Electron paramagnetic resonance and optical studies of the FA+centre in magnesium-doped calcium oxide

This centre consists of an electron trapped at an oxygen vacancy with a magnesium ion as one of its nearest neighbours. The EPR spectrum shows the expected tetragonal symmetry, and hyperfine interaction with the 10% abundant 25 Mg nucleus confirms the structure of the defect. Optical absorption bands associated with transitions from the '1s' ground state to the '2p' excited state split by the tetragonal field have been identified. Both bands show vibronic structure at low temperatures and their vibronic properties are compared with those of the F + band. Annealing studies and a comparison of the ratio of F A + to F + centres in various crystals show that the EPR and optical spectra are due to the same centre. A simple theoretical model gives the correct relative shifts of g/sub /// and g perpendicular to , but not their absolute values.