Luminescence-detected EPR of Oxygen-fluorine vacancy complexes in CaF₂

In several CaF₂ single crystals grown by the Bridgman method and doped with CaO or in addition with SrF₂ or NaFa luminescence band between 470 nm and 600 nm could be excited at 212 nm, its peak wavelength depending on the doping. With photo-luminescence (PL) detected EPR five spin triplet centres were identified. Their axial fine structure constants D varied from 87 mT to 690 mT whereby the most intense spectra had the smallest D value. Theoretical calculations of the fine structure tensors and superhyperfine interactions show that the most intense and probable triplet centre consists of a pair of an O_F-on a F- site next to a nearest neighbour F^- vacancy which, compared to the well-known F centre, is occupied by an excited electron in a shallow orbital and coupled to the 2p hole at the oxygen to form a S = 1 excited state. This centre can be excited optically at low temperature to a metastable new configuration where the F centre jumps into a 〈 111〉 position opposite the O_F.The other triplet centres have probably also an O-F centre nearest neighbour configuration but have in addition a cation impurity or vacancy nearby.