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

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.