Vibrational relaxation of the F center in NaI and NaBr.

Abstract

High-resolution Raman measurements have been performed to study the coupling 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 linewidth was measured and extrapolated to 0 K it turned out to be about 3 and 0.5 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively, for NaBr and NaI. These values are related to the lifetime of the local vibrational excitations, showing a relatively fast vibrational relaxation. Calculating the nonradiative transition probability during the vibrational relaxation in the electronic 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 predicts a high probability of reaching the RES in the case of NaBr. In the case of NaI the prediction strongly depends on the choice of the electronic matrix element for the interstate nonradiative transition. By comparison with similar cases of vibronic transition matrix elements reported in the literature, the efficiency of reaching the RES in NaI is expected in the range 0.05 to 1.