Optical Absorption and Luminescence of NaCl-NaBr Solid Solution

The optical absorption and luminescence of NaCl-NaBr solid solutions were studied in the whole concentration range at liquid nitrogen temperature. The change of the profile observed in the absorption spectra around 0.2 mole fraction of NaBr content is attributed to the percolation of minority composition in this system. With increasing the concentration of NaBr, the 5.15 eV emission band, which is known to arise from Br - -dimers, shifts gradually toward the low energy side to reach finally the location of the intrinsic emission band in pure NaBr, whereas the 4.0 eV emission band arising from Br - -monomers diminishes its intensity and fades out above 0.2 mole fraction of NaBr. This suggests that the relaxed excited state of the Br - -dimer, not of the Br - -monomer, in NaCl can be recognized as a proto-type of the self-trapped exciton in the pure NaBr.