Off-center self-trapped excitons and creation of lattice defects in alkali halide crystals

Self-consistent calculations and experimental evidence indicating that self-trapped excitons in alkali halides relax spontaneously to an off-center configuration are examined in relation to radiation-induced lattice defect formation. The theoretical results show that the shift of the diatomic halogen core in relaxation of the self-trapped exciton (STE) from $D_{2h}$ to $C_{2v}$ point-group symmetry is quite large (e.g., about 4 a.u. in KCl). The clear implication is that the STE triplet state which is responsible for the well-known π-polarized recombination luminescence is effectively a nearest-neighbor F-H pair, analogous in many ways to the F-H pair configuration which is known to constitute the STE luminescent state in alkaline-earth-metal fluorides. From this new perspective, in which conversion of an STE to an F-H pair reduces to conversion of a nearest-neighbor F-H pair to a more distant F-H pair, a number of formerly puzzling aspects of photochemical F-center generation can be rationalized quite simply.