A luminescence band with a half-width of about 0.6 eV at liquid-nitrogen temperature, which can be excited by uv light and x rays, has been observed in irradiated and additively colored MgO crystals at 3.13 eV. By combining ESR and optical studies, this emission band has been identified as due to $F^{+}$ centers (oxygen-ion vacancies each occupied by one electron). Furthermore, after resolving the $F^{+}$- and the $F$-band absorption in the vicinity of 5.0 eV, an analysis of the bands indicates that the oscillator strength of the $F$ center is about 1.25, when the oscillator strength of the $F^{+}$ center is taken to be 0.8.

• Received 14 April 1969

DOI:https://doi.org/10.1103/PhysRev.186.865