RESONANCE-RAMAN-SCATTERING SPECTROSCOPY OF THE SELF-TRAPPED EXCITONS IN ALKALI-HALIDES

We have, studied resonance Raman scattering tuned to the sigma(g) --> sigma(u) hole transition of the self-trapped excitons (STE's) in alkal i halides. The Raman spectrum is characterized by a sharp line of 137 cm-1 in NaBr, 361 cm-1 in NaCl, and 139 cm-1 in RbI. Each line has bee n assigned to the stretching vibration of the halogen molecular ion (X 2-) composing the STE. Comparison of the Raman frequencies with those of V(K) and H centers obtained by similar measurements indicates that the frequency of the STE in NaBr coincides with that of the V(K) cente r, while those in NaCl and in RbI are almost identical to the frequenc ies of the H centers. These results provide evidence that the X2- of t he STE in NaBr occupies two adjacent halogen sites and the X2- of the STE in NaCl and in RbI occupies a single halogen site.