Abstract
Kikuchi patterns from single crystals of magnesium oxide have been studied by electron microscopy at various accelerating voltages between 500 kV and 1 MV. In each case, the incident beam falls on the crystal near a zone axis [012] so that many non-systematic reflections were either strongly excited or not excited. In this paper it is shown that the critical voltage effect is very sensitive to the crystal orientation and, even when accidental reflections are weakly excited, the second-order Kikuchi lines vanish at a very different voltage than in the systematic case. Apart from this new value, a minimum intensity along the second-order Kikuchi lines and a reversal of the asymmetric contrast of the mid-line can be observed. These effects appear in the diffraction pattern at various distances of the systematic row, depending on the accelerating voltage and the excitation of the non-systematic reflections. They can be understood from the many-beam dynamical theory, and the theoretical profiles of the intensity of Kikuchi lines are generally in good agreement with experimental results.