Thermal bleaching of V bands at room temperature in alkali iodide crystals containing excess iodine

Thermal stability of V centers has been investigated at room temperature (RT) for additively colored alkali-iodide crystals by means of optical absorption measurements. V-2 and V-3 bands due to V centers in the crystal were gradually bleached by annealing at temperatures near RT. The bleaching rate of the bands is proportional to the square of the V-center concentration. The activation energies for bleaching of the V bands in KI and RbI crystals are estimated from the bleaching rates to be 0.85 +/- 0.04 eV and 0.82 +/- 0.03 eV, respectively. The V-2 bands in the crystals containing divalent cations bleach only weakly at RT. The effect of impurities, such as divalent cations, on the thermal stability of the V centers will be discussed.