Interpretation of valence loss spectra from composite media

Abstract

We discuss the interpretation of valence EELS data from composite media containing defects, clusters or precipates which may be difficult to resolve individually because of their small size or because of overlap effects. In some cases a well-defined collective loss (plasmon) peak is observed the position of which shifts slightly with changing defect concentration. In other cases discussed here the most noticeable effect is the appearance of new peaks in the spectrum which can be identified with interface as well as bulk excitations. Effective medium theories, although widely used to interpret optical absorption data from such samples, are only partially successful in explaining the energy loss spectra observed and do not account for the bulk loss peak from the minority phase. A simple dielectric excitation model where an appropriate average is made over the trajectories of a classical particle in the composite medium appears to offer a much better explanation and could be a useful way of characterising the fine-scale structure of composite samples. The connection between this trajectory approach and effective medium theory is discussed.