x-ray photoelectron spectroscopy

1. X-ray PhotoelectronX-ray Photoelectron
Spectroscopy (XPS)Spectroscopy (XPS)
Center for Microanalysis of MaterialsCenter for Microanalysis of Materials
Frederick Seitz Materials ResearchFrederick Seitz Materials Research
LaboratoryLaboratory
University of Illinois at Urbana-ChampaignUniversity of Illinois at Urbana-Champaign

2. Surface AnalysisSurface Analysis
The Study of the Outer-Most Layers of Materials (<100The Study of the Outer-Most Layers of Materials (<100 ΑΑ).).
ElectronElectron
SpectroscopiesSpectroscopies
XPS: X-rayXPS: X-ray
PhotoelectronPhotoelectron
SpectroscopySpectroscopy
AES: Auger ElectronAES: Auger Electron
SpectroscopySpectroscopy
EELS: Electron EnergyEELS: Electron Energy
Loss SpectroscopyLoss Spectroscopy
Ion SpectroscopiesIon Spectroscopies
SIMS: Secondary IonSIMS: Secondary Ion
Mass SpectrometryMass Spectrometry
SNMS: SputteredSNMS: Sputtered
Neutral MassNeutral Mass
SpectrometrySpectrometry
ISS: Ion ScatteringISS: Ion Scattering
SpectroscopySpectroscopy

3. Introduction toIntroduction to
X-ray PhotoelectronX-ray Photoelectron
Spectroscopy (XPS)Spectroscopy (XPS)

4. Introduction to X-ray PhotoelectronIntroduction to X-ray Photoelectron
Spectroscopy (XPS)Spectroscopy (XPS)
What is XPS?- GeneralWhat is XPS?- General
TheoryTheory
How can we identify elementsHow can we identify elements
and compounds?and compounds?
Instrumentation for XPSInstrumentation for XPS
Examples of materials analysis withExamples of materials analysis with
XPSXPS

5. What is XPS?What is XPS?
X-ray Photoelectron SpectroscopyX-ray Photoelectron Spectroscopy
(XPS), also known as Electron Spectroscopy(XPS), also known as Electron Spectroscopy
for Chemical Analysis (ESCA) is a widelyfor Chemical Analysis (ESCA) is a widely
used technique to investigate the chemicalused technique to investigate the chemical
composition of surfaces.composition of surfaces.

6. What is XPS?What is XPS?
X-ray Photoelectron spectroscopy,X-ray Photoelectron spectroscopy,
based on the photoelectric effect,based on the photoelectric effect,1,21,2
waswas
developed in the mid-1960’s by Kaideveloped in the mid-1960’s by Kai
Siegbahn and his research group at theSiegbahn and his research group at the
University of Uppsala, Sweden.University of Uppsala, Sweden.33
1. H. Hertz, Ann. Physik 31,983 (1887).
2. A. Einstein, Ann. Physik 17,132 (1905). 1921 Nobel Prize in Physics.
3. K. Siegbahn, Et. Al.,Nova Acta Regiae Soc.Sci., Ser. IV, Vol. 20 (1967).
1981 Nobel Prize in Physics.

7. XPS BackgroundXPS Background
XPS technique is based on Einstein’s idea about theXPS technique is based on Einstein’s idea about the
photoelectric effect, developed around 1905photoelectric effect, developed around 1905
– The concept of photons was used to describe the ejection ofThe concept of photons was used to describe the ejection of
electrons from a surface when photons were impinged upon itelectrons from a surface when photons were impinged upon it
During the mid 1960’s Dr. Siegbahn and his research groupDuring the mid 1960’s Dr. Siegbahn and his research group
developed the XPS technique.developed the XPS technique.
– In 1981, Dr. Siegbahn was awarded the Nobel Prize in Physics forIn 1981, Dr. Siegbahn was awarded the Nobel Prize in Physics for
the development of the XPS techniquethe development of the XPS technique

8. X-RaysX-Rays
Irradiate the sample surface, hitting the core electrons (eIrradiate the sample surface, hitting the core electrons (e--
) of the atoms.) of the atoms.
The X-Rays penetrate the sample to a depth on the order of a micrometer.The X-Rays penetrate the sample to a depth on the order of a micrometer.
Useful eUseful e--
signal is obtained only from a depth of around 10 to 100 Å on thesignal is obtained only from a depth of around 10 to 100 Å on the
surface.surface.
The X-Ray source produces photons with certain energies:The X-Ray source produces photons with certain energies:
– MgKMgKαα photon with an energy of 1253.6 eVphoton with an energy of 1253.6 eV
– AlKAlKαα photon with an energy of 1486.6 eVphoton with an energy of 1486.6 eV
Normally, the sample will be radiated with photons of a single energyNormally, the sample will be radiated with photons of a single energy
(MgK(MgKαα or AlKor AlKαα). This is known as a monoenergetic X-Ray beam.). This is known as a monoenergetic X-Ray beam.

9. Why the Core Electrons?Why the Core Electrons?
An electron near the Fermi level is far from the nucleus, moving inAn electron near the Fermi level is far from the nucleus, moving in
different directions all over the place, and will not carry informationdifferent directions all over the place, and will not carry information
about any single atom.about any single atom.
– Fermi level is the highest energy level occupied by an electron in aFermi level is the highest energy level occupied by an electron in a
neutral solid at absolute 0 temperature.neutral solid at absolute 0 temperature.
– Electron binding energy (BE) is calculated with respect to the FermiElectron binding energy (BE) is calculated with respect to the Fermi
level.level.
The core eThe core e--
s are local close to the nucleus and have bindings are local close to the nucleus and have binding
energies characteristic of their particular element.energies characteristic of their particular element.
The core eThe core e--
s have a higher probability of matching the energies ofs have a higher probability of matching the energies of
AlKAlKαα and MgKand MgKαα..
Core e-
Valence e-
Atom

10. Binding Energy (BE)Binding Energy (BE)
These electrons are
attracted to the proton
with certain binding
energy x
This is the point with 0 energy of
attraction between the electron and
the nucleus. At this point the
electron is free from the atom.
The Binding Energy (BE) is characteristic of the core electrons for each element. The BE is determined by the
attraction of the electrons to the nucleus. If an electron with energy x is pulled away from the nucleus, the attraction
between the electron and the nucleus decreases and the BE decreases. Eventually, there will be a point when the
electron will be free of the nucleus.
0
x
p+
B.E.

11. Energy LevelsEnergy Levels
Vacumm Level
Fermi Level
Lowest state of energy
BE
Ø, which is the work function
At absolute 0 Kelvin the electrons fill
from the lowest energy states up.
When the electrons occupy up to this
level the neutral solid is in its
“ground state.”

12. X-ray Photoelectron SpectroscopyX-ray Photoelectron Spectroscopy
Small Area DetectionSmall Area Detection
X-ray BeamX-ray Beam
X-ray penetrationX-ray penetration
depth ~1depth ~1µµm.m.
Electrons can beElectrons can be
excited in thisexcited in this
entire volume.entire volume.
X-ray excitation area ~1x1 cmX-ray excitation area ~1x1 cm22
. Electrons. Electrons
are emitted from this entire areaare emitted from this entire area
Electrons are extractedElectrons are extracted
only from a narrow solidonly from a narrow solid
angle.angle.
1 mm1 mm22
10 nm10 nm

13. XPS spectral lines areXPS spectral lines are
identified by the shell fromidentified by the shell from
which the electron waswhich the electron was
ejected (1s, 2s, 2p, etc.).ejected (1s, 2s, 2p, etc.).
The ejected photoelectron hasThe ejected photoelectron has
kinetic energy:kinetic energy:
KE=hv-BE-KE=hv-BE-ΦΦ
Following this process, theFollowing this process, the
atom will release energy byatom will release energy by
the emission of an Augerthe emission of an Auger
Electron.Electron.
Conduction BandConduction Band
Valence BandValence Band
L2,L3L2,L3
L1L1
KK
FermiFermi
LevelLevel
FreeFree
ElectronElectron
LevelLevel
Incident X-rayIncident X-ray
Ejected PhotoelectronEjected Photoelectron
1s1s
2s2s
2p2p
The Photoelectric ProcessThe Photoelectric Process

14. L electron falls to fill core levelL electron falls to fill core level
vacancy (step 1).vacancy (step 1).
KLL Auger electron emitted toKLL Auger electron emitted to
conserve energy released inconserve energy released in
step 1.step 1.
The kinetic energy of theThe kinetic energy of the
emitted Auger electron is:emitted Auger electron is:
KE=E(K)-E(L2)-E(L3).KE=E(K)-E(L2)-E(L3).
Conduction BandConduction Band
Valence BandValence Band
L2,L3L2,L3
L1L1
KK
FermiFermi
LevelLevel
FreeFree
ElectronElectron
LevelLevel
Emitted Auger ElectronEmitted Auger Electron
1s1s
2s2s
2p2p
Auger Relation of Core HoleAuger Relation of Core Hole

15. Why Does XPS Need UHV?Why Does XPS Need UHV?
Contamination of surfaceContamination of surface
– XPS is a surface sensitive technique.XPS is a surface sensitive technique.
Contaminates will produce an XPS signal and lead to incorrectContaminates will produce an XPS signal and lead to incorrect
analysis of the surface of composition.analysis of the surface of composition.
The pressure of the vacuum system isThe pressure of the vacuum system is << 1010-9-9
TorrTorr
Removing contaminationRemoving contamination
– To remove the contamination the sample surface is bombarded with argonTo remove the contamination the sample surface is bombarded with argon
ions (Arions (Ar++
= 3KeV).= 3KeV).
– heat and oxygen can be used to remove hydrocarbonsheat and oxygen can be used to remove hydrocarbons
The XPS technique could cause damage to the surface, but it isThe XPS technique could cause damage to the surface, but it is
negligible.negligible.

16. The Atom and the X-Ray
Core electrons
Valence electrons
X-Ray
Free electron
proton
neutron
electron
electron vacancy
The core electrons respond
very well to the X-Ray
energy

17. X-Rays on the SurfaceX-Rays on the Surface
Atoms layers
e-
top layer
e-
lower layer
with collisions
e-
lower layer
but no collisions
X-Rays
Outer surface
Inner surface

18. X-Rays on the SurfaceX-Rays on the Surface
The X-Rays will penetrate to the core eThe X-Rays will penetrate to the core e--
of the atoms in theof the atoms in the
sample.sample.
Some eSome e--
s are going to be released without any problem givings are going to be released without any problem giving
the Kinetic Energies (KE) characteristic of their elements.the Kinetic Energies (KE) characteristic of their elements.
Other eOther e--
s will come from inner layers and collide with other es will come from inner layers and collide with other e--
s ofs of
upper layersupper layers
– These e- will be lower in lower energy.These e- will be lower in lower energy.
– They will contribute to the noise signal of the spectrum.They will contribute to the noise signal of the spectrum.

19. X-Rays and the ElectronsX-Rays and the Electrons
X-RayElectron without collision
Electron with collision
The noise signal comes from
the electrons that collide
with other electrons of
different layers. The
collisions cause a decrease
in energy of the electron and
it no longer will contribute
to the characteristic energy
of the element.

20. What eWhat e--
s can the Cylindrical Mirror Analyzers can the Cylindrical Mirror Analyzer
Detect?Detect?
The CMA not only can detect electrons from theThe CMA not only can detect electrons from the
irradiation of X-Rays, it can also detect electrons fromirradiation of X-Rays, it can also detect electrons from
irradiation by the eirradiation by the e--
gun.gun.
The eThe e--
gun it is located inside the CMA while the X-Raygun it is located inside the CMA while the X-Ray
source is located on top of the instrument.source is located on top of the instrument.
The only electrons normally used in a spectrum fromThe only electrons normally used in a spectrum from
irradiation by the eirradiation by the e--
gun are known as Auger egun are known as Auger e--
s. Augers. Auger
electrons are also produced by X-ray irradiation.electrons are also produced by X-ray irradiation.

21. XPS Energy ScaleXPS Energy Scale
The XPS instrument measures theThe XPS instrument measures the
kinetic energy of all collectedkinetic energy of all collected
electrons. The electron signal includeselectrons. The electron signal includes
contributions from both photoelectroncontributions from both photoelectron
and Auger electron lines.and Auger electron lines.

22. KEKE = hv -= hv - BEBE -- ΦΦspecspec
Where:Where: BEBE= Electron Binding Energy= Electron Binding Energy
KEKE= Electron Kinetic Energy= Electron Kinetic Energy
ΦΦspecspec= Spectrometer Work Function= Spectrometer Work Function
If XPS spectra were presented on a kinetic energy scale,If XPS spectra were presented on a kinetic energy scale,
one would need to know the X-ray source energy used to collectone would need to know the X-ray source energy used to collect
the data in order to compare the chemical states in the samplethe data in order to compare the chemical states in the sample
with data collected using another source.with data collected using another source.
XPS Energy Scale- Kinetic energyXPS Energy Scale- Kinetic energy

23. XPS Energy Scale- BindingXPS Energy Scale- Binding
energyenergy
BEBE = hv -= hv - KEKE -- ΦΦspecspec
Where:Where: BEBE= Electron Binding Energy= Electron Binding Energy
KEKE= Electron Kinetic Energy= Electron Kinetic Energy
ΦΦspecspec= Spectrometer Work Function= Spectrometer Work Function
The binding energy scale was derived to make uniformThe binding energy scale was derived to make uniform
comparisons of chemical states straight forward.comparisons of chemical states straight forward.

24. Free electrons (those giving rise to conductivity) findFree electrons (those giving rise to conductivity) find
an equal potential which is constant throughout the material.an equal potential which is constant throughout the material.
Fermi-Dirac Statistics:Fermi-Dirac Statistics:
f(E) = 1f(E) = 1
exp[(E-Eexp[(E-Eff)/kT] + 1)/kT] + 1
1.01.0
f(E)f(E)
00
0.50.5
EEff1. At T=0 K:1. At T=0 K: f(E)=1 for E<Ef(E)=1 for E<Eff
f(E)=0 for E>Ef(E)=0 for E>Eff
2. At kT<<E2. At kT<<Eff (at room temperature kT=0.025 eV)(at room temperature kT=0.025 eV)
f(E)=0.5 for E=Ef(E)=0.5 for E=Eff
T=0 KT=0 K
kT<<EkT<<Eff
Fermi Level ReferencingFermi Level Referencing

25. Fermi Level ReferencingFermi Level Referencing
1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0
Ef
Fermi Edge of
TiN, room temperture
Binding energy (eV)
N(E)/E

26. Cylindrical Mirror Analyzer (CMA)Cylindrical Mirror Analyzer (CMA)
The electrons ejected will pass through a device called aThe electrons ejected will pass through a device called a
CMA.CMA.
The CMA has two concentric metal cylinders at differentThe CMA has two concentric metal cylinders at different
voltages.voltages.
One of the metal cylinders will have a positive voltage andOne of the metal cylinders will have a positive voltage and
the other will have a 0 voltage. This will create an electricthe other will have a 0 voltage. This will create an electric
field between the two cylinders.field between the two cylinders.
The voltages on the CMA for XPS and Auger eThe voltages on the CMA for XPS and Auger e--
s ares are
different.different.

27. Cylindrical Mirror Analyzer (CMA)Cylindrical Mirror Analyzer (CMA)
When the eWhen the e--
s pass through the metal cylinders, they wills pass through the metal cylinders, they will
collide with one of the cylinders or they will just passcollide with one of the cylinders or they will just pass
through.through.
– If the eIf the e--
’s velocity is too high it will collide with the outer’s velocity is too high it will collide with the outer
cylindercylinder
– If is going too slow then will collide with the inner cylinder.If is going too slow then will collide with the inner cylinder.
– Only the eOnly the e--
with the right velocity will go through the cylinderswith the right velocity will go through the cylinders
to reach the detector.to reach the detector.
With a change in cylinder voltage the acceptable kineticWith a change in cylinder voltage the acceptable kinetic
energy will change and then you can count how many eenergy will change and then you can count how many e--
ss
have that KE to reach the detector.have that KE to reach the detector.

28. Cylindrical Mirror Analyzer (CMA)Cylindrical Mirror Analyzer (CMA)
Slit
Detector
Electron Pathway through the CMA
0 V
+V
0 V 0 V
0 V
+V
+V
+V
X-Rays
Source
Sample
Holder

29. hv
Because the Fermi levels of the sample and spectrometer areBecause the Fermi levels of the sample and spectrometer are
aligned, we only need to know the spectrometer work function,aligned, we only need to know the spectrometer work function,
ΦΦspecspec, to calculate BE(1s)., to calculate BE(1s).
EE1s1s
SampleSample SpectrometerSpectrometer
ee--
Free Electron EnergyFree Electron Energy
Fermi Level, EFermi Level, Eff
Vacuum Level, EVacuum Level, Evv
Φsample
KE(1s) KE(1s)
Φspec
BE(1s)
Sample/Spectrometer Energy LevelSample/Spectrometer Energy Level
Diagram- Conducting SampleDiagram- Conducting Sample

30. hv
A relative build-up of electrons at the spectrometerA relative build-up of electrons at the spectrometer
raises the Fermi level of the spectrometer relative to theraises the Fermi level of the spectrometer relative to the
sample. A potential Esample. A potential Echch will develop.will develop.
EE1s1s
SampleSample SpectrometerSpectrometer
ee--
Free Electron EnergyFree Electron Energy
BE(1s)
Fermi Level, EFermi Level, Eff
Vacuum Level, EVacuum Level, Evv
KE(1s)
Φspec
Ech
Sample/Spectrometer EnergySample/Spectrometer Energy
Level Diagram- InsulatingLevel Diagram- Insulating
SampleSample

31. Binding Energy ReferencingBinding Energy Referencing
BEBE = hv -= hv - KEKE -- ΦΦspecspec- E- Echch
Where:Where: BEBE= Electron Binding Energy= Electron Binding Energy
KEKE= Electron Kinetic Energy= Electron Kinetic Energy
ΦΦspecspec= Spectrometer Work Function= Spectrometer Work Function
EEchch= Surface Charge Energy= Surface Charge Energy
EEchch can be determined by electrically calibrating thecan be determined by electrically calibrating the
instrument to a spectral feature.instrument to a spectral feature.
C1s at 285.0 eVC1s at 285.0 eV
Au4fAu4f7/27/2 at 84.0 eVat 84.0 eV

32. Where do Binding Energy ShiftsWhere do Binding Energy Shifts
Come From?Come From?
-or How Can We Identify Elements and Compounds?-or How Can We Identify Elements and Compounds?
Electron-electronElectron-electron
repulsionrepulsion
Electron-nucleusElectron-nucleus
attractionattraction
ElectronElectron
NucleusNucleus
BindingBinding
EnergyEnergy
Pure ElementPure Element
Electron-Electron-
NucleusNucleus
SeparationSeparation
Fermi LevelFermi Level
Look for changes hereLook for changes here
by observing electronby observing electron
binding energiesbinding energies

33. ElementalElemental ShiftsShifts
Binding Energy (eV)
Element 2p3/2 3p ∆
Fe 707 53 654
Co 778 60 718
Ni 853 67 786
Cu 933 75 858
Zn 1022 89 933
Electron-nucleus attraction helps us identify the
elements

34. Elemental ShiftsElemental Shifts

35. Binding Energy DeterminationBinding Energy Determination
The photoelectron’s binding energy will be
based on the element’s final-state configuration.
Conduction BandConduction Band
Valence BandValence Band
FermiFermi
LevelLevel
FreeFree
ElectonElecton
LevelLevel Conduction BandConduction Band
Valence BandValence Band
1s1s
2s2s
2p2p
Initial StateInitial State Final StateFinal State

36. The Sudden ApproximationThe Sudden Approximation
Assumes the remaining orbitals (often called the passive orbitals) areAssumes the remaining orbitals (often called the passive orbitals) are
the same in the final state as they were in the initial state (also calledthe same in the final state as they were in the initial state (also called
thethe frozen-orbital approximationfrozen-orbital approximation). Under this assumption, the XPS). Under this assumption, the XPS
experiment measures the negative Hartree-Fock orbital energy:experiment measures the negative Hartree-Fock orbital energy:
Koopman’s Binding EnergyKoopman’s Binding Energy
EEB,KB,K ≅≅ --εεB,KB,K
Actual binding energy will represent the readjustment of the N-1Actual binding energy will represent the readjustment of the N-1
charges to minimize energy (relaxation):charges to minimize energy (relaxation):
EEBB = E= Eff
N-1N-1
- E- Eii
NN

37. Binding Energy ShiftsBinding Energy Shifts
(Chemical Shifts)(Chemical Shifts)
Point Charge Model:Point Charge Model:
EEii = E= Eii
00
+ kq+ kqii ++ ΣΣ qqii/r/rijij
EEBB in atom i in givenin atom i in given
refernce staterefernce state
Weighted charge of iWeighted charge of i Potential at i due toPotential at i due to
surrounding chargessurrounding charges

38. XPS SpectrumXPS Spectrum
The XPS peaks are sharp.The XPS peaks are sharp.
In a XPS graph it is possible to see AugerIn a XPS graph it is possible to see Auger
electron peaks.electron peaks.
The Auger peaks are usually wider peaks in aThe Auger peaks are usually wider peaks in a
XPS spectrum.XPS spectrum.
Aluminum foil is used as an example on theAluminum foil is used as an example on the
next slide.next slide.

39. XPS Spectrum
O 1s
O because
of Mg source
C
Al
Al
O 2s
O Auger
Sample and graphic provided by William Durrer, Ph.D.
Department of Physics at the Univertsity of Texas at El Paso

40. Auger Spectrum
Characteristic of Auger graphs
The graph goes up as KE increases.
Sample and graphic provided by William Durrer, Ph.D.
Department of Physics at the Univertsity of Texas at El Paso

41. Identification of XPS PeaksIdentification of XPS Peaks
The plot has characteristic peaks for each elementThe plot has characteristic peaks for each element
found in the surface of the sample.found in the surface of the sample.
There are tables with the KE and BE already assignedThere are tables with the KE and BE already assigned
to each element.to each element.
After the spectrum is plotted you can look for theAfter the spectrum is plotted you can look for the
designated value of the peak energy from the graph anddesignated value of the peak energy from the graph and
find the element present on the surface.find the element present on the surface.

42. X-rays vs. eX-rays vs. e--
BeamBeam
X-RaysX-Rays
– Hit all sample area simultaneously permittingHit all sample area simultaneously permitting
data acquisition that will give an idea of thedata acquisition that will give an idea of the
average composition of the whole surface.average composition of the whole surface.
Electron BeamElectron Beam
– It can be focused on a particular area of theIt can be focused on a particular area of the
sample to determine the composition ofsample to determine the composition of
selected areas of the sample surface.selected areas of the sample surface.

43. Carbon-Oxygen BondCarbon-Oxygen Bond
Valence LevelValence Level
C 2pC 2p
Core LevelCore Level
C 1sC 1s
Carbon NucleusCarbon Nucleus
Oxygen AtomOxygen Atom
C 1sC 1s
BindingBinding
EnergyEnergy
Electron-oxygenElectron-oxygen
atom attractionatom attraction
(Oxygen Electro-(Oxygen Electro-
negativity)negativity)
Electron-nucleusElectron-nucleus
attraction (Loss ofattraction (Loss of
Electronic Screening)Electronic Screening)
Shift to higherShift to higher
binding energybinding energy
Chemical Shifts-Chemical Shifts-
Electronegativity EffectsElectronegativity Effects

44. Chemical Shifts-Chemical Shifts-
Electronegativity EffectsElectronegativity Effects
Functional
Group
Binding Energy
(eV)
hydrocarbon C-H, C-C 285.0
amine C-N 286.0
alcohol, ether C-O-H, C-O-C 286.5
Cl bound to C C-Cl 286.5
F bound to C C-F 287.8
carbonyl C=O 288.0

45. Electronic EffectsElectronic Effects
Spin-Orbit CouplingSpin-Orbit Coupling
2 8 4 2 8 0 2 7 62 8 82 9 0
B in d in g E n e r g y ( e V )
C 1 s
O r b i t a l = s
l = 0
s = + / - 1 / 2
l s = 1 / 2

46. Electronic EffectsElectronic Effects
Spin-Orbit CouplingSpin-Orbit Coupling
9 6 5 9 5 5 9 4 5 9 3 5 9 2 5
1 9 . 8
B in d in g E n e r g y ( e V )
C u 2 p
2 p 1 / 2
2 p 3 / 2
P e a k A r e a 1 : 2
O r b i t a l = p
l s = 1 / 2 , 3 / 2
l = 1
s = + / - 1 / 2

47. Electronic EffectsElectronic Effects
Spin-Orbit CouplingSpin-Orbit Coupling
3 7 03 7 43 7 8 3 6 6 3 6 2
6 . 0
B in d in g E n e r g y ( e V )
P e a k A r e a 2 : 3
A g 3 d
3 d 3 / 2
3 d 5 / 2
O r b i t a l = d
l s = 3 / 2 ,5 / 2
l = 2
s = + / - 1 / 2

48. Electronic EffectsElectronic Effects
Spin-OrbitCouplingSpin-OrbitCoupling
3 . 6 5
8 79 1 8 3 7 9
B in d in g E n e r g y ( e V )
P e a k A r e a 3 : 4
A u 4 f
4 f 5 / 2
4 f 7 / 2
O r b i t a l = f
l = 3
s = + / - 1 / 2
l s = 5 / 2 ,7 / 2

49. Electronic Effects- Spin-Orbit CouplingElectronic Effects- Spin-Orbit Coupling
Ti MetalTi Metal Ti OxideTi Oxide

50. Final State Effects-Final State Effects-
Shake-up/ Shake-offShake-up/ Shake-off
Monopole transition: Only the principleMonopole transition: Only the principle
quantum number changes. Spin andquantum number changes. Spin and
angular momentum cannot change.angular momentum cannot change.
Shake-up: Relaxation energy used toShake-up: Relaxation energy used to
excite electrons in valence levels toexcite electrons in valence levels to
bound states (monopole excitation).bound states (monopole excitation).
Shake-off: Relaxation energy used toShake-off: Relaxation energy used to
excite electrons in valence levels toexcite electrons in valence levels to
unbound states (monopole ionization).unbound states (monopole ionization).
Results from energy made available in the relaxation of the finalResults from energy made available in the relaxation of the final
state configuration (due to a loss of the screening effect of thestate configuration (due to a loss of the screening effect of the
core level electron which underwent photoemission).core level electron which underwent photoemission).
L(2p) -> Cu(3d)L(2p) -> Cu(3d)

51. Final State Effects-Final State Effects-
Shake-up/ Shake-offShake-up/ Shake-off
Ni MetalNi Metal Ni OxideNi Oxide

52. Final State Effects- Multiplet SplittingFinal State Effects- Multiplet Splitting
Following photoelectron emission, theFollowing photoelectron emission, the
remaining unpaired electron mayremaining unpaired electron may
couple with other unpaired electrons incouple with other unpaired electrons in
the atom, resulting in an ion withthe atom, resulting in an ion with
several possible final stateseveral possible final state
configurations with as many differentconfigurations with as many different
energies. This produces a line whichenergies. This produces a line which
is split asymmetrically into severalis split asymmetrically into several
components.components.

53. Electron Scattering EffectsElectron Scattering Effects
Energy Loss PeaksEnergy Loss Peaks
Photoelectrons travelling through thePhotoelectrons travelling through the
solid can interact with other electrons insolid can interact with other electrons in
the material. These interactions can resultthe material. These interactions can result
in the photoelectron exciting an electronicin the photoelectron exciting an electronic
transition, thus losing some of its energytransition, thus losing some of its energy
(inelastic scattering).(inelastic scattering).
eph + esolid
e*ph + e**solid

54. XPS of Copper-Nickel alloyXPS of Copper-Nickel alloy
- 1 1 0 0 - 9 0 0 - 7 0 0 - 5 0 0 - 3 0 0 - 1 0 0
0
2 0
4 0
6 0
8 0
1 0 0
1 2 0
Thousands
B in d in g E n e r g y (e V )
N(E)/E
P e a k
A r e a
M c t- e V /s e c
R e l.
S e n s .
A t o m ic
C o n c
%
N i 2 .6 5 4 .0 4 4 4 9
C u 3 .6 5 5 .3 2 1 5 1
C u 2 p
C u 3 p
N i 2 p
N i 3 p
N i
L M M N i
L M M N i
L M M
C u
L M M
C u
L M M
C u
L M M

55. Comparison of SensitivitiesComparison of Sensitivities
A T O M I C N U M B E R
2 0 4 0 6 0 8 0 1 0 0
5 E 1 3
5 E 1 6
5 E 1 9
H N e C o Z n Z r S n N d Y b H g T h
1 %
1 p p m
1 p p b
0
R BS
A E S a n d X PS
S IM S
P IX EPIX E

56. Instrumentation for X-rayInstrumentation for X-ray
PhotoelectronPhotoelectron
SpectroscopySpectroscopy

57. Introduction to X-ray PhotoelectronIntroduction to X-ray Photoelectron
Spectroscopy (XPS)Spectroscopy (XPS)
What is XPS?- General TheoryWhat is XPS?- General Theory
How can we identify elements andHow can we identify elements and
compounds?compounds?
Instrumentation for XPSInstrumentation for XPS
Examples of materials analysis withExamples of materials analysis with
XPSXPS

58. Instrumentation for XPSInstrumentation for XPS
Surface analysis by XPS requiresSurface analysis by XPS requires
irradiating a solid in an Ultra-high Vacuumirradiating a solid in an Ultra-high Vacuum
(UHV) chamber with monoenergetic soft X-(UHV) chamber with monoenergetic soft X-
rays and analyzing the energies of therays and analyzing the energies of the
emitted electrons.emitted electrons.

59. Remove adsorbed gases fromRemove adsorbed gases from
the sample.the sample.
Eliminate adsorption ofEliminate adsorption of
contaminants on the sample.contaminants on the sample.
Prevent arcing and high voltagePrevent arcing and high voltage
breakdown.breakdown.
Increase the mean free path forIncrease the mean free path for
electrons, ions and photons.electrons, ions and photons.
Degree of VacuumDegree of Vacuum
1010
1010
1010
1010
1010
22
-1-1
-4-4
-8-8
-11-11
Low VacuumLow Vacuum
Medium VacuumMedium Vacuum
High VacuumHigh Vacuum
Ultra-High VacuumUltra-High Vacuum
PressurePressure
TorrTorr
Why UHV for Surface Analysis?Why UHV for Surface Analysis?