Recombination Luminescence from Self-Trapped Excitons in Ammonium Halide Crystals

Luminescence due to intrinsic self-trapped exciton annihilation has been observed in ammonium halide single crystals under x irradiation at low temperatures. A broad emission band appears at 4.86 eV for NH₄Cl, 4.20 eV for NH₄Br, and 3.74 eV for NH₄I. The source of the luminescence is identified primarily through an experiment which relates the polarization of the luminescence to the orientation of self-trapped holes or Vₖ centers. As in the alkali halides, the self-trapped exciton may be characterized as a bound pair of nearest-neighbor halide ions in an excited state. The transition in NH₄Cl is σ-polarized with respect to the pair axis, and its lifetime under pulsed x-ray excitation is on the order of 10^-8 sec. The NH₄Br emission band consists of two overlapping transitions. There is a fast σ-polarized component similar to that found in the chloride. The other component is π-polarized and has a longer lifetime of 7.7×{}10^-7 sec. Comparison is made between these transitions and similar transitions previously observed in the alkali halides. It is concluded that the self-trapped exciton states are essentially the same in the two materials; in particular, a triplet state evidently initiates the long-lived emission band.