Channelling Effects in Electron Induced X-Ray Emission from Diatomic Crystals

The variation of current density distribution over the unit cell with direction of the incident beam gives rise to anomalous absorption and other orientation effects in electron diffraction. One of these, the electron induced x-ray emission, was demonstrated to depend on the diffraction condition already in 1962 by Duncumb [1] and a theoretical treatment based on the Bloch wave considerations was given by Hirsch, Howie and Whelan [2], The possibility of obtaining crystallographic information from this effect was discussed by Gjonnes and Hoier [3] who showed theoretically that the orientation dependence could, at least in principle, be used for determination of e.g. solute atoms within the unit cell. Relatively few experiments on x-ray emission under anomalous absorption or channelling conditions have been reported, however, despite the widespread use of x-ray spectrometers as auxiliary equipment to electron microscopes [4—8]. Compared to the similar effect in ion beam channelling which has already been utilized in solid state studies (e.g. Kool et al. [9]), electrons suffer from two disadvantages: The quite strong Bremsstrahlung which gives an appreciable background, and the strong diffuse scattering which tends to smear out the effect. On the other hand, the versatility of the electron beam instruments, e.g. the possibility of focussing the beams onto a very small area, simultaneous observations of diffraction patterns and images etc., and also the fact that the same effect can be observed from the energy loss of the electrons, may suggest a more detailed study of the channelling effects in x-ray emission, particularly