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Photoemission Spectroscopy

Photoemission Spectroscopy (also known as Photoelectron Spectroscopy, PES) probes the energy levels of electrons, or more in general, the nature of chemical bonding and electron motion in a substance. PES is based on the Photoelectric Effect, which means that when light impinging on a surface is absorbed it induces the emission of electrons. Together with the related Auger spectroscopy, the PES technique is commonly referred as Electron Spectroscopy for Chemical Analysis (ESCA) and was pioneered by Swedish physicist Kai Siegbahn.

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Basic Features of PES

The electronic transitions are selected by tuning the energy of the incident radiation (a). The electrons bring information via the Kinetic Energy, the direction of the momentum (the angle between the impinging radiation and emitted electrons, and their angle with the surface) and spin (b). The basic relation is $E_k=h\nu-\Phi-E_b$, with $\Phi$ the work function, $\nu$ the light frequency, and $E_b$ the binding energy. Photoelectrons suffer elastic/inelastic scattering when leaving the sample. The escape depth in solids is only few Ångström. This sometimes makes it problematic to separate surface and bulk contributions. In an atom excited by X rays, the main structures in the spectrum are due to photoelectrons. There are however also other peaks, of similar width but different energy, unrelated to incident photons, due to Auger recombination (c). In a proper theoretical formulation, PES and the Auger recombination should be treated on equal footing as a single coherent process.

In a theory of energy and angle resolved photoemission, a basic quantity of interest is the spectral function $A(q,E)=-\frac{1}{\pi}\frac{Im \Sigma(q,E)}{[E-\epsilon_q-Re\Sigma(q,E)]^2+Im\Sigma(q,E)^2}$, where $\epsilon_q$ is the one-body part and $\Sigma(q,E)$ accounts for the self-energy corrections. The quantity $A(q,E)$, to be compared to experimental PES spectra, is computed at the ab-initio level, and incorporates the information about the electronic energy bands, about the effects of correlation between electrons, the structure of the Fermi surface, the role of lattice vibrations (and in a more general form) about spin ordering.

Light sources

Traditional radiation sources are X-ray (XPS) and ultraviolet radiation (UPS). The use of synchrotron radiation has made such division somewhat conventional. In synchrotron sources the photon energy can be chosen continuously over a wide energy range. Other qualities of synchrotron light are high brightness and intensity, tunable polarization, the feasibility of short time-scale, time-resolved experiments.

Photoemission and the ETSF

What

  • Reliable quasiparticle energies and band-gaps.
  • Core and valence photoemission, angle resolved photoemission, thermal effects and electron-phonon coupling.
  • Photoemission beyond the sudden approximation, lifetimes of electrons and holes, dependence of spectra on photon energy, spectral functions.
  • Auger spectra.

Where

  • Metals, semiconductors, molecules, surfaces, nanosystems, including e.g. transition metals and their alloys, transition-metal oxides, graphite, etc.

How

  • Density functional theory.
  • Many-body techniques: GW, T-matrix-approximation.

Beamline Coordinator

Dr. Claudio Verdozzi
Lund University, Lund, Sweden
Claudio [dot] Verdozzi [at] teorfys [dot] lu [dot] se

References