Local-field effects on inelastic electron scattering

The wave-vector-dependent local-field corrections to the dielectric response of a solid are discussed. These effects are explicitly calculated in the point-dipole approximation which is shown to be valid even if only the initial state is well localized. A generalization to arbitrary wave vector of the classical Clausius-Mossotti equation is given and the relationship of this new equation to lattice-dynamical calculations is discussed. Our results indicate that local fields can have a substantial influence on the wave-vector dependence of the dielectric-response functions as measured by inelastic electron scattering. In some cases they can dominate other wave-vector-dependent effects. Their importance for insulators and also for metals in the vicinity of their x-ray thresholds is emphasized. In particular the predicted spectra for CsCl are given for various directions of the wave vector. A comparison of actual data with these results can lead to a better understanding of the initial states contributing to the interband transitions.