Electronic structure of the self-trapped exciton in alkali fluorides and chlorides

Abstract

The authors report pseudopotential calculations for the relaxed exciton in alkali fluorides ahd chlorides, with emphasis on NaCl. These calculations supplement earlier Hartree-Fock calculations by permitting investigation of a number of specific features. The more extended and higher energy excitations of the electron associated with the exciton are studied and a wider range of host lattices and crystal geometries considered. The most important result is that the origin of the sigma (singlet) and pi (triplet) luminescence bands can be understood: the two bands derive from different orbital states, contrary to previous assumptions. Estimates of hyperfine constants, the sigma - pi splitting and oscillator strengths are also given and agree well with experiment. The results suggest that there should be an additional sigma -polarized band of the self-trapped exciton in the infrared.