Deficiencies in the single particle picture of valence band photoemission

Abstract

The applicability of the single particle theoretical model of photoemission from solids and solid surfaces will be evaluated by comparison of data and theory for simple two- and three-dimensional systems. For nearly free electron metals like Mg, Al and Be the agreement between band theory and measured dispersion for both the occupied and unoccupied bulk bands is fairly good. Yet quantitative discrepancies do exist, arising from finite hole and electron lifetimes, electron-plas-mon coupling and inadequacies of local density theory. Many of these effects can qualitatively be explained with an interactive electron gas theory. The two-dimensional dispersion of intrinsic and extrinsic surface states (or resonances) is reproduced fairly well by single particle calculations. At this stage of our development most of the discrepancies between theory and experiment for surface states can not be blamed on deficiencies of either the single particle approach or the local density approximation. Several clear experimental examples do exist where single particle theory can not be applied. These include multielectron satellite lines in adsorbate spectra, intensity variations due to spatially varying electromagnetic fields and k dependent adsorbate line widths.