The examination of single crystals by reflection electron microscopy
The crystallographic structure, and hence the chemical composition of surfaces, is readily found by reflection electron diffraction, but the distribution of surface constituents and the fraction of the surface which contributes to the diffraction pattern cannot be determined by this means. The possibility of obtaining this information from reflection electron micrographs was first suggested by Haine and Hirst (1) who observed bright spots in some images which were attributed to electrons diffracted through the objective aperture by suitable oriented crystallites. In such circumstances, the total angle of deviation of the electrons β must be twice the Bragg angle for a particular set of planes in the crystallites. Since β is only dependent upon the interplanar spacing d,and the electron wavelength λ, it should be possible to determine the distribution of each surface constituent by examining the specimen at various values of β. If these effects are to be obtained with single crystals or oriented microcrystalline films, the specimen must be oriented so that normal to the diffracting planes is co-planar with and bisects the angle between the incident electron beam and the electron optical axis of the microscope.