H₂O⁻defects in alkali halides

Abstract

The production of H₂O⁻ defects in alkali halides doped with OH⁻ impurities was studied at different temperatures. It was found that H₂0⁻ defects can be formed either directly by the thermal annealing of U₂ centers that get captured by OH⁻ defects or indirectly after the extinction of the U_2x center, an intermediate specimen composed by an U₂ center pre-captured by an OH⁻defect. Several new properties of the H₂O⁻ defect were found and studied. These centers are photodissociated at low temperatures with an efficiency that decreases with increasing temperature and at the expenses of very low activation energies (0.01 eV). The total H₂O⁻ absorption oscillator strength was found to be 0.62 and its integrated absorption did not vary with temperature in this thermal stability range. The structure of the H₂O⁻ band was decomposed and best fit by a sum of 5 gaussians. The integrated absorption of the individual component bands changed with temperature indicating a redistribution of electronic transitions with a possible modification of the con-figurational symmetry of the H₂O⁻ center at high temperatures but with conservation of the total electronic charge. Close to its thermal dissociation limit the H₂O⁻ center smoothly changes into an F center that independently traps an OH molecule and a H atom.