The scattering of electrons by atoms and crystals. II. The effects of finite source size

Abstract

The new formulation of physical optics developed by Cowley & Moodie (1958) is applied to determine to what extent the intensity distributions in electron diffraction patterns, calculated on the assumption of perfectly coherent incident radiations should be modified to take into account the finite size and incoherence of practical electron sources. A model of an ideally incoherent source is set up. Expressions are derived for the intensity distribution in diffraction patterns obtained under various experimental conditions, including cases where partial coherence is introduced in the illumination of the specimen by the presence of an aperture. Numerical calculations for representative cases indicate that coherence considerations may become important with any extension of present-day high-resolution techniques. (auth)