$V_K$ Center in N${\mathrm{H}}_4$Br and N${\mathrm{H}}_4$Cl Single Crystals

The EPR spectrum of $\mathrm{Br}_2^{}{}_{}{}^{-}$ in N${\mathrm{H}}_4$Br single crystals has been observed and fitted with an axial spin Hamiltonian. Optical transitions of the $V_K$ center in N${\mathrm{H}}_4$Br and N${\mathrm{H}}_4$Cl have been identified and an interpretation of the radiation-induced 380-mμ absorption band, first observed by Rüchardt in both ammonium halides, is presented. These measurements show that the $V_K$ center in the ammonium halides is essentially the same as in the alkali halides. A description is given of a procedure for determining the polarization of the two observed $\mathrm{Br}_2^{}{}_{}{}^{-}$ optical transitions, and they are both found to be largely $\sigma$-polarized. These polarizations can be explained by the generally accepted energy-level scheme.