STABLE GEOMETRIES OF THE SELF-TRAPPED EXCITON IN ALKALI-HALIDE CRYSTALS

According to recent works, the self-trapped excitons (STE) in alkali h alides are grouped in three different types. In some (e.g. Rbl), a pop ulation transfer is observed among coexisting types as temperature is raised. In others (NaBr and NaI) extra type emission bands have been o bserved under dilational strain. We studied the adiabatic potential en ergy surface (APES) of the STE under the effect of expanded or compres sed lattices, and of rotation of the V(k) core in several directions, based on both extended-ion and ab initio Hartree-Fock methods. A criti cal study of the zero field splitting parameter D of the spin Hamilton ian is made in assessing the effect of rotation. It is shown that: (a) the potential barrier separating the adjacent local minima on the APE S becomes smaller as the lattice is dilated; (b) the APES associated w ith rotation of the halogen molecule-ion axis from [110] toward [001], upto about 30-degrees, is flat; Rotations in other directions are sti ff, however; (c) the observed anomaly of the D parameter in NaCl is co mpatible with the rotation described in (b) above. The geometries of t he three types are discussed on the basis of this study.