TEMPERATURE DEPENDENCE OF F BAND IN MAGNESIUM OXIDE

The position and width of the F band in magnesium oxide have been measured in the temperature range 4-400°K. The data have been analysed in terms of the simplest adequate 'configuration coordinate' model. The width results give an effective frequency of 7·8 × 1012 s-1, 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% 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.