Zero-phonon lines under uniaxial stress in magnesium oxide

Zero-phonon lines at 3628 Å, 4684 Å, 4700 Å, 5248 Å, 5680 Å, 6419 Å, 6490 Å and 7058 Å observed in the optical absorption spectrum of magnesium oxide after fast-neutron irradiation have been examined at 77 °K and 4 °K. The half-widths of all the lines varied markedly according to the particular irradiation temperature, dose and the subsequent heat treatment of the specimen. This behaviour is associated with the presence of strain fields associated with interstitial dislocation loops produced by irradiation. The phonon-assisted structure observed on the high-energy side of the zero-phonon lines has been analysed and compared with the phonon spectrum determined by Peckham using slow-neutron scattering. The splittings of the zero-phonon lines under uniaxial stress show that defects are present in magnesium oxide with the same symmetries as the M, R and N 1 centres observed in the alkali halides. As a result, it is suggested that analogous aggregate defects may be present in magnesium oxide although the situation is probably complicated by the divalent nature of the magnesium oxide lattice.