Spin-Lattice Relaxation of Atomic Hydrogen in CaF2Involving Local Modes

The spin-lattice relaxation of atomic hydrogen and deuterium in Ca${\mathrm{F}}_2$, measured by Feldman, Castle, and Murphy, shows an exponential dependence, which is ascribed to a Raman process involving local modes. The magnitude of the pre-exponential factor can be compared to the ordinary Raman process for these centers, which was also observed. This ratio is compared with the predictions of the author's theory of spin-lattice relaxation involving local modes. It appears that the theory overestimates the effect of local modes on spin-lattice relaxation by a factor of order 5. This may arise from a comparable error in the theoretical estimate of the decay rate of a local mode by anharmonic processes, which enters the expression for the spin-lattice relaxation rate. Other possible sources of error are also discussed.