The Theory of Electron Diffraction

It is shown that the omission of an angle-dependent phase factor in the scattering amplitude constitutes a significant error of the Born approximation, as customarily applied to electron diffraction experiments. Some general properties of the scattering amplitude are discussed in relation to the Born approximation and used to derive a simple estimate of the required phase. The theory, thus corrected, is found to remove the need for assuming rather distorted structures in some molecules containing heavy atoms. The effect discussed is present quite generally in the interference of waves scattered by differing potentials and becomes more prominent as the particle energy is lowered. In the Appendix a semiclassical procedure is used to treat the analogous effect in proton diffraction.