Energy Levels of Divalent Thulium in CaF2

From the fluorescence and absorption spectra of Ca${\mathrm{F}}_2$: ${\mathrm{Tm}}^{2+}$ an energy-level scheme for the system is proposed. The cubic-crystal-field parameters are determined, and evidence is presented that the $f-f$ transitions are of magnetic-dipole origin. A fluorescent band is observed that appears to originate in closely coupled ${\mathrm{Tm}}^{2+}$—${\mathrm{Tm}}^{3+}$ pairs. The large crystal-field splitting in Ca${\mathrm{F}}_2$: ${\mathrm{Tm}}^{2+}$ is confirmed by the absorption spectra of its isoelectronic systems of Ca${\mathrm{F}}_2$: ${\mathrm{Yb}}^{3+}$ and Ca${\mathrm{F}}_2$: ${\mathrm{Ce}}^{3+}$.