By using the Green's function method, the localized state energy was calculated for an electron which was trapped in the chain-O vacancy. For reasonable parameter values, it lay above the chain-O and plane-O band top. The formation of a stable chain F+ center was proposed, as in the mechanism of photo-induced conductivity.

P.Rubin, N.Kristoffel: Journal of Physics - Condensed Matter, 1998, 10[8], L127-9