Optical spectroscopy of and in

The configuration ions and have been incorporated in strontium fluorovanadate (SVAP) crystals grown using the Czochralski technique. Such crystals provide fourfold-coordinated sites for occupancy by the and ions, which are located near the centres of their respective tetrahedrally bonded molecular ions and . The optical absorption spectra of these ions are in the range 500 nm to 1100 nm and feature a number of overlapping bands related to the splittings of the and excited states by the distorted tetrahedral crystal field. In terms of point-ion crystal field theory strong- and weak-field descriptions are appropriate to and molecular ions respectively. This is confirmed for by photoluminescence spectra which reveal sharp zero-phonon lines attended by vibronic structure characteristic of coupling to molecular vibrations of both and the host ion. Luminescence from the ion is quenched by efficient nonradiative decay at all temperatures between 10 and 300 K.

The energy level structure of these ions has been calculated using group theory assuming reductions in symmetry from to to . A comparison of the calculated level structure with the absorption and luminescence spectra shows that and occupy distorted tetrahedral sites in SVAP. Selection rules derived for all transitions assuming symmetry account for all the features in the absorption spectra of and and in the photoluminescence spectrum of .