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 N${\mathrm{H}}_4$Cl, 4.20 eV for N${\mathrm{H}}_4$Br, and 3.74 eV for N${\mathrm{H}}_4$I. 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 $V_k$ 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 N${\mathrm{H}}_4$Cl is $\sigma$-polarized with respect to the pair axis, and its lifetime under pulsed x-ray excitation is on the order of ${10}^{-8\mathrm{}}$ sec. The N${\mathrm{H}}_4$Br emission band consists of two overlapping transitions. There is a fast $\sigma$-polarized component similar to that found in the chloride. The other component is $\pi$-polarized and has a longer lifetime of 7.7×${10}^{-7\mathrm{}}$ 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.

• Received 12 August 1968

DOI:https://doi.org/10.1103/PhysRev.176.1070