Optical absorption and luminescence experiments were used to study the photo-conversion of neutral O vacancies (F centers) in single crystals which had been thermochemically reduced at high temperatures. In crystals with an undetectable concentration of hydride ions and a moderate concentration of F centers (about 1017/cm3), excitation with ultra-violet light produced positively charged anion vacancies (F+ centers) and electrons which were subsequently trapped at impurities. Under continuous excitation, the F+ centers released holes which were trapped at cation vacancies that were charge-compensated by impurities. In crystals with high concentrations of hydride ions and F centers (about 1018/cm3), the electrons from the F-to-F+ photo-conversion were trapped mainly at the hydride ions, to form H2- ions which were metastable at room temperature.

Photoconversion of F-Type Centers in Thermochemically Reduced MgO Single Crystals. R.González, M.A.Monge, J.E.M.Santiuste, R.Pareja, Y.Chen, E.Kotomin, M.M.Kukla, A.I.Popov: Physical Review B, 1999, 59[7], 4786-90