Abstract

The electronic states of a semiconducting or of an insulating crystal often are treated as though the lattice were rigid, quite indifferent to the presence of carriers. However, there are well known departures from this picture, in varying degrees. In polar AB crystals, conduction-band electrons have enhanced mass, existing as polarons; and in many halide salts the hole is actually immobilized. Tields associated with an electron in the vicinity of such a hole, a self-trapped exciton (STE), can give rise to substantial displacements of nuclei and even to photochemistry. There are near-unity yields of transient lattice defects which can be recognized in optical absorption or in emission; similar transients are found in alkaline earth halides. Such absorption may cover much of the spectrum, from the near-infrared to the near-ultraviolet. For the alkali halides, there are minor yields of stable or long-lived defects; these populations have been studied for many years by conventional spectroscopic techniques.

© 1984 Optical Society of America