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.