Pressure dependence of F-center hyperfine interactions

We report a study of the effects of intense hydrostatic stress (10 Kbar) on the ENDOR spectra of F-centers in several alkali halides. In LiF, KCl, and KBr, the first shell contact hyperfine interaction constant, aI, increases with compression, while the second shell term, aII, decreases. In LiCl, however, daII/dP and daI/dP are both less than zero. We explain our experimental results, including the anomaly cf LiCl, with a model involving (1) ionic motion in a non-constant spin-density gradient and (2) ion-motion-induced perturbation of the F-center envelope function.