V centers in irradiated alkali halide crystals

Abstract

Resonant Raman scattering (RRS) allows a determination of the nature of stable V centers created under X or γ irradiation in alkali halide crystals. In the early stage of irradiation at low temperature, trihalide ions are formed. By increasing the radiation dose, polyhalide ions, characterized by the vibrational mode of the halogen molecule in X − n (X: halogen; n = 5 …), appear. These centers are precursors of halogen clusters created at room temperature (or above). They exhibit the structure of an iodine crystal in KI and that of a “solid like” aggregate of bromine molecules in KBr. Thermal annealing almost restores the crystal except for voids which remain after the disappearance of halogen aggregates. The stabilization of halogen interstitials into V centers is the major process in the production of stable electron and hole defects obtained in alkali halide crystals exposed to ionizing radiations. The nature of the V centers observed after irradiation of pure or doped crystals is discussed in this paper together with effects due to temperature, irradiation dose and thermal annealing.