Jahn-Teller effect onFcenters in barium sulfide

Abstract

Crystals of BaS were grown and additively colored in Ba vapor. A broad $F$ band was observed in the optical absorption spectra with three partially resolved peaks at 1.7, 2.33, and 2.75 eV. The shape of this band was found to be independent of the density of coloration, indicating that the band was due to transitions of a single center. Calculations of the shape of the $F$-center absorption were carried out using a nine-dimensional Monte Carlo integration. This procedure not only made possible the inclusion of all of the linear and quadratic terms in the interaction Hamiltonian but also the mixing of nondegenerate with degenerate excited states. Assuming an $s^2\to \mathrm{sp}$-type transition, a good fit to the experimental band shape was obtained based on the ${}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$-orbital triplet excited state being split by the Jahn-Teller interaction, primarily the $E_g$ modes, and being perturbed by an ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ excited state coupled by the $T_{1u}$ modes. A luminescence was observed in the colored crystals but it was shown that no radiative transitions were to be expected from the ${}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ excited state of the $F$ center and that the luminescence observed was probably due to an unknown impurity.