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 F19 nuclear spins. An additional 44.4 mT hfc to the S29i 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 sp3 orbital, bonded to a glass network by 2 bridging oxygen atoms and to a F atom.