Anion-Vacancy Processes Occurring after Room-TemperatureF-Light Bleaching or Irradiation of NaCl

The thermally activated processes occurring in the dark after room-temperature $F$-light bleaching of x- or $\gamma$-irradiated NaCl have been investigated. It has been shown that, near room temperature, the concentration of free-anion vacancies decays exponentially with time. The kinetics of the process has been studied by putting the crystal in the dark for a variable time after bleaching and then subjecting it to a short illumination or irradiation pulse which detects the disappearance of the vacancies. It has been observed that the decay in vacancy concentration is closely associated with a decrease in the $F^{\prime }$-band intensity and with increases in the $F$- and $M$-band intensities. The experimental information has led to the conclusion that vacancies become trapped at $F^{\prime }$ centers according to a first-order reaction, giving rise to $F$ and $M$ centers through the mechanisms $F^{\prime }+\mathrm{anion}\mathrm{vacancy}\to 2F$, and $F^{\prime }+\mathrm{anion}\mathrm{vacancy}\to M$, respectively. The activation energy for the trapping process has been found to be $E=0.48\mathrm{}\pm 0.06$ eV. These phenomena also occur after an x or $\gamma$ irradiation, and are shown to provide a coherent picture of the previously reported anomalous coloring behavior appearing at the start of a new irradiation period.