Low-Temperature Paramagnetic Relaxation of Color Centers in KCl

The spin-lattice relaxation of color centers in KCl was experimentally studied at liquid helium temperatures by following the recovery of the magnetization after saturation of the electron spin resonance. X-irradiated and additively colored crystals were both studied and found to behave in the same manner. For crystals containing only $F$ centers, the recovery rates were roughly proportional to the $F$-center concentration. A tendency toward concentration independence at the lower concentrations was observed. The most dilute system studied had about 8×${10}^{16}$ $F$ centers/cc, and showed a relaxation rate at 2°K of 2.2×${10}^{-3\mathrm{}}$ ${\sec }^{-1\mathrm{}}$. Measurements at 1.4°K indicated a linear temperature dependence for the relaxation rate. A crystal containing 3×${10}^{18}$ $F$ centers/cc showed an initial recovery rate of about 0.1 ${\sec }^{-1\mathrm{}}$. The recovery was non-exponential, and the terminal recovery rate was about 0.03 ${\sec }^{-1\mathrm{}}$. Measurements at the same temperatures as those used with the dilute system showed the relaxation rate for this concentrated system to be temperature independent. Samples of intermediate concentration gave a smooth transition between these two types of behavior. Measurements in magnetic fields of 3500 gauss and 1750 gauss indicate a field-independent relaxation rate for all samples. After optical bleaching in the $F$ absorption band, with concomitant production of $M$, $R$, and $N$ centers, crystals showed an increased recovery rate for both the initial and terminal portions of the recovery. As bleaching progressed, the recovery became more nonexponential and less temperature dependent. Increases in relaxation rates by factors of several hundred were observed with no signs of saturation of the effect. Subsequent thermal bleaching of $M$, $R$, and $N$ centers restored the relaxation rate to its value prior to optical bleaching. Details of sample preparation are given, as is a discussion of possible explanations of the results.