Cluster multiple-scattering theory for medium-energy electron diffraction

Abstract

A theory of medium-energy (100–5000-eV) electron diffraction (MEED) is developed from a multiple-scattering, curved-wave theory of photoelectron diffraction. It may be called ‘‘near-field expansion in clusters.’’ Only selected important scattering events are included and these are computed in times proportional to electron wave number by using a generalized scattering-factor method (conventional low-energy electron-diffraction methods require computations proportional to at least the fourth power of the wave number, while the ‘‘chain’’ method for MEED scales as at least the square of the wave number). This removes the most serious barrier to a multiple-scattering analysis for surface-structure determination. A direct summation over atoms and scattering paths is used, avoiding any assumptions of periodicity in the surface structure. The theory allows a clearer understanding of the relationship between diffraction intensities and surface structure than heretofore possible.