Creation of lattice defects by electronic excitation in alkali halides

The main purpose of this article is to review our understanding of the predominant mechanism for defect creation in alkali halides i.e. the evolution of Frenkel (F-H) pairs from self-trapped excitons. Consideration is first given to other models of defect formation since they may play a secondary role in alkali halides and major roles in other solids. Recent experimental work is surveyed with emphasis on studies of non-radiative transitions from highly-excited self-trapped excitons, of F-centre formation and of the photochemical processes associated with impurities and with the surfaces of alkali halides. Brief descriptions are given of the photochemical process in silver halides and of recombination-induced defect migration in compound semiconductors. Finally a critical assessment is made of our understanding of the dominant mechanisms for defect formation in alkali halides with particular attention paid to the identification of the precursor state, the excited state of the self-trapped exciton from which an F-H pair is evolved, the potential energy surface and the reaction dynamics. Problems which might be of interest in future are suggested.