Optical Absorption and Electron Spin Resonance after Optical and Thermal Bleaching of F Centers

A correlation is observed between changes in the half-width and peak position of the F band and the presence of M and R centers in x-irradiated KCl crystals. The changes in the shape of the F band caused by optical bleaching are attributed to absorption bands of M and R centers located in the spectral region of the F band. The narrowing of the electron spin resonance line which occurs during optical bleaching of x-irradiated KCl crystals is removed by thermal treatment at 140^∘{}C. The narrowing is tentatively attributed to the presence of a second, narrow resonance line, due to either R₃ or N centers, which is superimposed on the line attributed to F centers. The results also suggest that M, R, and V₃ centers are not paramagnetic. Both the optical absorption and the electron spin resonance measurements indicate that F centers are present even after prolonged optical bleaching and that the character of the F center is not changed during bleaching.