Auger Spectroscopy of Solid Surfaces: Electron versus Ion Excitation

Auger transitions involving outer electrons in free atoms and in solids are compared with particular reference to L 23 deexcitation in elements Na to Si. Under electron or x-ray excitation the elemental solid state spectra are band-like in character with breadth twice that of the conduction valence band, and a shape determined in detail by matrix element, surface and electronic relaxation effects. This is in strong contrast to the L 23 VV spectrum of Cu which is quasi-atomic in character and the M 45 VV spectrum of various Ag alloys which show mixed atomic-band like behaviour. Under ion excitation a superposition of a solid state spectrum and an excited atom spectrum is observed. Various theories of the origin of the quasi-atomic spectrum are considered, and attempts are made to account for the energies in the atomic spectrum and the relative intensities of atomic and solid state features. Calculations of the detailed motions of atoms in the surface region following inner shell ionisation are reviewed, and the sensitivity of atomic yield to inner core lifetime and the angle of incidence of the ion beam is discussed.