Luminescence due to Br - -ions in NaCl has been investigated at liquid nitrogen temperature by varying the amount of Br - -ions from 1×10 -3 to 1.4×10 -2 mole fraction. Two emission bands, having their maxima at about 4.0 eV and 5.15 eV, were observed with UV-light excitation. The 4.0 eV luminescence is known to enhance linearly with increasing Br - -ion concentration. Ratio of the intensity of the 5.15 ev luminescence to that of the 4.0 eV luminescence was found to vary in proportion to the Br - -ion concentration, suggesting that the intensity of the 5.15 eV luminescence varies with square of Br - -ions concentration. This means that the 5.15 eV luminescence is caused by (Br - ) 2 -center. Presence of the absorption band of (Br - ) 2 -center was also confirmed in the low energy side of the absorption band due to Br - -ion monomer.
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Nakagawa et al. (1972) studied this question.
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