The diffraction of electrons by single crystals

Abstract

The Diffraction of Electrons by Single Crystals . 1. The diffraction of electron waves has been observed both with single crystals and with polycrystalline aggregates. So far most of the work with single crystals has been done with slow electrons of a few hundred volts energy. While the results are in general agreement with wave mechanics, they differ considerably from the predictions of the simple theory, even when the latter is extended to include a consideration of the inner potential of the crystal. Part of the discrepancy is caused by undue simplification of the theory, and better results have been got with the more elaborate theory of Morse, but there are a number of cases in which beams corresponding to half order spectra and reflection by “forbidden” crystal planes have been recorded, and for these the present theory is quite unable to account. Experiments with fast electrons on polycrystalline aggregates have shown results in full agreement with the simple theory, and it is hoped that electron diffraction will become a valuable method for the study of surface films Clearly, however, this cannot be done satisfactorily as long as there is any doubt of the ability of the existing theory to explain the diffraction by known structures. It is therefore of importance to see whether any abnormalities exist for single crystals with fast electrons. So far the only work in this connection is that of Kikuchi and others on mica, that of Niskikawa and Kikuchi on calcite, and some work by Emslie at Aberdeen on galena and calcite. Kikuchi’s work shows complete agreement with theory, but has unusual features on account of the large spacing of mica across the cleavage face. The work on calcite is complicated by charging up of the crystal, and galena, as the present author has found, changes under the influence of the rays, so that it is difficult to draw precise conclusions. No metals have been investigated. The present work has, therefore, been undertaken. It consists of a detailed investigation of the diffraction of electrons of from 30,000 to 50,000 volts by single crystals of rocksalt, copper and silver.