Charge distribution and energy levels of trapped electrons in ionic solids

The charge distribution and energy levels of electrons trapped at two types of lattice defect in ionic solids (interstitial ions and F-centres) are calculated using a variation method which takes account of the fact that the field acting on the electron near such a defect depends upon the wave function of the electron itself. The model used for the interstitial ion consists of a point charge in a uniform polarizable medium while that for the F-centre consists of a potential well whose dimensions are decided by the charges and induced dipoles on the surrounding ions. The 1s-2p optical excitation energies are determined for F-centres in typical alkali- and silver-halides. Calculations of thermal activation energies from the 2p state to the conduction band and of thermal dissociation energies (1s state to conduction band) for the silver halides indicate that F-centres should play a more important part than interstitial ions in photoconductive processes in such materials.