Linear dichroism in the absorption and triplet-state excitation spectra ofF2centers in KCl

The absorption and triplet-state excitation spectra of aligned and unaligned $F_2$ centers in KCl at liquid-nitrogen temperature have been measured by means of the polarization modulation of visible light. A new technique involving the selective deexcitation of the triplet state of $F_2$ centers of specific orientations has been developed. Evidence for both an indirect and a direct triplet-state excitation mechanism is presented. The results have permitted the identification of the symmetry representations of the excited states of a number of transitions. In particular, the results indicate that the $M_4$ band at 634 nm terminates on a ${}_{}{}^3{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ state which includes a spin-orbit admixture of ${}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$. The spin-orbit coupling parameter has been evaluated at 0.062 ± 0.003 eV. The triplet-triplet transition at 505 nm is shown to include both polarizations perpendicular to the defect axis and thus terminates on a ${}_{}{}^3{}_{}{}^{}\Pi _g^{}{}_{}{}^{}$ level. An energy-level diagram for the $F_2$ center is presented.