Optical Absorption and Electron Spin Resonance after Optical and Thermal Bleaching ofFCenters

Abstract

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 $R_3$ or $N$ centers, which is superimposed on the line attributed to $F$ centers. The results also suggest that $M$, $R$, and $V_3$ 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.