X-Ray Photoelectron Spectroscopy

Photoelectron spectroscopy with characteristic x-rays was developed into a useful analytical technique by Kai Siegbahn and collaborators at Uppsala in the late sixties. The most important applications depend on the ability to measure small shifts in core-electron binding energies, which are related to the valence state and chemical bonding of the atom. In metals the core-electron binding energy is a function of the cohesive energy of the solid. Changes in the cohesive energy at the surface of a bulk metal, or in a small metal cluster, find expression in the core-electron binding energy. More detailed analysis of core-electron photoemission spectra from metals reveals the contributions of the collective excitations which accompany the ejection of an energetic electron. These include the screening response of the conduction electrons, which modifies the shape of primary line, and the excitation of plasmons and interband transitions, which manifest themselves as satellites of this line. The technique can also be used to obtain the valence band density of states of metals and alloys, and is particularly well adapted to the elemental analysis of thin (20 A) surface layers.