Spin-Lattice Relaxation ofFCenters in Alkali Halides: Theory and Optical Measurements to 50 kG

We have measured the ground-state spin-lattice relaxation of $F$ centers in KBr and KI over the range of magnetic field from 0 to 50 kG, and at the temperature of 1.6°K. Essentially continuous measurement over this large field range was made possible by a detection technique in which the magnetic circular dichroism of the optical absorption band was monitored. In the range 10 to 50 kG, the relaxation rate is predominantly that of single $F$ centers. Here, the field dependence of the relaxation rate followed very closely a curve of the form $(A{H}^3+B{H}^5)coth\left(\frac{g\beta H}{2KT}\right)$. The first term represents a mechanism involving phonon modulation of the hyperfine contact interaction with neighboring nuclei, and is the larger term for $H_0\lesssim 25$ kG. The second term is due to relaxation via the Kronig-Van Vleck process. A theoretical evaluation of that part of the rate due to the hyperfine mechanism has produced close agreement with experimental results. In the evaluation, the interaction with the second shell of nuclei (halogens) was found to be of greatest importance for KBr and KI.