ultraviolet photoelectron spectroscopy abbr., UPES (rus. спектроскопия, ультрафиолетовая фотоэлектронная abbr., УФЭС) — a type of photoelectron spectroscopy where excitation of photoelectrons is ensured through ultraviolet emission and which is used to probe the filled electron states in the valence and conduction bands in the sample's surface layer.

Description

Unlike molecular electronic spectroscopy which is used to study matter by its photon absorption and emission spectra corresponding to transitions of electrons from one energy level to another, the main tool applied by ultraviolet photoelectron spectroscopy (UPES) to study samples' structure is the registration of photoemission i.e. electrons that have left the material due to photoionisation.

As laboratory energy sources, the UPES method uses discharge lamps, usually helium. In these sources, depending on their gas pressure and discharge current, one of two strong lines with photon energies of 21.2 eV (He I) and 40.8 eV (He II) is generated. Because of the fact that the UPES method uses relatively low-energy photons, only valence levels are excited in the photoemission process. In addition to that, except for levels corresponding to occupied states of the studied surface, the photoelectron spectrum can also be affected by occupied orbitals of adsorbed molecules. Due to the large cross-section of photoemission associated with valence states at excitation energies used in UPES this method is a powerful tool to study the structure of valence bands on a material's surface and its modifications caused by various processes occurring on the surface, such as adsorption, growth of thin films, chemical reactions, etc.

Depending on the task, the UPES method is typically used in one of the two modes: UPES with angular integration and UPES with angular resolution.

Ideally, the angular integration UPES detects all photoelectrons emitted into the sample's upper half-surface. The resulting data is used to determine the distribution of states' density in the valence band.

The angular resolution UFES detects photoelectrons emitted only to a certain pre-selected direction. This type of measurement records not only an electron's energy, but also its wave vector, which makes it possible to determine the dispersion law for surface states.

Authors

  • Zotov Andrey V.
  • Saranin Alexander A.

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