S01Level ofPr3+in Crystals of Fluorides

The ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ level of the trivalent praseodymium ion ${\mathrm{Pr}}^{3+}$ in Ca${\mathrm{F}}_2$ and La${\mathrm{F}}_3$ crystals has been identified in the absorption spectrum and in the excitation spectrum of orange fluorescence. It appears as a small peak near the $4f\to 5d$ band of ${\mathrm{Pr}}^{3+}$ at 47 200 ${\mathrm{cm}}^{-1\mathrm{}}$ (2120 Å) in Ca${\mathrm{F}}_2$ and 46 900 ${\mathrm{cm}}^{-1\mathrm{}}$ (2135 Å) in La${\mathrm{F}}_3$. The identification of this peak as a transition to the ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ level rather than as part of the $4f\to 5d$ transition is based on its small spectral shift when observed in different environments, its low intensity, and its relatively narrow width—all of which are characteristics of a shielded $4f$ transition.