Low-temperature fluorescence in sapphire

The polarized fluorescence band at 3.0 eV which can be produced by photoexcitation of the 6.1-eV $F$ band in high-purity $\alpha$-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ has been studied over the temperature range 4.4-300 K. The centroid and zeroth moment of the band remain relatively constant with increasing temperature, but the half-width follows a hyperbolic cotangent relation with an effective frequency of 345 ${\mathrm{cm}}^{-1\mathrm{}}$. A Huang-Rhys factor of 14.7 is estimated for the band. After pulse excitation at 4.4 K the fluorescence decays following a curve which can be analyzed as the sum of two exponentials with lifetimes of 24 and 160 ms, respectively. Both the lifetime and the intensity of each component are temperature dependent between 15 and 40 K. Above 50 K the intensity decays following a single exponential with a lifetime of 34 ms. A schematic electronic structure is suggested for the $F$ center, which is consistent with the experimental results and the expected nature of the center.