Ultraviolet-Absorption Spectra of Europium and Ytterbium in Alkaline Earth Fluorides

The ultraviolet-absorption spectra of europium and ytterbium ions in Ca${\mathrm{F}}_2$ crystals have been measured at room and liquid-nitrogen temperatures. Their spectra are similar to each other in the general location of their absorption bands. Both spectra consist of: (i) $4f\to 5d$ bands, between about 24 000 ${\mathrm{cm}}^{-1\mathrm{}}$ and about 50 000 ${\mathrm{cm}}^{-1\mathrm{}}$, due to isolated R${\mathrm{E}}^{2+}$ with crystal field strength ∼17 000 ${\mathrm{cm}}^{-1\mathrm{}}$ between $e_g$ and $t_{2g}$ bands; (ii) $4f\to 5d$ bands, shifted ∼9 000 ${\mathrm{cm}}^{-1\mathrm{}}$ to higher energy, due to R${\mathrm{E}}^{2+}$ ions surrounded by R${\mathrm{E}}^{3+}$ neighbors; (iii) a $4f\to 6p$ broad and weak band of ${\mathrm{Eu}}^{2+}$ in Ca${\mathrm{F}}_2$ with maximum at ∼71 000 ${\mathrm{cm}}^{-1\mathrm{}}$; and (iv) $4f\to 5d$ bands above ∼64 000 ${\mathrm{cm}}^{-1\mathrm{}}$ due to isolated R${\mathrm{E}}^{3+}$ and cluster-ion R${\mathrm{E}}^{3+}$. The structure in the absorption spectra of both isolated ${\mathrm{RE}}^{2+}\left(4\mathrm{}{f}^n\right)$ and ${\mathrm{RE}}^{3+}\left(4\mathrm{}{f}^n\right)$ ions can be interpreted as formed through interaction between a $5d$ electron having $e_g$ or $t_{2g}$ crystal field symmetry and electrons in the ground multiplet of the $4f^{n-1\mathrm{}}$ core.