Hardness for Formation of [F-H] Center Pairs in NaI and NaBr Single Crystals

By using a time resolving spectroscopic technique in picosecond range with a ruby laser, absorption spectra of NaI and NaBr single crystals have been observed to explore formation of [F-H] center pairs just after the excitation of crystals. Although the formation of self-trapped excitons at the lowest triplet state (STE L ) was observed, no F band due to [F-H] center pairs was detected in both crystals at 110 ps after the excitation at 80 and 373 K. We observed no F band in NaI 3 min after the excitation even at 373 K, while trace of the F band in NaBr at 296 K. Formation mechanism of [F-H] center pairs is discussed in connection with the configuration of the STE, i.e. with the “on-center” type STE L in these crystals.