V Centers in KI Crystals Containing Excess Iodine

Optical and thermal bleaching of the V 2 and V 3 bands were investigated for additively- or electrolytically-colored KI crystals. After V 3 -light irradiation at 85 K, absorption bands were formed at 430 and 800 nm in company with an increase near the fundamental-absorption edge; while, after the same irradiation at 125 K, the α band and a band with a peak at about 330 nm were formed simultaneously. Changes in intensities among these bands were caused with subsequent optical or thermal treatments; changes in the V 2 and V 3 bands were always accompanied. Upon irradiation with polarized light, the band at about 330 nm exhibited the [100] anisotropy, whose maximum was shifted to 325 nm. On the assumption that cation vacancies are introduced in halogen-excess crystals, probable models of V centers are discussed and then the photochemical and thermal changes are interpreted by them.