Luminescence ofFandF+centers in magnesium oxide

Time-resolved spectra of luminescence from F and $F^{+}$ centers in MgO excited at 248 nm are reported over 8 decades of time and intensity, from 20 ns to 2 s. The decay time of F and $F^{+}$ luminescence has been measured at temperatures ranging from 90 to 573 K. The luminescence yield and spectra were studied as a function of excitation-power density at 248 nm over a span of 10 decades, from 1 mW/${\mathrm{cm}}^2$ to 38 MW/${\mathrm{cm}}^2$. Higher pump-power density favors F luminescence over $F^{+}$ luminescence in thermochemically reduced crystals, and the luminescence yield saturates above about 50 kW/${\mathrm{cm}}^2$ of pump intensity. Transient absorption spectra were measured in the microsecond-to-seconds time range. The luminescence data show that the decay kinetics of excited F and $F^{+}$ centers are dominated by ionization from the excited state and charge recapture from traps which include the $H^{\mathrm{-}}$ center in thermochemically reduced MgO. It is furthermore inferred that electrons are the charge carriers having primary influence on kinetics of both F and $F^{+}$ centers. This implies that the $F^{+}$ first-excited state lies close to the conduction-band edge. We confirm that excitation at 248 nm also causes release of valence holes, and propose a mechanism which may explain the hole release, the increase of the F-to-$F^{+}$ intensity ratio with increasing pump intensity, and the saturation of luminescence with increasing pump intensity.