Kinematical electron diffraction

In a conventional TEM, electrons are emitted from an electron gun and focused by the condenser lens as a beam, which illuminates the specimen. The electron beam interacts with the specimen and is scattered (or diffracted) by the crystal atoms. Thus, the electron wave at the exit face of the specimen contains information about the potential distribution in the specimen. Since electrons are charged particles, their interaction with a solid is rather strong in comparison with either X-rays or neutrons, so that multiple scattering effects are always present in electron diffraction. This means that electron diffraction must be described by dynamical scattering theory, especially when quantitative structural analysis is necessary. However, many characteristics of electron diffraction can be qualitatively treated based on the kinematical scattering theory, which is actually a single-scattering theory. The purpose of this chapter is to outline some basic concepts of kinematical electron diffraction theory and to introduce the imaging theory of TEM, which will be applied in the future chapters for REM imaging. A systematic kinematical treatment of electron diffraction for perfect and imperfect crystals has been given by Cowley (1981). A complete description of dynamical electron diffraction theories has been given by Wang (1995).