RCenter in KCl; Stress Effects in Optical Absorption

The $R_2$ optical absorption band in KCl shows a prominent zero-phonon line at liquid-helium temperature. This line may be split by application of uniaxial stress to the crystal, and the qualitative dependence of this splitting upon the direction of the stress and the polarization of the measuring light implies that the absorption is to be associated with a defect with a [111] axis of symmetry. The dependence of the intensities of the split components upon stress implies that the ground state is degenerate. The ground and lowlying excited states of van Doorn's $F_3$ model are discussed in terms of a continuum model for this center, as well as the implications of the Jahn-Teller effect which results from the coupling of these electronic states to the lattice vibrations. These considerations, in addition to being consistent with the results on the zero-phonon line, predict a stress-induced dichroism in all of those broad-band absorptions of the center which have transition moments in the plane of the triangle of $F$ centers. An experimental study of the stress-induced dichroism has allowed the identification of six distinct transitions of the $R$ center: a transition in the region of the $N$ bands, one near the $M$ band, the familiar $R_1$ and $R_2$ bands, a broad transition underlying the $R_1$ and $F$ bands, and a narrow band on the violet side of the $F$ band. The symmetry of the corresponding excited states is also obtained from the nature of the stress-induced dichroism. These experimental results are consistent with the qualitative predictions of the continuum $F_3$ model. These conclusions, supplemented by results on the spin quartet states of the $R$ center and by polarized bleaching experiments, give the relative locations of ten distinct states of the $R$ center.