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

Abstract

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 $^{3}\ensuremath{\Pi}_{u}$ state which includes a spin-orbit admixture of $^{1}\ensuremath{\Sigma}_{u}^{+}$. The spin-orbit coupling parameter has been evaluated at 0.062 \ifmmode\pm\else\textpm\fi{} 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 $^{3}\ensuremath{\Pi}_{g}$ level. An energy-level diagram for the ${F}_{2}$ center is presented.