Photoconductivity and luminescence of electron irradiated CaO

We report an experimental investigation of the electronic structure of $F$ and $F^{+}$ centers in electron-irradiated CaO using the spectral response and temperature dependence of photoconductivity, with associated luminescence and fluorescence lifetime measurements made on the same samples. Photoconductivity and optical-absorption measurements were made over the spectral range from 2 eV (620 nm) to 4.5 eV (275 nm). Photoconductivity and fluorescence measurements were made over the temperature range from 65 to 350 K. The photoresponse of $F^{+}$ centers increased sharply with increasing temperature between 210 and 300 K, where the zeroth moment, $M_0$, of the $F^{+}$ luminescence was decreasing with increasing temperature. Below 200 K, photoconductivity from the $F^{+}$ center was masked by photoconductivity from an unidentified impurity. The thermal activation energy for $F^{+}$-center photoconductivity was 0.24 eV. The photoresponse of $F$ centers showed no significant temperature dependence between 65 and 300 K. The $F$-center photoresponse could be optically bleached at all temperatures between 77 and 300 K and, then, fully restored by $\gamma$ irradiation at room temperature.