Correlation of Electron Paramagnetic Resonance and Optical-Absorption Spectra of Ca F 2 : Yb 3 +

An account is given of a combined optical and paramagnetic resonance investigation of trivalent ytterbium ions in ten different sites in calcium fluoride. Correlation between site geometry and optical absorption has been obtained for six of these by examining crystals prepared by different methods. A single optical-absorption line was observed for all but one of these sites, so that it was impossible to deduce the magnitudes of the Stark splittings. However, there is some evidence to indicate that they are large (of the order of hundreds of ${\mathrm{cm}}^{\ensuremath{-}1}$), rather than small, as suggested by Low. A new rhombic paramagnetic resonance is also reported and is thought to arise from pairs of ${\mathrm{Na}}^{+}$ and ${\mathrm{Yb}}^{3+}$ ions on adjacent ${\mathrm{Ca}}^{2+}$ sites.