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.
2. Outline
• Introduction
• X-ray Photoelectron Spectrometry
o InstrumentationInstrumentation
o X-ray Sources
o Spectral 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
electrons in 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 all materials
7. X-ray Photoelectron Spectrometry
(XPS)
• X-ray Photoelectron Spectroscopy (XPS), also known
as Electron Spectroscopy for Chemical Analysis
(ESCA) is a widely used technique to investigate the
chemical composition of surfaces.
• Photoelectron spectroscopy is used for solids, liquids
and gases, but has achieved prominence as an
analytical technique for solid surfaces
8. …..XPS
• X-ray Photoelectron spectroscopy, based on the
photoelectric effect, was developed in the mid-1960’s
by Kai Siegbahn and his research group at the
University of Uppsala, Sweden.
9. A) What elements are present in the surface region.
B) Often, also the chemical state of the elements can be
determined,
Photoemission, what’s it used for ?
C) The surface geometry can be qualitatively determined.
D) The band-structure of the solid can be measured.
10. X-ray Beam
X-ray penetration depth
Electrons are extracted only from a
narrow solid angle.
1 mm2
10 nm
X-ray penetration depth
~1mm.
Electrons can be excited in
this entire volume.
X-ray excitation area ~1x1 cm2.
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
FreeFree
ElectronElectron
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
FreeFree
ElectronElectron
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
17. A Typical XPS spectrum
1. Sharp peaks due to photoelectrons created within the first few
atomic layers (elastically scattered).
2. Multiplet splitting (occurs when unfilled shells contain unpaired
electrons).
3. A broad structure due to electrons from deeper in the solid
which are ineslastically scattered (reduced KE) forms the
background.
4. Satellites (shake-off and shake-up) are due to a sudden change
in Coulombic potential as the photoejected electron passes
through the valence band.
18. 5. Plasmons which 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).
22. • Arise when a core electron is removed by a
photoionization.
• There is a sudden change in the effective charge due to
the loss of shielding electrons.
Satellites
This perturbation induces a transition in which an
electron from a bonding orbital can be transferred to
an anti-bonding orbital simultaneously with core
ionization.
23. Two types of satellite are detected.
1. Shake-up: The outgoing electron interacts with a valence
electron and excites it (shakes it up) to a higher energy
level. As a consequence the energy of core electron is
reduced and a satellite structure appears a few eV below
Satellites….
reduced and a satellite structure appears a few eV below
(KE scale) the core level position.
2. Shake-off: The valence electron is ejected from the ion
completely (to the continuum). Appears as a broadening of
the core level peak or contribute to the inelastic
background.
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. •Chemical shift arises in the initial state from the displacement of
the electronic charge from the atom towards its ligands, reducing
the electrostatic potential at the atom.
•There is a final state shift due to the polarization of the ligand by
the core on the central atom.
Chemical shift
•Core electron BE in molecular systems exhibits chemical shifts
which are simply related to various quantitative measures of
covalency.
•Greater the electronegativity of the ligands, the greater the BE of
the core electron of the ligated atom.
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. Aspherical 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
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
