Experimental measurements and theoretical calculations are reported which may resolve some of the remaining difficulties associated with photoemission from anion vacancies in thermochemically reduced MgO crystals. The intensities of the 2.3and 3.2-eV luminescence bands are found to be strongly influenced by both the concentrations of H ioris and anion vacancies present, and also by the intensity of the -5.0-eV exciting light. Theoretical calculations predict that for a 'A~g relaxation of the surrounding ions the 'T&"and T&"electronic states of the F center are almost degenerate and -0.05 eV above the 'A~g state. The 'Tj"-+'A&~and the T&"~'A&~transition energies are predicted to be 2.2 and 2.9 eV, respectively.

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The first unambiguous evidence is presented that H ions are the main electron traps responsible for the long-lived phosphorescence observed at 2.3 ev near room temperature in thermochemically reduced MgO.

The efficiency of the 3.0-eV photoluminescence band which results from optical excitation into the 6.1-eV F band in single-crystal sapphire is much weaker for F centers introduced by fast neutron bombardment than for those produced thermochemically (i.e., by either deliberate additive coloration or by coloration during growth). The luminescent efficiency has been monitored at room temperature following successive isochronal annealing stages up to 580 °C for samples exposed to ∼1017 neutrons cm−2. The relative emission intensity per F center increased rapidly as the F centers were thermally destroyed and reached a value at the upper end of the annealing range which was 40 times the preanneal value. It is suggested that the initial relatively weak emission is due to concentration quenching which results from the highly inhomogeneous distribution of lattice defects, i.e., displacement cascades, produced by fast neutron bombardment.

The excited states of the F+ center in neutron-or proton-irradiated SrO have been studied by measuring the temperature dependence of the yield and lifetime of the F+ fluorescence over the temperature range from 5 to 140 K. Both the fluorescence yield and lifetime decrease in two stages; one over the temperature range from 5 to 60 K, and the other over the range 60 to 140 K. The results are interpreted in terms of a three-level model of the F+ center which is suggested by recent theoretical work. The lowest radiative level, which has a lifetime of 0.42 p, sec, is found to be 0.05 eV below the conduction band. Another level is found only 0.005 eV above this radiative level. In order to account for the results, nonradiative decay to the ground state appears to occur from this second level.