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Abstract. The position and width of the F band in magnesium oxide have been measured in the temperature range 4-400 OK. The data have been analysed in terms of the simplest adequate 'configuration coordinate' model. The width results give an effective frequency of 7.8 x 1012 s -l , which is close to a peak in the phonon density of states and to the value extrapolated from data for the alkali halides. There is evidence that the effective frequency is reduced by about 5 "4 in the excited state of the F centre. The Huang-Rhys factor is about 39, and luminescence is predicted at about 2.4 ev. The band shape indicates the existence of three small absorption bands on the high-energy side of the main F absorption band. These appear to be associated with the F centre, but their nature is not clear.

The alkaline earth oxides have the same crystal structure as the alkali halides and consequently defects with electronic states and symmetry properties analogous with the M-and R-centers are observed after neutron irradiation. The divalent nature of the host lattice also offers additional potential for electron trapping : in this respect the R-like centres have been especially interesting. The application of uniaxial stress techniques has confirmed that optical zero-phonon lines at 5 248 Å, 6 419 Å and 6 490 Å are associated with such centres. The line at 5 248 Å is strongly dichroic under stress at 4 °K and the results are indicative of E → A transitions. The lack of such dichroic behaviour in the 6 419 Å and 6 490 Å lines confirm that these lines are associated with A → E transitions. Calculation of the electronic states of theses trigonal defects suggest that the 5 248 Å, 6 419 and 6 490 Å lines are due to clusters of three anion vacancies lying in a (111) plane and containing 5, 6 and 7 electrons respectively. In addition to defects which are readily recognised on account of their M- and R-like characteristics, there are defects that are not analogous with any defects so far identified in the alkali halides. Two such defects have spin triplet states (S = 1) at 300 OK but have spin singlet states at 4 °K. Models for these defects involve exchange coupled pairs of F-centres in next nearest neighbour association along the [100] axes (tetragonal symmetry) and [110] axes (orthorhombic symmetry). A third defect, designated the Fz-centre by Wertz, has optical properties in accord with those expected for a single electron bound to an anion-cation vacancy pair. More recent paramagnetic resonance measurements at Q-band strongly support the proposed model for this defect