Paramagnetic Point Defect in Fluorine-Doped Silica Glass: The E′(F) Center

Fluorine-doped silica is a key material used in all low-loss and/or radiation-resistant optical fibers. Surprisingly, no fluorine-related radiation-induced point defects have been identified. By using electron paramagnetic resonance, we report the first observation of F-related defects in silica. Their fingerprint is a doublet with 10.5 mT splitting due to hyperfine coupling (hfc) to ^{19}F nuclear spins. An additional 44.4 mT hfc to the ^{29}Si nucleus indicates that this defect belongs to the "E^{'} center" family and has a structure of a fluorine-modified Si dangling bond: 3-coordinated Si atoms with an unpaired electron in an sp^{3} orbital, bonded to a glass network by 2 bridging oxygen atoms and to a F atom.