Plasmon excitations at diffuse interfaces

Energy losses experienced by a fast electron probe moving through a dielectric medium have been studied both numerically and analytically, where the response function varies continuously with position in one transverse direction. The frequent assumption that the loss spectrum should exhibit a peak determined by the plasmon energy in a homogeneous medium with the composition found locally at the probe position can be incorrect. In free electron systems, inhomogeneous effects can cause spectral shape changes as well as peak shifts. Computations for diffuse interfaces between semiconductors with differing band gaps are also reported. Prospects for improved spatial resolution in valence loss spectroscopy at higher momentum transfer are discussed.