Optical Behavior of V Centers in KI Crystals at Low Temperatures

Photochemical conversion and reorientation of V centers in KI crystals colored additively or electrolytically are studied at temperatures of 19–140 K. When V centers are dissociated at 19 K by irradiation with light in the 270 nm region, I 2 - -like centers are formed. The I 2 - -like centers begin to bleach at about 40 K forming centers of another type which possess <110 > symmetry and give rise to absorption bands with peaks at 358, 430, 625 and 808 um. The V 2 and V 3 bands observed in quenched crystals are composed of absorption bands due to two kinds of V centers with <110 > and <111 > symmetries. Besides these centers, other V centers with <100 > or near-<110 > symmetry are also formed by optical excitation; after the optical excitation, dipoles of V centers reorient into proper directions at a given temperature of the sample. Based on the model of the I 3 - molecule-ion bound to a cation vacancy, possible structures of the four kinds of V centers are discussed.