Electron Damping in Surface Studies

Abstract

Electron correlation is responsible for finite lifetimes of excited electrons in crystals. Lifetime energy dependence can be obtained for infinite jellium model and only very recently the first results for an infinite crystal have been evaluated (GW approximation). Here, a phenomenological approach, based on Green functions, is presented. Broadening of local densities of electron states as well as that of angular-resolved photoemission (ARUPS) peaks and very-low-energy-electron diffraction (VLEED) profiles, due to the imaginary component of the optical potential is reviewed and interpreted. Anisotropy of electron damping on crystal surfaces has been found in VLEED as a result of electron channeling along the densely packed (111) surface atomic planes in fcc crystals. Interpretation of peak widths in VLEED-and ARUPS-profiles provides a mean to learn about damping of electrons, excited on crystal surface. Quasi-particle lifetimesOne-particle approximation in the electron theory of crystals is surprisingly successful. This fact is explained by dressing the bare particles to form the quasi--particles with only a weak interaction (effective screening of correlation and exchange interactions).Quasi-particles, describing excited states of the system of electrons, have been studied in detail for a simplified model of jellium, where individual positive charges of ions are replaced by a uniform positive charge background.It has been found that the dispersion relation E(k) of electrons gets somewhat modifled (effective mass renormalization) and also that their lifetime (expressed as an imaginary part of electron self-energy) is in general finite. The electron lifetime is infinite at the Fermi level but shortens when electron is excited above the EF ; during its collisions with other electrons, electron-hole pairs can be created, and starting from the plasmon threshold energy also collective oscillations of the system can be induced. Numerical results for such an interacting electron gas have been obtained in the random phase approximation for electron densities typical for metals [1].(33)