Intensities in Filtered and Unfiltered Electron Diffraction from Thin Films

Abstract

LOSS electrons are frequently considered to be unimportant in electron diffraction from thin films. Indeed, their effects are usually ignored, because few diffraction instruments are capable of eliminating them. The basis for this belief is that if the thickness of the films is of the order of 100 Å, this is less than the mean free path Λ for all electron scattering processes, and that therefore only a small fraction of the electrons will suffer two scattering acts. For electrons which have lost energy by collective excitation, two such acts are needed to put a loss electron among the diffracted ones, because collective cross-sections are small-angle; but there are also core excitations which are not small-angle. Films about 100 Å thick should in general be prepared in the high vacuum of the diffractometer and a support film is necessary; films thinner than 100 Å usually need a support film in any case. Of the practicable substrates carbon is one of the lightest and strongest; the situation in which loss electrons might most probably be neglected is therefore that of a heavy element a few angstroms in average thickness supported by a thin carbon membrane.