VKCenters and Recombination Luminescence in Rubidium Iodide and Sodium Iodide

$V_K$ centers are introduced into both NaI and RbI by low-temperature irradiation of crystals containing electron-attaching impurities. These centers are identified by optical-absorption spectra and by the anisotropic absorption of oriented centers. $V_K$ centers are stable in NaI below about 58°K, and in RbI below about 125°K. These temperatures permit an estimate of the thermal activation energy for migration of the $\mathrm{I}_2^{}{}_{}{}^{-}$ as 0.15 and 0.32 eV, respectively. Recombination of electrons with $V_K$ centers in both crystals produces the "intrinsic" luminescence characteristic of the pure crystal. The connection with exciton states is discussed briefly.