Bleaching and Recovery of F Centers in KCl

F centers in KCl are optically bleached at room temperature with a minimum light energy of 20.1±{}1.0 ev per F center. The F band is recovered on further room-temperature x raying with a minimum energy of 69±{}5 ev per F center. The increase in the M and R bands during optical bleaching of the F band is nullified upon the recovery of the F band by further x raying. The recovery kinetics are of second order in the number of optically bleached F centers with a rate-constant of (7.9±0.8)×10^-20 cm³/F center-sec. The rate law requires an energy transfer from the bulk lattice and is consistent with an exciton mechanism.