Electron Spin Double Resonance Studies ofFCenters in KCl. I

This paper describes the investigation of the $F$-center electron-spin-resonance spectrum in KCl at 300°K using a double-frequency (DESR) technique. The inhomogeneously broadened resonance is saturated by an applied rf pump field and the spectrum is simultaneously probed by a weak detector field applied at a different frequency. The $F$-center concentrations in the samples studied were $n_F\lesssim 3\times {10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The techniques used in obtaining the experimental data are discussed and a theoretical treatment of the response of the spin system to two simultaneously applied fields is developed. The $T_1-T_2$ spin-packet model of the $F$-center resonance is specifically treated and a comparison of the experimental data with the theoretically predicted results indicates that this model, which has formed the basis of previous treatment, cannot be used in a consistent interpretation of the resonance properties of the KCl $F$-center system. A general expression is derived which relates the DESR signal area to the absorption intensity in a corresponding ESR experiment. The application of this relation to the data obtained in the experiments gives a KCl $F$-center spin-lattice relaxation time at 300°K of $T_1=0.3\mathrm{}\times {10}^{-4\mathrm{}}$ sec with an estimated uncertainty of 15%. A phenomenon which has not been previously considered in relation to the $F$-center absorption is observed in the existence of significant contributions to the absorption intensity from "forbidden" transition processes. These processes are attributed to a simultaneous electron and nuclear spin flip via the anisotropic terms in the hyperfine interaction. The large intensity associated with certain of these forbidden transitions is shown to imply a sufficiently rapid nuclear relaxation rate that the inhomogeneous broadening interactions may not be validly treated as a static phenomenon.