Delayed Luminescence of theMCenter inF−to−MConversion in NaF

A "delayed luminescence" has been observed in x-irradiated NaF samples after bleaching with $F$ light near room temperature. The delayed luminescence (D.L.) is attributed to the latent formation of $M$ centers in their excited state and their subsequent luminescence. An analysis of the decay of this D.L. indicates that there are two types of migrating center involved in $F-\mathrm{t}\mathrm{o}-M$ conversion, corresponding to two competing optical bleaching processes. It is proposed that these two migrating centers are ($\alpha ,\tilde{\alpha }$) centers (vacancy pairs) and $\alpha$ centers (anion vacancies). The activation energies for individual migrating centers are found from the temperature dependence of the exponential decay of the D.L. to be $E_1=0.75\mathrm{}\pm 0.02$ eV and $E_2=0.77\mathrm{}\pm 0.08$ eV. The temperature dependence of the time constant for the growth of the D.L. when the $F$ light is turned on indicates that the activation energy for $F-\mathrm{t}\mathrm{o}-M$ conversion is $E_g=0.62\mathrm{}\pm 0.02$ eV.