THE "OPTICAL POTENTIAL" AND MULTIPLE DIFFUSE SCATTERING IN DYNAMICAL ELECTRON DIFFRACTION AND IMAGING

The theory for the absorption potential (or optical potential) in electron diffraction was established many years ago by Yoshioka. However, few studies have been devoted to examining the approximations originally introduced when the potential was derived. In this paper, the absorption potential first proposed by Yoshioka is revised for dynamic electron diffraction with consideration of the effects arising from thermal diffuse scattering and point defect scattering. A rigorous theoretical proof is given to show that the inclusion of this “potential” in the dynamical calculation automatically recovers the contributions made by the high order diffuse scatter-ing, although the calculation is done using the equation derived for single diffuse scattering. If Yoshioka’s approximation is made, i.e., the Green’s function is replaced by its form in freespace, then the inclusion of the optical potential in dynamical calculations still recovers the multiple diffuse scattering terms except the dynamic Bragg reflection after each diffuse scattering event. This conclusion establishes the basis for expanding the conventional diffraction theories developed under the first order diffuse scattering approximation to cases where the specimen thickness is large and the degree of disorder is high. It has been shown that the “optical potential” depends also on the structure of the crystal. The Fourier coefficients of this function are given in the Bloch wave representation for transmission electron diffraction.