Effect of substitutional hydride ions on the charge states of oxygen vacancies in thermochemically reduced CaO and MgO

Different methods of thermochemical reduction of CaO and MgO crystals can yield dramatically different optical-absorption and -emission properties. In general, thermochemical reduction produces oxygen vacancies and substitutional ${\mathrm{H}}^{\mathrm{-}}$ (hydride) ions with the vacancies mainly in the two-electron charge state (F center). Removal of protons before or during thermochemical reduction results in anion vacancies that are primarily in the one-electron ${\mathit{F}}^{+}$ state. Data are presented to relate the effect of hydride concentration on the charge states of the anion vacancies. Photobleaching experiments permit identification of three absorption bands as due to electron donors in thermochemically reduced CaO. Finally, we provide experimental results which indicate that the intrinsic F and ${\mathit{F}}^{+}$ centers in MgO or CaO crystals are not viable candidates for tunable lasers.