Spin-Lattice Relaxation ofFCenters in KCl: IsolatedFCenters

Spin relaxation of $F$ centers in KCl has been observed by a field-sweep-inversion recovery technique between 2.1°K and 150°K in external magnetic fields from 1 Oe to 10 000 Oe. The intrinsic spin relaxation time is best fitted by: $\frac{1}{T_1}=A{H}^{2}T+B{\left(\frac{T}{\theta }\right)}^7{J}_6\left(\frac{\theta }{T}\right),$ where $A=5.1\mathrm{}\times {10}^{-12\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{Oe}}^{-2\mathrm{}}$ ${\left(\mathrm{°}\mathrm{K}\right)}^{-1\mathrm{}}$, $B=7.8\mathrm{}\times {10}^4$ ${\sec }^{-1\mathrm{}}$, $\theta =210\mathrm{}°$K, and $J_6\left(\frac{\theta }{T}\right)$ is a tabulated "transport" integral. These results are interpreted in terms of spin-lattice relaxation of isolated $F$ centers by means of phonon modulation of the electron-nuclear hyperfine interaction.