Dynamical effects in high-voltage electron diffraction

Relativistic versions of dynamical theories are surveyed and it is shown that electron diffraction phenomena become more dynamical at very high voltages. The results of diffraction experiments obtained with a 500 kV electron microscope are outlined. Specimens were magnesium oxide wedges and molybdenite films. The dynamical many-beam interactions were enhanced at 500 kV. The variation of Bragg width with the accelerating voltage was in agreement with the relativistic theory. The intensity reversal of Kikuchi bands, which is not remarkable below 100 kV, was clearly observed at 500 kV. The 3n + 1,0,0 reflections of molybdenite were found to be vanishingly weak, although it is not yet clear whether or not this is due to a dynamical effect. The appearance of many diffraction spots at high voltages seems to be due mainly to the decrease of the background intensity.