Abstract
Studies of the polarization of luminescence from FA+ centres in CaO have been carried out to confirm the presence and position of the second absorption band of this centre, the FA2+ band. It has been shown that there is polarization in the FA1+ luminescence band of the centre when excited at 325 nm, consistent with the presence of an A to E absorption transition in this region. Complete polarized excitation spectra show that this FA2+ absorption band peaks at 332 nm, confirming the earlier assignment by Boas et al. (1973). A new emission band peaking at 430 nm has been found in Mg-doped CaO crystals, which is shown to be an A to A transition at a defect of tetragonal symmetry. The centre has an A to A absorption band at 406 nm and an A to E absorption band at 317 nm, and it is proposed that the defect is an FAA+ centre: and F+ centre perturbed by two neighbouring Mg impurities.
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