EMBEDDED MOLECULAR CLUSTER MODELING OF TL-0(1), IN-0(1) AND GA-0(1) CENTERS IN POTASSIUM-CHLORIDE

Ab initio, embedded-cluster RHF-SCF-LCAO molecular-orbital calculations were performed on both ground and excited states of the Tl-0(1) center in potassium chloride. Effective core potentials and valence orbitals were employed on the thallium atom and its nearest-neighbor chlorine ions. The remaining ions in the cluster were represented either by bare effective core potentials or by point ions. Spin-orbit effects were calculated perturbatively in the intermediate-coupling regime. External interactions of the molecular cluster were represented by pair potentials, and simultaneous relaxation of the cluster and surrounding lattice was accomplished with a modified lattice-statics program. Optical absorption energies for transitions to the first two excited states, the optical emission energy and a ground-state vibrational frequency were determined successfully. A simplified model based on a single-center expansion and fixed point-ion lattice, validated for the Tl-0(1) center, was also employed to determine the optical absorption energies of the analogous In-0(1) and Ga-0(1) centers in potassium chloride and to explain the non-radiative de-excitation of these centers.