A b i n i t i o study of CN− impurity centers in alkali halides: Lattice stabilization of excited electronic states

Abstract

The lowest electronic states of 1Σ+, 3Σ+, 1Π, and 3Π symmetry in the cyanide anion are calculated at the multiconfiguration self-consistent field (MCSCF) level using simple point-charge models to simulate the ionic environment in the cubic alkali halide crystals. The electrostatic potential of the lattice is essential to stabilize the excited states against autodetachment, yet the resulting spectroscopic properties are remarkably insensitive to gross changes in the lattice, including deletion of all but the six nearest-neighbor cations. The lowest excited state—the state responsible for an observed UV emission spectrum of CN− in some alkali halides—is shown to be 3Σ+, as in the isoelectronic N2 and NO+ molecules, rather than 3Π, as in CO. The properties of the ground electronic state are further examined at the SCF level in clusters of six alkali ions. The cations produce a ‘‘ compression’’ of the anion, decreasing the internuclear distance and increasing the vibrational frequency from the point-charge results and thus yielding better agreement with experiment. Attempts to determine the orientational potential of the ground state of CN− in the lattices remain inconclusive, due to basis set limitations; however there are strong indications that in the 〈100〉 orientation favored by CN− in the sodium halides, the anion prefers an off-center location with the Na–N distance appreciably shorter than the Na–C distance. An examination of methods used to extract spectroscopic constants from pointwise tabulated potentials indicates that fits to closed-form potentials are better than fits to polynomials in (R-Re ) and to methods which entail numerical solution of the vibrational wave equation for the tabulated potential.