Spectroscopic studies of ${\mathrm{Eu}}^{3+}$ and ${\mathrm{Dy}}^{3+}$ centers in ${\mathrm{ThO}}_2$

Luminescence and excitation spectra of ${\mathcal{R}}^{3+}$-doped thorium oxide $({\mathcal{R}}^{3+}={\mathrm{Eu}}^{3+}{,\mathrm{}\mathrm{Dy}}^{3+})$ are reported and analyzed. ${\mathrm{ThO}}_2$ was synthesized and grown as crystal using the flux technique. Usually, ${\mathcal{R}}^{3+}$ ions substitute ${\mathrm{Th}}^{4+}$ as a cubic site, but charge compensation often results in the presence of other centers. Doping concentration also has an important effect on the formation of the crystallographic sites. Using selective dye laser excitation at 12 K and selection rules, site symmetries of ${\mathrm{Eu}}^{3+}$ and ${\mathrm{Dy}}^{3+}$ ions in ${\mathrm{ThO}}_2$ are determined and fluorescence from different centers is isolated. Results show that under conditions investigated, ${\mathrm{Eu}}^{3+}$ ions have ${\mathrm{O}}_h$ and ${\mathrm{C}}_{3v}$ site symmetry in ${\mathrm{ThO}}_2$ while for ${\mathrm{Dy}}^{3+}$ only ${\mathrm{C}}_{3v}$ exists. Lifetimes were measured and discussed. A crystal-field (cf) calculation has been performed on the reduced ${}^7{F}_J$ basis of the ground ${}^{7}F$ term of the ${\mathrm{Eu}}^{3+}$ ion. The crystal-field parameter set is then used to calculate ${\mathrm{Dy}}^{3+}$ energy levels in ${\mathrm{ThO}}_2$ as a test of its consistency.