Formation of the self-trapped exciton via thermally induced defect reactions in alkali halides

Low-temperature thermoluminescence has been studied in KBr, NaCl, and KI by means of emission-spectrum measurements of glow peaks and of detailed analysis as to the quantitative correlation between emission intensities and the amount of reacted defects. It is found that the luminescence center is the self-trapped exciton which results in both $\sigma$ and $\pi$ emissions in these salts below 100 K and that the self-trapped exciton is produced at the temperature where the annihilation of the $F$ center takes place. It is, however, shown that the recombination between $F$ and $H$ centers can excite only the lowest triplet exciton in KBr. The quantum yield of the exciton formation by $F-H$ recombination is estimated less than 0.03. The energetic correlation between the exciton and the $F-H$ pair state in the defect-annihilation process in KBr is discussed.