Luminescence of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>F</mml:mi></mml:math>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 \ifmmode\pm\else\textpm\fi{} 4 msec. We attribute this long lifetime to the forbidden triplet-to-singlet transition of the $F$ center. The $F\ensuremath{\leftrightarrow}{F}^{+}$ conversion can be shown by studying the $F$ and ${F}^{+}$ emission bands in these crystals.

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