Laser-excited spectra and reorientation of the S−2 ion in alkali halides

Resonance Raman and emission spectra are reported for the S−2 molecular ion in several alkali halide crystals. At 6 K, the emission consists of one or more vibronic series of sharp zero-phonon lines with associated isotope structure and phonon sidebands. The Raman spectra exhibit several overtones of the ground-state S−2 vibration. In NaI:S−2, nine overtones are observed, each with isotope structure and phonon side-bands. The Raman and emission spectra are analyzed and compared for NaI, KI, and RbI. These data support the model of a single S−2 site with Πg splitting. Time dependence of the polarized emission intensity is seen at 6 K in KCl, KBr, NaBr, KI, and RbI, but not in RbCl, RbBr, or NaI. This phenomenon is explained and analyzed in terms of the reorientation kinetics of the optically-excited ions. The multiple-order Raman scattering efficiencies are shown to agree reasonably well with the theory of Hizhnyakov and Tehver.