Optical properties of CaO crystals containing hydrogen

Optical-absorption measurements show that substitutional ${\mathrm{H}}^{\ensuremath{-}}$ ions (that is, protons on anion sites each with two electrons) are thermally more stable than anion vacancies when thermochemically reduced CaO samples are annealed in a reducing atmosphere. Electron irradiation decreases the concentration of ${\mathrm{H}}^{\ensuremath{-}}$ ions and increases the concentration of oxygen vacancies with a cross section much larger than for elastic collision damage. The room-temperature lifetime of the 2.1-eV $F$-center luminescence decreases as the concentration ratio of $F$ centers to ${\mathrm{H}}^{\ensuremath{-}}$ ions increases.