Many-Electron Effects in Spectra of Electrons Bound in Atoms and Solids

Abstract

The study of elementary excitations and not so elementary excitations in solids forms the main theme of this Advanced Study Institute. It is the purpose of these lectures to give a brief introduction to the analog questions in atomic systems and with a particular point of view. Rather than just reviewing recent progress made in atomic theory I wish to emphasize that these problems can be discussed using essentially the same concepts and similar theoretical methods which are used for solids. Atomic theory has to a large extent followed its own different path, partly because many problems for light atoms can be solved with direct numerical methods but also because of a certain reluctance in applying the more powerful methods used in solid state theory. In removal of one electron, for example, one can in principle find the threshold energy by comparing two Hartree-Fock calculations which is possible for a system which is small enough but becomes impractical for a larger system. Similarly, autoionizing resonances can be handled by direct numerical methods for the lightest atoms. For heavier atoms and of course of extended systems the more powerful methods of many-body theory are needed to develop a theory from which the line-shape parameters can be calculated. The examples just given refer to cases where techniques used in many-body solid state theory could with advantage be applied to atomic systems, but where the simplest cases can be treated by brute force.