Luminescence of the $F$ center in sapphire

An $F$-center emission band (3 eV) has been observed in subtractively colored ${\mathrm{Al}}_2$${\mathrm{O}}_3$ crystals upon exciting with $F$ light (6.1 eV). The lifetime of the 3 eV luminescence is 36 ± 4 msec. We attribute this long lifetime to the forbidden triplet-to-singlet transition of the $F$ center. The $F\leftrightarrow F^{+}$ conversion can be shown by studying the $F$ and $F^{+}$ emission bands in these crystals.