F+↔FCenter Conversions in Magnesium Oxide

$F$ centers are observed to be converted to $F^{+}$ centers by light in the $F$ band. Heating the crystals results in the reverse of this process, accompanied by $F$-band luminescence. Thermal-luminescence measurements reveal several glow peaks, the most prominent being at 500 °K. We estimate the trap depth for this glow peak to be about 1.1 eV. This trap was identified by a combined thermoluminescence-electron-spin-resonance study to be ${\mathrm{Fe}}^{2+}$, the process being $F+{\mathrm{Fe}}^{2+}\leftrightarrow F^{+}+{\mathrm{Fe}}^{1+}$.