Symmetry of a colour centre in magnesium oxide

The symmetry of the centre responsible for the 9570 cm -1 (10 450 Å) line, which appears in the absorption spectrum of neutron-irradiated magnesium oxide, has been investigated at 77 °K by analysis of the polarization spectra of the line under uniaxial stress. The line is associated with a [110] linear electric dipole oscillator in a centre which has orthorhombic symmetry and [110] orientation. The peaks in the phonon-assisted spectrum of the line can be explained by taking 280 cm -1 and 380 cm -1 as the energies of the fundamental phonon modes. There is some evidence for a second zero-phonon line at 10 070 cm -1 having phonon energies of 380 cm -1 and 490 cm -1 .