Introduction

Kinematic theories describe the motion of physical processes without consideration of the forces involved. In electron diffraction the kinematic approach has come to mean singlescattering analysis since in this view symmetry and energy conservation, and not the details of the potential, largely determine the diffraction pattern (see Chapter 6). But in fact singlescattering analysis is more than this. The strength of the interaction is included by means of a scattering factor, the mean potential is included by the refraction of the incident angle when a beam enters the crystal and some multiple-scattering processes are included in the diffraction of disordered systems by considering diffraction from blocks of atoms. In addition inelastic processes, related to the imaginary part of the potential, are included by a factor that describes absorption. As a result kinematic theory is an exceedingly useful approximate analysis that serves as a starting point for much of the dynamical theory. In contrast, the exact dynamical theory, which will be described in Chapters 12–14, is an analysis in which the potential is included from the beginning and in which multiple scatterings are the main diffraction process. But the results and trends of dynamical theory are difficult to visualize in simple ways. In this chapter we present the basic kinematic theory for electron diffraction from surfaces.