Polarized luminescence of FA+centres in calcium oxide

Abstract

Studies of the polarization of luminescence from F_A⁺ centres in CaO have been carried out to confirm the presence and position of the second absorption band of this centre, the F_A2⁺ band. It has been shown that there is polarization in the F_A1⁺ 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 F_A2⁺ 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 F_AA⁺ centre: and F⁺ centre perturbed by two neighbouring Mg impurities.