RCenter in KCl: Electron-Spin-Resonance Studies of the Ground State

A calculation of the $g$ tensor is carried out for the ${}_{}{}^2{}_{}{}^{}E$ ground state of the $R$ center in KCl. The method employs linear combinations of atomic orbitals as wave functions, made up of $F$-center $1s$ states orthogonalized to the occupied states of the crystal, and takes into account the dynamical Jahn-Teller effect and the "many-center" nature of the problem. The positive sign of the $g$ shift $\delta g_{\parallel }$ is correctly predicted, and the magnitude is in reasonable agreement with experiment. From studies of the stress dependence of $\delta g_{\parallel }$, a value is obtained for the strength of the Jahn-Teller coupling, which is found to be $k^2=3.0\mathrm{}\pm 0.5$ in the notation of Longuet-Higgins, Öpik, Pryce, and Sack. The effects of random internal strains are found to play an important role, broadening the signal beyond observability at zero applied stress. Estimates of the $R$-center spin-lattice relaxation rate fall considerably short of the experimental value, for which an upper limit is obtained from saturation plots. Experiments to determine the effect of the presence of the $R$ center on the spin-lattice relaxation rate of the $F$ center indicate that the $R$ center forms a significant channel for $F$-center relaxation at large $R$ concentrations, and may not be dismissed as a possible relaxation channel at very low $R$ concentrations.