The Influence of Electron‐Trapping Impurity on Defect Creation and Bleaching in KCl Irradiated at 78 °K

The excitonic mechanism for creation of halogen Frenkel‐type defects in alkali halides predicts an early positive curvature in the defect growth curve. The model also predicts that annihilation of VK‐centers by the optical release of electrons from traps would produce Frenkel pairs in the halide sublattice. The first prediction has been checked in KCl: Tl+ and KCl:Pb++ irradiated at 78 °K with 1.7 MeV Van de Graaff electrons, and the second upon ir bleaching of an irradiated Tl+‐doped crystal at 78 °K. If one takes the F‐center concentration as a measure of the Frenkel‐defect density, both tests were positive. A careful monitoring of the α‐band region before and after bleaching, however, indicates that the F‐center concentration is not a valid measure of the vacancy density. In a crystal containing 48 ppm Tl°, the α‐center concentration after irradiation at 80 °K was 2.5 times that for F‐centers. Moreover, ir bleaching of Tl° and VK, while enhancing the F‐band ≈ 30%, destroyed nearly all of the α‐centers. The last observation reveals that more empty halide vacancies are destroyed than F‐centers are produced by the bleaching light.