Sharp-Line Fluorescence, Electron Paramagnetic Resonance, and Thermoluminescence of ${\mathrm{Mn}}^{4+}$ in $\alpha$-${\mathrm{Al}}_2$${\mathrm{O}}_3$

Sharp-line fluorescence, paramagnetic resonance in the ground state, and optical absorption due to ${\mathrm{Mn}}^{4+}$ in $\alpha$-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ have been observed (analog of ${\mathrm{Cr}}^{3+}$). The ${\mathrm{Mn}}^{4+}$ valence state is obtained by charge compensation with ${\mathrm{Mg}}^{2+}$. The increased charge of ${\mathrm{Mn}}^{4+}$ compared to ${\mathrm{Cr}}^{3+}$ results in a stronger crystal field and greater covalency whose effects are clearly seen in both the optical and paramagnetic resonance results. The ground state splitting is 0.39 ${\mathrm{cm}}^{-1\mathrm{}}$, almost the same as in ruby, while the metastable ${}_{}{}^2{}_{}{}^{}E$ state splitting is 80 ${\mathrm{cm}}^{-1\mathrm{}}$. When the crystals are irradiated with ultraviolet (<3000 Å), more than 50% of the ground state of ${\mathrm{Mn}}^{4+}$ is depopulated, as long-lived traps are filled which decay via the ${}_{}{}^2{}_{}{}^{}E$ states through sharp-line thermoluminescence. The possible application to light masers is briefly discussed.