A study of the 2.3-eV absorption band in MgO crystals irradiated with electrons and gamma rays at 7, 80, and 305°K has been made. This broad band, which has been designated as $V_1$, has a half-width of 0.96 eV at 5 and 78°K and could be optically bleached even at liquid-helium temperature. When specimens were bleached with polarized light, it was found that there was no dichroism in any of the $〈100〉$, $〈110〉$, or $〈111〉$ directions at 5°K. The colorability of the crystals was strongly dependent on their thermal treatment. The concentration of centers responsible for the 2.3-eV absorption could be enhanced by quenching the crystals from high temperature prior to irradiation and could be suppressed by slow cooling of the specimens to room temperature. The effects of these heat treatments were found to be reversible. The presence of certain impurities in the crystals was observed to suppress the 2.3-eV coloration, but no correlation between the ${\mathrm{Fe}}^{3+}$ concentration or the total Fe concentration and the maximum coloration in a quenched crystal was observed. The afterglow luminescence was also surveyed in the energy range 1.3-5.0 eV.

• Received 22 August 1966

DOI:https://doi.org/10.1103/PhysRev.154.842