The nature of Kikuchi lines in the Bragg case in reflexion high-energy electron diffraction

The behaviour of the Kikuchi lines parallel to the shadow edge, which are formed under diffraction conditions in the Bragg case, is experimentally studied for zincblende (ZnS), MgO and rock salt. A line of this group generally has an asymmetric intensity profile across it. Sometimes it shows contrast reversal from an excess appearance in the central range to a defect appearance in the side ranges along the line. These peculiarities become noticeable with the increase in the glancing angle of the incident beam with the crystal surface. It is pointed out that theoretically the form of the line term in the intensity formula for Kikuchi patterns in the Bragg case differs from that in the Laue case. The variation of the line profile, including the effect of contrast reversal, takes place by the change in the intensity formula, in the ratio of the band term to the line term, depending on the scattering vector appropriate to the inelastic electron scattering involved. By assuming approximate forms for the scattering factors for Kikuchi patterns, the observed results are qualitatively explained.