Optical potentials and damped one-electron functions in photoemission and electron scattering

Recently a new approach to photoemission, EXAFS and electron scattering has been developed. The correlations between target electrons are separated from the active electron-target electron correlations, and the damping effects are coming out explicitly. The theory uses van Hove diagonal Green's functions and projection operator techniques, not the usual diagram expansions. By omitting from the start terms in the Hamiltonian which lead to reabsorption of the active electron (photoelectron or scattered electron), remarkably simple expressions are obtained. These expressions contain optical potentials which damp the electron propagation, and “fluctuation potentials” which determine the inelastic scattering. The fluctuation potentials also appear in the expressions for the optical potentials. On the RPA level of approximation the fluctuation potentials are obtained from the dielectric response function of the target. The theory is quite general, but its main advantages are for extended systems, where the damped propagators and damped one electron functions are important effects. The present theory should make it possible to quantitatively attack a number of correlation effects in different spectroscopies.