Spin-Lattice Relaxation ofFCenters in KCl: InteractingFCenters

Spin relaxation of $F$ centers in KCl has been observed by a field-sweep inversion recovery technique at liquid-helium temperatures. Intrinsic relaxation behavior was reported previously. Various kinds of extrinsic behavior are reported here. They include distorted resonance line shapes, unusually fast relaxation, and unusual field and temperature dependences of the relaxation. Such effects were always seen at low temperatures and low fields after the following treatments: high $F$-center concentrations, light exposure, the addition of certain impurities, plastic deformation, and low-temperature gamma irradiation. A model is presented which explains all of these effects and is consistent with the extensive lore involving the properties of $F$ centers. This model identifies the extrinsic relaxation process as one compounded of spatial diffusion of Zeeman energy in the $F$-center system and cross relaxation from $F$ centers to rapidly relaxing centers. These rapidly relaxing centers are composed of loosely bound clusters of $F$ centers, which can be formed in a variety of ways.