Photon electron spectroscopy (PES)
XPS
UPS
The three-step model
infos
photo-ionization effect is used to release electrons
from a sample surface. The kinetic energy distribution of the emitted photoelectrons is
studied and used to determine the composition and electronic state of surface
monochromatic sources of radiation to achieve a higher energy resolution
core (inner shell) electron.
valence shell electrons
kinetic energy distribution of the emitted photoelectrons can be measured usingelectron energy analyser and a photoelectron spectrum can thus be recorded.
distribution of momentum with respect to the crystal axes of the sample can be measured with an angle-resolved spectrometer.
2) the photoelectron propagates to the surface. The information about the original binding energy of the electron is only conserved if it reaches the surface unscattered, i.e without energy loss
3) the photoelectron overcomes the potential barrier to the vacuum
1) excitation of the photoelectron by the incident photon
Binding Energy
binding energy of a core level electron depends on the chemical and physical environment
Electrons of atoms at the surface can have different binding energies than those of the bulk.
2 distinct core level lines --> multiplet – splitting of the final state --> two final states with different energy are possible (J = L ± S) --> photoelectrons of two different kinetic energies appear.
Depends also on the formal oxidation state of the atom. A peak shift is expected for different oxidation states within different compounds. A higher positive oxidation state exhibit a higher binding energy due to the extra coulombic interaction between the photoemitted electron and the ion core.
Allows the identification of elements present at a surface and its oxidation state
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Constant potential (Jellium model): the photoelectron emission process can not occur, since conservation of momentum and energy are not met at same time:The photon momentum is so small that transition can
only occur if the momentum of the electron involved is conserved. This kind of transitions
do not exist within this model.
Periodic potential: Conservation of momentum is the consequence of symmetry of space. In empty space we
have complete translational invariance. Moving in a periodic potential, the momentum is conserved up to a reciprocal lattice vector. Without any translational symmetry there is no conservation of momentum.
An electron leaving the surface always encounters symmetry parallel to the surface. But perpendicular to the surface, the translational symmetry is broken
Properties of surface states
The energy of the surface states is located within the energy gap of the bulk states.
The feature in an EDC corresponding to a surface state is independent of k⊥ but shows dispersion as a function of k||
react sensitively to contamination
Soft criterion: features corresponding to surface states in EDCs appear narrower than bulk states
“Tamm” – states: are more strongly localized and are described in the „Tight Binding Model“. Ex: materials with directional bonds (transition metals, semiconductors,
insulators).
Shockley – states:strongly deslocalized and it's described in a model of electrons in a periodic potential Ex:surface states of pure metals (Be, Cu), originate from s- and
p- electrons.
strongly affected by adsorbates: surface state of gold shifts to higher binding energy with increasing Na-coverage. This shift is due to a charge transfer from the Na to the gold surface state
with the inverse process
(electron in → photon out) information on the unoccupied states can be obtained.