CONSEQUENCES OF BLOCHS THEOREM ON DYNAMICAL THEORY OF ELECTRON DIFFRACTION CONTRAST

Some consequences of Bloch's theorem on the diffraction of fast electrons in solids are considered. It is shown that the absorption coefficients of the various crystal wavefields excited in a crystal by the incident electron are periodic functions of crystal orientation. As a result it is shown that the two‐beam theory, when applied to the calculation of the absorption coefficients, results in a poor approximation. Recently several authors have shown that the higher‐order even reflections, such as the 222 reflection in Al or the 400 reflection in MgO, become vanishingly weak at certain critical values of the incident beam potential. From an investigation of the symmetry properties of the crystal wavefields a simple physical picture of this effect is obtained, and it is shown that at the critical voltage a reversal of the crystal wavefield symmetry occurs. Copyright © 1969 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim