Resonance-Raman-scattering spectroscopy of the self-trapped excitons in alkali halides

We have studied resonance Raman scattering tuned to the ${\mathrm{\ensuremath{\sigma}}}_{\mathit{g}}$\ensuremath{\rightarrow}${\mathrm{\ensuremath{\sigma}}}_{\mathit{u}}$ hole transition of the self-trapped excitons (STE's) in alkali halides. The Raman spectrum is characterized by a sharp line of 137 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in NaBr, 361 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in NaCl, and 139 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in RbI. Each line has been assigned to the stretching vibration of the halogen molecular ion (${\mathit{X}}_{2}^{\mathrm{\ensuremath{-}}}$) composing the STE. Comparison of the Raman frequencies with those of ${\mathit{V}}_{\mathit{K}}$ and H centers obtained by similar measurements indicates that the frequency of the STE in NaBr coincides with that of the ${\mathit{V}}_{\mathit{K}}$ center, while those in NaCl and in RbI are almost identical to the frequencies of the H centers. These results provide evidence that the ${\mathit{X}}_{2}^{\mathrm{\ensuremath{-}}}$ of the STE in NaBr occupies two adjacent halogen sites and the ${\mathit{X}}_{2}^{\mathrm{\ensuremath{-}}}$ of the STE in NaCl and in RbI occupies a single halogen site.