Polarised luminescence in neutron- and proton-irradiated α-Al2O3

The polarisation properties of the optical absorption and luminescence bands of a defect centre in fast neutron- and proton-irradiated alpha -Al 2 O 3 have been studied at low temperatures. The centre has an absorption band at 358 nm and a mirror image emission band at 379 nm. Both bands show resolved vibronic structure with the zero-phonon line at 368.4 nm and a Huang-Rhys factor of about 3.9. These bands are due to allowed electric-dipole transitions involving the same excited electronic level of a low-symmetry defect. The plane-polarised light experiments show that the transition dipole moment of the defect is set at 40 degrees to the c axis of the crystal and the bands are assigned to a defect centre consisting of two next-nearest-neighbour oxygen ion vacancies which have trapped one or more electrons.