Embedded Molecular Cluster Modeling of the TL(0)(1) Center in Potassium Chloride.

The Tl^0(1) center in potassium chloride was modeled using an embedded molecular cluster technique. Although initial investigations of the laser -active center included vacancy-centered orbitals and lattice relaxation effects, in the most successful approach, the ground and first two excited states of the Tl^ {0}(1) center were studied using a fixed -lattice, single-center-expansion, molecular-cluster model. Ab initio Restricted Hartree-Fock, Self-Consistent Field (RHF -SCF) calculations were performed on a 28 atom cluster as a function of the thallium atom position using the MELD program. Effective core potentials were employed in conjunction with double-zeta quality basis sets on the thallium atom. A second, diffuse, p-type orbital was added to enhance the flexibility of the single-center wavefunction. All other ions were treated strictly as point charges restricted to their perfect lattice positions. Spin-orbit effects were calculated in the intermediate coupling regime. Optical absorption energies to the first two excited states, the optical emission energy, and the ground state vibrational frequency were determined in good agreement with experiment. A method was developed to calculate Cl ^{-} ion pseudopotentials from MELD calculations of the wavefunctions and energy eigenvalues of the Cl^{2-} ion. The unstable, doubly-negative chlorine ion is confined by a "Watson" well. The method of calculation, based on that used by Hay and Wadt, is modified to allow for the limited, Gaussian-orbital basis set employed by MELD. The ground and first excited state of the F-center in potassium chloride were modeled using an embedded molecular cluster technique. Ab initio RHF-SCF calculations were performed on a 26 atom cluster using the MELD program. Five vacancy-centered s- and p-type orbitals were included to model the F-center wavefunctions. The nearest potassium ions were represented by effective core potentials while the remaining ions in the cluster were represented as point charges. The external interactions were represented by Buckingham pair potentials, and the relaxed configurations were determined in each electronic state by the HADESR modification of the HADESIII lattice statics program. The optical absorption energy and the ground vibrational frequency were determined in good agreement with experiment.