can be effectively monochomatized by reflectionfrom bent quartz crystal to produce, so called, monochromaticX-ray sources. One of the advantages in using monochromaticX-rays is that the beam natural width is narrow compared to theunfiltered X-ray line and therefore improves the resolution ofthe photoelectric peaks in the XPS spectrum. A furtherconsequence of filtering the X-rays prior to irradiating thesample is that minor resonance lines in the X-ray spectrum areremoved from the excitation mechanism. If unfiltered, theseminor X-ray lines produce additional satellite peaks so called"ghost" peaks in XPS spectrum and these appear at kineticenergies characteristic of the energy separation between theprimary X-ray lines.

The photoelectrons from material excited by X-ray beamtravel through electrostatic transfer lens to electrostatichemispherical mirror analyser (Figure 5, 4). A conventionalhemispherical analyser consists of a multi-element electrostaticinput lens, a hemispherical deflector with entrance and exit slits,and an electron detector (i.e., a channeltron or a multi-channel

5

Figure 5: The picture of the XPS spectrometer: (1) X-raysource, (2) monocromator, (3) UHV chamber, (4) hemisphericalmirror analyser [9].

Table 1: A few common anode materials used for X-ray source[4].

Figure 6: Hemispherical mirror analyser [10].

detector). The heart of the analyser is the deflector whichconsists of two concentric hemispheres (of radius R1 and R2)(Figure 6). In the constant analyser energy mode thesehemispheres are kept at a potential difference ΔV . The role of theelectrostatic lens is deceleration and focusing the photoelectronsonto the entrance slit. Electrons entering the analyser with anenergy Eo at a radius Ro =(R1+R2)/2, follow a circular path withconstant radius. This energy Eo is defined as the pass energy. Thepotentials applied to the inner and outer hemispheres are definedby the pass energy chosen and the dimensions of the analyser. Onthe end of the analyser the electrons hit the electron detectorwhere their energy is measured. By scanning the lens retardingpotential we can effectively record the photoemission intensityversus the photoelectron kinetic energy.

Alternatively, an electron energy distribution may be obtained byvariation of the pass energy, keeping the ratio between the passenergy and the retardation voltage constant. This is done in theconstant retard ratio mode. Since the spatial divergence of theelectron trajectories in the analyser increases with decreasingpass energy, the energy resolution in the constant retard ratiomode is proportional to the detected energy, whereas in theconstant analyser energy mode the energy resolution is constantover the entire spectrum.

Interpretation of XPS spectrum XPS counts electrons ejected from a sample surface when

irradiated by x-rays. If solid sample is analysed then the zero ofthe spectrum is at Fermi energy. A spectrum representing thenumber of electrons recorded at a sequence of energies includesboth a contribution from a background signal and also resonancepeaks characteristic of the bound states of the electrons in thesurface atoms. We notice that the background intensity rises withthe falling kinetic energy. That is because of the secondaryelectrons that elastically lost their energy on their way into thevacuum. The resonance peaks above the background are thesignificant features in an XPS spectrum shown in Figure 7.

Two kinds of spectra, survey and high resolution spectrum

are measured. First of all, the survey spectrum (Figure 6) that issampled with lower energy resolution is measured on scale ofbinding energies normally between 1-1000 eV. In this region theplot has characteristic peaks for each element found on thesurface of the sample.

The peak intensities measure how much of a material is atthe surface, while the peak positions indicate the elemental andchemical composition. Other values, such as the full width athalf maximum (FWHM) are useful indicators of chemical statechanges and physical influences.

If the chemical bonds or electronic states need to beanalysed, the high resolution spectrum has to be measured in anarrow energy range around each individual peak that we areinterested in.

Photoelectric peaks shift in EB with difference of X-rayphoton energy hν, when a different excitation source is used.The dependence of the energy spectrum on the anode used toproduce the X-rays means that the photoelectric peaks move inposition when the same sample is analysed but using a differentanode to produce the X-rays.

We display XPS spectra using binding energy for theabscissa. The kinetic energy scale is reported relative to thephoton energy of the excitation source and so photoelectric linepositions with respect to a binding energy scale becomeindependent of the X-rays used to excite the sample. In theinvestigation of molecules and solids, one is not usuallyinterested in the absolute binding energy of particular core level,but in the change in binding energy between two differentchemical forms of the same atom.

Electron binding energies for the elements in their naturalform, together with their cross section have been tabulated foridentification of the peaks in the spectrum.

When a core electron is removed, leaving a vacancy, anelectron from a higher energy level may fill the vacancy,resulting in a release of energy. Although most of the time thisenergy is released in the form of an emitted photon, the energycan also be transferred to another electron, which is ejected fromthe atom. This second ejected electron is called an Augerelectron (Figure 1). This are not photoelectrons. They are seenas wide low peaks before elements peak (Figure 6). Auger peaksalways appear at the same kinetic energy. They accompanyevery XPS spectrum. They are not of our interest in this method.

Semi-quantitative analysis of concentration of elements onsample surface is possible by measuring the peak areas ofspecific elemental peaks. From the area of the peaks, atomicconcentrations of the elements can be calculated. Characteristicpeaks are superimposed on a background, thereforebackground has to be subtracted before calculation ofelemental concentration. A variety of background algorithms areused to measure the peak area; none of the used algorithms arecorrect and therefore represent a source for uncertainty whencomputing the peak area.

Concentration of elements is correlated with the intensity ofthe peak weighted by sensitivity factor S. Atomic concentrationCi of the element i can be calculated by [11]:

C x=nx

∑ ni

=I x /S x

∑ ( I i / S i)(19)

ni represents the number of atoms, Ii the area of the i-th elementspeak and Si sensitivity factor of the element

S= f σ D λ (20)f = X-ray flux

6

Figure 7: Survey and high resolution spectra of Ni [4].

σ = photoelectron cross-section

D = detector efficiency

λ=inelastic electron mean free path.

The accuracy of the final quantitative result for thecomposition of the surface atom layer depends on the accuracywith which the S is known. When calculating S, λ represents thebiggest uncertainty. There are two methods currently used forobtaining λ: experimental determination and theoreticalprediction. In theoretical prediction λ is actually defined by theEquation (4) which gives the probability of the electrontravelling a distance d in a semi-infinity space, through the solidwithout undergoing scattering. The equations of absorption arenot sufficient to model with a good precision the electron traveldistances in the matter. Many experiments were conducted inorder to determine with precision the distance covered by theelectrons as a function of matter nature and their kinetic energy.From these data, a function passing through as many points aspossible was calculated. According to the material types and thedegree of accuracy, various empirical relations and algorithmswere developed. Universal electron scattering curve is used todescribe this dependency (Figure 4).

Initial state effects An electron spectrum is essentially obtained by monitoring a

signal representing the number of electrons emitted from asample over a range of kinetic energies. The energy for theseelectrons, when excited using a given photon energy, depends onthe difference between the initial state for the electronic system

and the final state. If both initial and final states of the electronicsystem are well defined, a single peak appears in the spectrum.

Position of orbitals in atom is sensitive to chemicalenvironment of atom. For different chemical bonds of theelement, different peak components can appear. Sometimes thereis no additional peak but the energy shift of the main elementpeak is observed. These energy shifts called also chemical shifts

are used to interpret the bonding of the elements.

Using a very simplified model we discuss how thischemical shifts arise. As an example we choose the lithium 1slevels in lithium metal and lithium oxide. Figure 8 gives asimplified representation of the electronic structure of these twosystems.

In lithium metal the lithium 2s electrons form a band, andtheir wave function is therefore only partly at the site of aparticular lithium atom. However, in lithium oxide each lithiumatom donates its 2s electron totally in 2p shell of oxygen suchthat a closed 2p6 configuration is obtained.

Thus in lithium oxide the Li 2s electron has no part of thiswave function near the lithium atom. The 1s electron in lithiumoxide therefore feels a somewhat stronger Coulomb interactionthan in Li metal, where the lithium nucleus is screened from the1s2 shell trough the 2s valence electron. Therefore the bindingenergy of the Li 1s level is larger in Li20 than in Li metal and achemical shift between two compounds is observed. If dealingwith complex samples like organic macromolecules, containinga lot of different chemical bonds then multiple peaks overlap(Table 2). Exact qualitative and quantitative interpretation isdifficult in this situation.

For example, analysing the C1s peak of organic polymerPET, C-O group is shifted to by eV relative to theunfunctionalized carbon, C-C/C-H (carbon atoms in phenyl ring

and methylene bond to H) by -0,4 eV and O-C=O by 3,7 eV(Figure 9). This shifts are tabulated in the books of standards. If

7

Figure 8: Schematic drawing of the electron configuration ofLi metal an Li2O, and the correspondin XPS spectra [8].

Figure 9: Spectrum of C1s carbon peak taken from a samplePET (polyethylene terephthalate). The spectrum is fitted with peakthat belong to different carbon bonds [12].

Table 2: Binding energies of carbon peaks in the spectrum of organic compounds [13].

known chemical bond are expected on the surface then fitting ofexpected peaks on the measured spectrum can be done .

The photemission process is a many-electron event. Duringthe photoemission, all the other electrons in the system mustreadjust their position and energy since they are no longer intheir ground electronic states. This initial sate effects give rise toasymmetric peak shapes as well as peak splitting.

The simplest type of splitting is the spin-orbit splitting. Corelevels in XPS use the nomenclature n l j where n is the principalquantum number, l is the angular momentum quantum numberand j = l + s (where s is the spin angular momentum number andcan be ±½). All orbital levels except the s levels (l = 0) give riseto a doublet with the two possible states having different bindingenergies. This is known as spin-orbit splitting. This splitting isdue to “spin-orbit coupling”, between the electron spin and theangular momentum vector of the orbital, which can either beparallel or anti-parallel. The anti-parallel alignment is more“favourable”, has a lower energy and therefore appears at higherbinding energy.

The degeneracy of each of these levels is 2j+1 (Table 3). Therelative intensities of the peaks is a function of the degeneracy. Incase of 2p orbital the total angular momentum is either j=1/2with degeneracy 3 or j=3/2 with degeneracy 4. The intensityratio is 2:4 = 1:2 . The splitting of the Ni 2p1/2 an 2p3/2 peaks canbe seen on Figure 7 in the high resolution spectrum.

Final state effectsMultiplet splitting arises when an atom contains unpaired

valence electrons. Some of the transition metal electronic statesgive rise to significant intensity components in their 2p spectradue to multiplet splitting [14]. When a core electron vacancy isformed by photoionization there can be coupling between theunpaired electron in the core with the unpaired outer shellelectron. The 2p hole, created by phtoelectron emission, and the3d hole of the missing valence electron, have radial wavefunctions that overlap significantly [15]. This wave functionoverlap is an atomic effect that can be very large. Coupling ofelectrons from e 2p and 3d orbitals can create a number of finalstates, which will be seen in the spectrum as a multi-peakenvelope (Figure 10). There is a finite probability that an ionafter photoionization will be left in a specific excited energy statea few eV above the ground state through excitation of the ion bythe outgoing photoelectron. Such features are denoted satellitepeaks.

To valence electrons associated with an atom the loss of acore electron by photoemission appears to increase nuclear

charge. This major perturbation gives rise to substantialreorganization of valence electrons witch may involveexcitations to a higher unfilled level. The energy required forthis transition is not available to the primary electron and thusthe two electron process leads to discreet structure on the lowkinetic energy side of the main peak. This peaks are calledshake-up satellites. Shake off satellites are similar in origin onlyinstead of the valence electron being excited to an unoccupiedenergy level, it is lost to the continuum, resulting in a doublyionized final state. For solids, the shake off satellites do notappear in most of the cases in the spectrum as distinct peaksbecause they usually fall into the energy region of the inelasticsecondary electrons.

Sometimes satellites are also observed on the high kineticenergy side of the main peak but his effect is very rare. Theseare the shake-down satellites.

All these satellites are final state effects that arise during atomrelaxation and creation of photoelectron following core-holecreation in contrast to chemical shifts which results from initialstate effects.

When a photoelectron is emitted from a solid it is a finalprobability that it may excite a collective oscillation in theconduction electron gas. This excitation called pasmon is acoherent superposition of electron-hole pairs that represents awave-like disturbance of the charge density. The photoelectronsthat are responsible for plasmon excitations lose an amount ofenergy that correspond to an integer times the plasmon energywp, and therefore appear to the high binding energy side of themain photoemission peaks [Figure 11]. Plasmons are rather widefeatures in photoemission spectra and are seldom mistaken to bedirect photoemission peaks. This effect occurs during transportof electron to surface and is treated as final effect.

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Table 3: Spin orbit splitting .

Orbital l j 2j+1(degeneracy) Electron level

1s 0 1/2 1 1s

2s 0 1/2 1 2s

2p 1 1/2 2

2p 1 3/2 4

3d 2 3/2 4

3d 2 5/2 6

4f 3 5/2 62j+1(degeneracy)

4f 3 7/2 8

Iratio

2p1/2 } 1/22p3/2

3d3/2 } 2/33d

5/2

4f5/2

4f7/2

Figure 10: Multiple splitting of Cr 2p3/2 peak for Cr2O3

specimen, Cr metal 2p3/2 peak is positioned on 574.2 eV [15].

ConclusionX-ray photoelectron spectroscopy (XPS) is a powerful

technique widely used for the surface analysis of materials. Itdoes not just provide a qualitative and quantitative informationon the elements present on the surface but it also givesinformation on electronic structure of material on the surface,chemical state and bonding of those elements.

XPS is one of a number of surface analytical techniques thatbombard the sample with photons, electrons or ions in order toexcite the emission of photons, electrons or ions (Table 9). InXPS, the sample is irradiated with low-energy (~1.5 keV) X-rays, in order to provoke the photoelectric effect. Surface atomcore electrons are excited. The energy spectrum of the emittedphotoelectrons is determined by means of a high-resolutionelectron spectrometer witch sees the surface in the area of acircle with 40 μm of diameter.

Photoemission provides information on electron bindingenergies, chemical states, electronic states and quantitativeelemental composition.

Counting and measuring of kinetic energy of the emittedphoto-electrons enables determination of their binding energysince the energy of X-rays photons is known. The measuredspectrum consists of characteristic peaks which correspond toelectronic energy levels of the investigated material. Becauseeach element has a unique set of binding energies, XPS can be

used to identify and determine the concentration of theelements at the surface.

Further advantage of XPS is the ability to determine thechemical state of the investigated material. Chemical state isdetermined from the chemical shift (slight shift of the elementalbinding energy) which is due to different chemicalenvironment of the elements in compounds. From thisinformation oxidation state of metals, bonding in polymers oreven proteins can be analysed.

This method is sensitive to virtually all elements and wasfirst named as Electron Spectroscopy for Chemical Analysis(ESCA) but this acronym is now not used any more.

XPS is very surface sensitive method which is due to veryshort distance that photoelectrons can travel in the solidmaterial. Only photoelectrons produced in the surface regionwith a thickness of about few nanometres can escape fromthe material. Since XPS is surface sensitive technique, it canalso be used for depth profiling in combination with ion etching(sputtering) or with variation of the specimen incoming X-raymeasurement angle , the depth of the information gathered canbe varied by 1-10 nm. This can be seen from Equation 4.

Photoionization has the advantage over electron impactionization in that it is more likely to eject inner shell electrons atthe same probability as outer shell electrons and that itconsistently ejects only a single electron. Another benefit ofXPS is that incident photon beams are less destructive thanelectron bombardment of the sample, particularly when dealingwith organic materials.

Table 4: Table of available surface analytical techniques depending on excitation and emission particles [17].

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Figure 11: Plasmon loss structure in a spectrum of aluminummetal [16].

Bibliography[1] H. Hertz, "Ueber den Einfluss des ultravioletten Lichtes auf die electrische Entladung", Annalen der Physik 31, p983

(1887).

[2] A. Einstein, Concerning an Heuristic Point of View Toward the Emission and Transformation of Light, Annalen der Physik17, p132 (1905)

[3] F. Reinert , S. Hüfner, Photoemission spectroscopy—from early days to recent applications, Universität Würzburg,Universität des Saarlandes, New Journal of Physics 7 , p5(2005)

[4] N. Winograd, S.W. Gaarenstroom, X-ray Photoelectron Spectroscopy, Department of chemistry, The Pennsylvania StateUniversity University, University Park, Pennsylvania, Physical methods in modern chemical analisys, vol 2, p128-152,(1980)

[5] J.F. Watts, X-ray photoelectron spectroscopy, Surface science techniques, p12 Loughborough University of Technology,Leicesterhier, Pergamon, UK, (1994)

[6] F.Schwable , Quantum mechanics, p289 Springer-Verlag, Berlin, (1993)

[7] G.Kladnik, Elecronic structures and charge transfer at nanostructures an hybrid interfaces, dissertation p17-21, Universitiyof Ljubljana, Faculty of mathematics and physics, Ljubljana (2012)

[8] S. Hüfner, Photoelectron Spectroscopy, Springer-Verlag, Berlin Heidelberg, 3rd edition, p39-51 (2003)

[9] J. Vohs, Introduction to X-Ray Photo-Electron Spectroscopy (XPS), University of Pennsylvania, (2006), found at:www.pire-ecci.ucsb.edu/pire-ecci-old/.../papers/vohs1.pdf (21.03 2015)

[10] http://arpes.stanford.edu/facilities_ssrl.html, (28.1.2013)

[11] Moulder JF, Stickle WF, Sobol PE, Bomben KD and Chastain J, King RC (eds), Handbook of X-ray PhotoelectronSpectroscopy, p25, Physical Electronic, Minesota (1995)

[12] C. Wang, P. Lai, S. H. Syu, J. Leu, Effects of CF4 plasma treatment on the moisture of poly(ethylene terephthalate) flexiblefilms, Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, (2011).

[13] Nianqiang Wu, X-ray Photoelectron Spectroscopy, found at: http://www2.cemr.wvu.edu/~wu/mae649/xps.pdf, (28.1.2013).

[14] M.C. Biesinger, L.W.M. Lau, A.R. Gerson, R.St.C. Resolving surface chemical states in XPS analysis of first row transitionmetals, oxides and hydroxides: Sc, Ti, V, Cu and Zn, Surface Science, vol. 257, p2717–2730 (2010)

[15] F. Groot, Multiplet effects in X-ray spectroscopy, Elsevier, Coordination Chemistry Reviews 249, p31–63 (2005)

[16] A.P. Grosvenor, M.C. Biesinger, R.St.C. Smart, N.S. McIntrye, New interpretations of XPS spectra of nickel metal andoxides, Surface Science vol. 600, p1771–1779 (2006).

[17]R. Paynter, XPS Theory, INRS-Énergie, Matériaux et Télécommunications (2004), found at: csacs.mcgill.ca/francais/docs/.../XPS_Paynter_t.pdf (24.02.2015).

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