F+andF′Centers in Magnesium Oxide

Single crystals of MgO have been additively colored by heating them in Mg vapor at temperatures from 1600 to 1800°C and pressures up to 4000 Torr. The $F^{+}$ and $F^{\prime }$ optical absorption bands (due to anion vacancies containing one and two electrons, respectively) were observed to be extremely similar but distinguishable, and were observed at liquid-nitrogen temperature to peak at 4.96 and 5.03 eV, respectively. The oscillator strength for the $F^{\prime }$ center is estimated to be 0.70±0.05. A photoconversion process $F^{+}\leftrightarrow F^{\prime }$ was studied. The luminescence bands for these centers were observed at 3.1 and 2.4 eV, respectively. The density of $F^{\prime }$ centers $N_{F^{\prime }}$ produced by the additive coloring process was observed to be related to the density of atoms in the vapor $N_{\mathrm{Mg}}$ by the equation $N_{F^{\prime }}=1270\mathrm{}{N}_{\mathrm{Mg}}{e}^{-\frac{Q}{\mathrm{kT}}}$, where $Q=1.53\mathrm{}$ eV. The temperature dependence of these two bands was measured and is discussed.