Laser bleaching of F centres in electron-irradiated KBr

Abstract

Experiments are described in which fast electron-induced colour centres in single crystal KBr are optically bleached by illumination at room temperature with light from a focused helium–neon laser. Illuminating power densities up to 10 kw cm⁻² are used to bleach F-centres present in concentrations of 10¹⁸−10¹⁹ centres cm⁻³. The kinetics of bleaching under these conditions are similar for a wide variety of samples, dF/dE varying approximately as E ⁻¹ over six decades of absorbed energy E. The mechanism of bleaching at room temperature for crystals coloured at liquid nitrogen temperature involves only vacancy–interstitial recombination, whereas crystals coloured at room temperature bleach solely by the random aggregation of F centres. A new modulation technique is used to show that R⁺ centres are formed during illumination, and this points to the importance of F–α and M–α mechanisms for aggregation in the later stages of bleaching.