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

A calculation of the g tensor is carried out for the ²E 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 δg_∥ is correctly predicted, and the magnitude is in reasonable agreement with experiment. From studies of the stress dependence of δg_∥, a value is obtained for the strength of the Jahn-Teller coupling, which is found to be k²=3.0±0.5 in the notation of Longuet-Higgins, \"Opik, 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.