Smooth parabolas in transmission electron diffraction patterns

Perturbation theory is used to analyse the geometry of various diffracted features in large-angle high-energy electron diffraction patterns taken in transmission. Particular attention is paid to smooth parabolic features and their straight-line envelopes. It is shown that the features lie in positions determined by a `modified Bragg condition' that takes account of dynamical interactions in the crystal. The results have considerable bearing on the interpretation of almost identical resonance features seen in reflection high-energy electron diffraction.