Single crystals of MgO have been additively colored by heating them in Mg vapor at temperatures from 1600 to 1800'C and pressures up to 4000 Torr. The F+ and F' optical absorption bands (due to anion vacancies containing one and two electrons, respectively) were observed to be extremely similar but distinguishable, and were observed at liquid-nitrogen temperature to peak at 4.96 and 5.03 eV, respectively.The oscillator strength for the F' center is estimated to be 0.70&0.05. A photoconversion process F+~F' was studied. The luminescence bands for these centers were observed at 3.1 and 2.4 eV, respectively. The density of F' centers 1' produced by the additive coloring process was observed to be related to the density of atoms in the vapor $«by the equation EF =1270K«e @'~~, where Q=1.53 eV. The temperature dependence of these two bands was measured and is discussed.

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