Effects of Random Fields on Radiative Decay of Color Centers

Transitions which are normally forbidden have been observed in a variety of defects in solids because of the electric fields associated with lattice vibrations or with random impurities. The radiative decay of an $F$ center is a good example of this. We ask in this paper: When will the contribution of random impurities be important, and when will the decay be intrinsic? Several qualitatively distinct examples are analyzed, and tables for the transition probabilities in terms of dimensionless parameters characteristic of the host and the defects are given. Random fields should dominate in III-V and group-IV hosts, but they should prove less important in the alkali halides. Effects of random fields on related phenomena are also discussed.