VIBRATIONAL-RELAXATION OF THE F-CENTER IN NAI AND NABR

High-resolution Raman measurements have been performed to study the co upling of lattice modes to the F center in NaBr and NaI. In both cases , a prominent line near or in the region of the energy gap between the acoustic and optical phonon bands dominates the Raman spectrum. Its l inewidth was measured and extrapolated to 0 K it turned out to be abou t 3 and 0.5 cm-1, respectively, for NaBr and NaI. These values are rel ated to the lifetime of the local vibrational excitations, showing a r elatively fast vibrational relaxation. Calculating the nonradiative tr ansition probability during the vibrational relaxation in the electron ic excited state, the efficiency of reaching the relaxed excited state (RES) of the F center is analyzed. The calculations show that, if the experimental value of the vibrational lifetime is used, the model pre dicts a high probability of reaching the RES in the case of NaBr. In t he case of NaI the prediction strongly depends on the choke of the ele ctronic matrix element for the interstate nonradiative transition. By comparison with similar cases of vibronic transition matrix elements r eported in the literature, the efficiency of reaching the RES in NaI i s expected in the range 0.05 to 1.