Photoelectron spectroscopy

Photoelectron spectroscopy
- a single photon in/ electron out process
• X-ray Photoelectron Spectroscopy (XPS)
- using soft x-ray (200-2000 eV) radiation to

examine core-levels.
• Ultraviolet Photoelectron Spectroscopy (UPS)
- using vacuum UV (10-45 eV) radiation to
examine valence levels.

1. Photoelectron Spectroscopy
&XPS UPS
Addis Ababa University
Department of Chemistry
PhD Program
Presenter: Tesfaye Haile
14
th
May 2015

2. Outline
•Introduction
•X-ray Photoelectron Spectrometry
oInstrumentationInstrumentation
oX-ray Sources
oSpectral interpretations
•Ultraviolet Photoelectron Spectroscopy

3. Photoelectron spectroscopy detects the kinetic
energy of the electron escaped from the surface.
Introduction
energy of the electron escaped from the surface.

4. Photoelectron spectroscopy
-a single photon in/ electron out process
•X-ray Photoelectron Spectroscopy (XPS)
-using soft x-ray (200-2000 eV) radiation to
Introduction
-using soft x-ray (200-2000 eV) radiation to
examine core-levels.
•Ultraviolet Photoelectron Spectroscopy (UPS)
-using vacuum UV (10-45 eV) radiation to
examine valence levels.

5. The VUV, XUV, and soft x-ray regions
Soft x-rays
5 nm > l > 0.5 nm
Strongly interacts with core
electronsin materials
Vacuum-ultraviolet (VUV)
180 nm > l > 50 nm
Absorbed by <<1 mm of air
Ionizing to many materials
Extreme-ultraviolet (XUV)
50 nm > l > 5 nm
Ionizing radiation to allmaterials

6. Part-I
X-ray Photoelectron Spectrometry X-ray Photoelectron Spectrometry
()XPS

7. X-ray Photoelectron Spectrometry
(XPS)
•X-rayPhotoelectronSpectroscopy(XPS),alsoknown
asElectronSpectroscopyforChemicalAnalysis
(ESCA)isawidelyusedtechniquetoinvestigatethe
chemicalcompositionofsurfaces.
•Photoelectronspectroscopyisusedforsolids,liquids
andgases,buthasachievedprominenceasan
analyticaltechniqueforsolidsurfaces

8. …..XPS
•X-rayPhotoelectronspectroscopy,basedonthe
photoelectriceffect,wasdevelopedinthemid-1960’s
byKaiSiegbahnandhisresearchgroupatthe
UniversityofUppsala,Sweden.

9. A)Whatelementsarepresentinthesurfaceregion.
B)Often,alsothechemicalstateoftheelementscanbe
determined,
Photoemission, what’s it used for ?
C)Thesurfacegeometrycanbequalitativelydetermined.
D)Theband-structureofthesolidcanbemeasured.

10. X-ray Beam
X-ray penetration depth
Electrons are extracted only from a
narrow solid angle.
1 mm
2
10 nm
X-ray penetration depth
~1mm.
Electrons can be excited in
this entire volume.
X-ray excitation area ~1x1 cm
2
.
Electrons are emitted from this entire area

11. XPS spectral lines are identified
by the shell from which the
electron was ejected (1s, 2s, 2p,
etc.).
Conduction BandConduction Band
FermiFermi
LevelLevel
Free Free
Electron Electron
LevelLevel
Incident XIncident X--rayray
Ejected PhotoelectronEjected Photoelectron
The Photoelectric Process
The ejected photoelectron has
kinetic energy:
KE = KE = hvhv--BEBE--
Following this process, the atom
will release energy by the
emission of an Auger Electron.
Valence BandValence Band
L2,L3L2,L3
L1L1
KK
LevelLevel
1s1s
2s2s
2p2p

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

13. XP SPECTROMETERS

14. XP SPECTROMETERS….

15. X-ray Sources Available

16. XPS Photoelectron Binding Energies versus Atomic Number (Z)

17. A Typical XPS spectrum
1. Sharp peaks due to photoelectrons created within the first few
atomic layers (elastically scattered).
2. Multipletsplitting(occurs when unfilled shells contain unpaired
electrons).
3. A broad structure due to electrons from deeper in the solid
which are ineslasticallyscattered (reduced KE) forms the
background.
4. Satellites (shake-off and shake-up) are due to a sudden change
in Coulombicpotential as the photoejectedelectron passes
through the valence band.

18. 5. Plasmonswhich are created by collective excitations of the
valence band
‧Extrinsic Plasmon: excited as the energetic PE propagates through the
solid after the photoelectric process
A Typical XPS spectrum…
‧Intrinsic Plasmon: screening response of the solid to the sudden
creation of the core hole in one of its atom
The two kinds of Plasmon are indistinguishable.
6. Auger peaks produced by X-rays (transitions from L to K shell:
O KLL or C KLL).

19. Sharp Peak (core level)

20. MultipletSplitting

21. MultipletSplitting

22. •Arisewhenacoreelectronisremovedbya
photoionization.
•Thereisasuddenchangeintheeffectivechargedueto
thelossofshieldingelectrons.
Satellites
Thisperturbationinducesatransitioninwhichan
electronfromabondingorbitalcanbetransferredto
ananti-bondingorbitalsimultaneouslywithcore
ionization.

23. Twotypesofsatellitearedetected.
1.Shake-up:Theoutgoingelectroninteractswithavalence
electronandexcitesit(shakesitup)toahigherenergy
level.Asaconsequencetheenergyofcoreelectronis
reducedandasatellitestructureappearsafeweVbelow
Satellites….
reducedandasatellitestructureappearsafeweVbelow
(KEscale)thecorelevelposition.
2.Shake-off:Thevalenceelectronisejectedfromtheion
completely(tothecontinuum).Appearsasabroadeningof
thecorelevelpeakorcontributetotheinelastic
background.

24. Shake-up
satellites distinct
peaks a few eV
below the main
Line
Shake-off
satellites broad
featureat lower
energy w.r.t. to
main line

25. Plasmons
This feature is specific
to clean surfaces.
The photoelectron
excites collective
oscillations in theoscillations in the
conduction band (free-
electron gas), so called
Plasmons.
(discrete energy loss).

26. •Chemicalshiftarisesintheinitialstatefromthedisplacementof
theelectronicchargefromtheatomtowardsitsligands,reducing
theelectrostaticpotentialattheatom.
•Thereisafinalstateshiftduetothepolarizationoftheligandby
thecoreonthecentralatom.
Chemical shift
•CoreelectronBEinmolecularsystemsexhibitschemicalshifts
whicharesimplyrelatedtovariousquantitativemeasuresof
covalency.
•Greatertheelectronegativityoftheligands,thegreatertheBEof
thecoreelectronoftheligatedatom.

27. Basic concept: The core electrons feel an alteration in the chemical
environment when a change in the potential (charge distribution)
of the valence shell occurs.
For example assume that the core electrons are inside a hollow
spherical charged shell. Each core electron then sees a potential. A spherical charged shell. Each core electron then sees a potential. A
change in Q by ΔQ gives a change in V.
where ΔV is the chemical shift

28. Oxidized metal surfaces
•Oxidized and clean Cr 2p spectra (left). Oxidized and clean Cu 2p spectra (right).
•The oxide layer resulted in extra peaks (shoulder at higher BE—left of the main line).
•Satellites are also seen on the Cu 2p spectra.

29. The core level binding energies are found to depend on the chemical
state of the atom under investigation.
Oxidized metal surfaces….
WHY ?
Strange as the core levels do NOT
take part in the bonding
Ti
Ti
+

30. XPS: Chemical Shifts
Peaks appear in XPS spectra
for distinguishable atomic
and molecular orbitals.
Effects that cause chemical
shifts in XPS spectra:shifts in XPS spectra:
Oxidation states
Covalent structure
Neighboring electron
withdrawing groups
Anything else that can
affect ionization/orbital
energies

31. Electronic Effects
Spin-Orbit Coupling

32. End of XPS !!! End of XPS !!!

33. Part-II
Ultraviolet Photoelectron SpectroscopyUltraviolet Photoelectron Spectroscopy
()UPS

34. Ultraviolet Photoelectron Spectroscopy
(UPS)
•Similar to XPS but using vacuum UV (10-45 eV)
radiation to examine valence levels.
•In UPS the source of radiation is normally a noble gas •In UPS the source of radiation is normally a noble gas
discharge lamp.
frequently a He-discharge lamp emitting
He I = 21.2 eV ( ~ 58.4 nm )
He II = 40 eV ( ~ 30.4 nm )

35. •Such radiation is only capable of ionising electrons
from the outermost levels of atoms -the valence
levels.
•The advantage of using such UV radiation over X-rays •The advantage of using such UV radiation over X-rays
is the very narrow line width of the radiation and the
high flux of photons available from simple discharge
sources.

36. Ultraviolet photoelectron spectra of atoms
The He I (21.22 eV) spectrum of argon
Molecular Photoelectron Spectroscopy, p. 41, John Wiley, London, 1970

37. Thank You !!!Thank You !!!