A study of the structure of the self-trapped exciton in alkali halides by ab initio methods

Hartree-Fock calculations have been performed for various possible geometries of the self-trapped exciton (STE) in several alkali halide crystals. The computations treat a four- to twelve-ion cluster embedded within an ionic lattice representation that may or may not respond self-consistently to the charge distribution in the cluster. Both the triplet and open-shell singlet excited states of the exciton are considered. The energy of each state decreases as the molecular component of the exciton moves from the on-centre D_2h symmetry to an off-centre C_2v symmetry site. This distortion is accompanied by a separation of the electron and hole components of the exciton leading to a nearest neighbour F-H pair. These findings are in general accord with earlier analyses and one-electron calculations presented by K.S. Song and co-workers (1989).