OPTICAL BLEACHING OF V-CENTERS AT LIQUID-NITROGEN TEMPERATURE IN KI CRYSTALS CONTAINING EXCESS IODINE

Optical absorption, luminescence and ESR of V-type centers have been studied for KI crystals containing excess iodine. Colored crystals were irradiated previously with V3 light at liquid nitrogen temperature (LNT) to convert the primary V centers into other V-type centers. Dichroism is induced in the absorption bands of the crystal at around 420 and 800 nm by irradiation with the [110]- or [100]-polarized light. ESR spectrum of 11 equally spaced lines is detected at LNT under a magnetic field along a [110] axis of the crystal. Absorption bands, luminescence and ESR lines due to these centers disappear at temperatures above 110 K and original V centers recover in pure KI crystals. We conclude that V(K) and V(L) centers are generated simultaneously by the optical bleaching of the primary V centers.