Spin-orbit coupling and stress spectra of trigonal colour centres in magnesium oxide

Two independent zero-phonon lines are seen at 6420 Å and 6490 Å in the absorption spectra of single crystals of magnesium oxide which have been neutron irradiated and thermally annealed. The 6490 Å line is split by 9 cm -1 owing to spin-orbit interaction. The polarization spectra of the two colour centres were examined under uniaxial stresses of up to 50 kg mm -2 and are in agreement with the theoretical splitting patterns for electric dipole transitions in trigonal centres. Models for the centres include the analogue of the R centre in the alkali halides and an R centre with a trigonally disposed cation vacancy.