The fine structure of beams in electron diffraction emerging from wedge‐shaped crystals. Multiple beam case

The fine structure of electron diffraction beams emerging from a wedge‐shaped crystal is analysed on the basis of the dynamical multiple beam theory. First the basic principle of the Fourier‐transforms method and the method of the scattering matrix is summarized. The theory is applied to a wedge as well in a particular as in a general orientation. Assuming n strongly diffracted beams it is found that the transmitted and each strongly diffracted beam is composed of n + 1 sub‐beams with slightly different orientations. The geometry of the fine structure configuration is discussed and the influence of absorption effects on the intensity of the beams is examined. It is emphasized that the intensity of the different sub‐beams is directly related to the strength of excitation of the different Bloch wave fields in the crystal. The method is discussed explicitly for a particular multiple beam situation in MgO where the 400 reciprocal lattice vector is in the exact Bragg orientation and where only systematic reflections are supposed to be active. Taking account of absorption effects the situation is discussed within the three beam dynamical theory (000, 200, 400). Afterwards the influence of the systematic interactions (600, 500) is investigated. Three‐ and five‐beam calculations are compared with experiments. It is found that the observed fine structures (of the different beams) can be explained adequately by three‐beam theory.