ABSTRACT
This chapter considers the interactions with lattice vibrations that are responsible for the strong Reststrahl absorption bands in the far-infrared. The introduction of defects into a crystal lattice has two primary effects on the normal modes of lattice vibration. One is a perturbation of their frequencies, and the other is a perturbation of their mode strengths. If a large number of similar atoms are brought together to form a crystal, each single atomic energy level is broadened by the interatomic interactions into a band of electronic levels that can be thought of as a continuum of energy states. For inner-shell electrons the bandwidths are narrow. The outermost electrons are subjected to large interaction energies of the same order of magnitude as the excitation energies of the outer electrons in the individual isolated atoms. The chapter concentrates on nonmetals, for which the individual bands are separated in energy and are formally completely filled or empty.
