X-ray stimulated luminescence in MgO

Studies have been made of the emission spectrum of MgO crystals induced by X-irradiation at 90 K. Two bands (half-widths ～0.8 eV) were observed to peak at 4.95 and 3.2 eV, respectively, in high purity crystals. Doping with 100 ppm or greater of Fe, Co, Cr, Cu, Mn, and Ni suppressed the luminescence, though in the MgO:Ni crystal the 2.3 eV Ni²⁺ band due to the ¹T_2g→³A_2g transition was observed. In deuterium-doped crystals the ratio of the intensity of 3.2–4.95 eV emission was found to be 1.2 as compared to 8 for the undoped crystals. Prior exposure of the pure crystals to ionizing radiation enhances the 4.95 eV band by a factor of three while not affecting the 3.2 eV band. This enhancement of intensity decays in several stages upon standing at room temperature in a way that reflects the thermal stability of the various components of the composite V-band absorption. These facts together with the observation that the 210 K thermoluminescence peak is composed entirely of 4.95 eV emission indicate that this luminescence band is associated with the recombination of an electron with a hole located in a V-type center, i.e. O?□ + e → (O²?□)1 → O²?□ + 4.95 eV, where the square indicates that the perturbing positive ion vacancy is adjacent to the oxugen ion which has captured the hole. In MgO:Li⁺, which exhibits no V-type centers upon irradiation, the 4.95 eV band was absent and a 2.9 eV emission which may be associated with recombination at the Li]⁰ center was observe.