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Luminescence from Self-Trapped Excitons in NH4Cl Crystals

Intrinsic luminescence of NH 4 Cl crystals has been studied under excitation with UV light in the exciton band region at 65 K. We observe a broad emission band peaking at 4.88 eV with a half-width of 0.67 eV, which is in good agreement with that arising from the recombination of electrons with V K centers. As in the alkali halides, such a coincidence confirms that the exciton in NH 4 Cl is self-trapped in the configuration of “ V K +electron.” Measurements of the excitation spectrum suggest that the 4.88 eV emission ( σ emission) originates from the lowest singlet state of the self-trapped exciton.