The lowest $4f\to 5d$ ultraviolet absorption band of eleven (Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb) of the fourteen trivalent rare-earth (RE) ions in host Ca${\mathrm{F}}_2$ crystals has been measured. The room temperature absorption cross section [${\mathrm{cm}}^2$] and half-width [${\mathrm{cm}}^{-1\mathrm{}}$] of these bands for the first-half series of ${\mathrm{RE}}^{3+}$ are larger than that of the second-half series (from ${\mathrm{Tb}}^{3+}$ on), possibly because of some spin-forbiddenness in the transition of the latter. Typically, they are ∼5×${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$ and ∼1700 ${\mathrm{cm}}^{-1\mathrm{}}$, respectively, for the former and ∼3×${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$ and ∼1300 ${\mathrm{cm}}^{-1\mathrm{}}$, respectively, for the latter at typical concentration of ∼${10}^{-4\mathrm{}}$. The oscillator strength is estimated to be ∼${10}^{-2\mathrm{}}$ for the first half-series and ∼4×${10}^{-3\mathrm{}}$ for the second half-series ${\mathrm{RE}}^{3+}$. At liquid nitrogen temperature the band sharpens and shifts ∼200 ${\mathrm{cm}}^{-1\mathrm{}}$ toward lower wave number, probably because of increasing crystal field in the contracted lattice; a zero-phonon line and some vibrational structure are also developed in ${\mathrm{Ce}}^{3+}$-and ${\mathrm{Pr}}^{3+}$-doped Ca${\mathrm{F}}_2$. The location of the band ranges from ∼33 000 ${\mathrm{cm}}^{-1\mathrm{}}$ (${\mathrm{Ce}}^{3+}$) to ∼71 000 ${\mathrm{cm}}^{-1\mathrm{}}$ (${\mathrm{Yb}}^{3+}$), with that of ${\mathrm{Gd}}^{3+}$ and ${\mathrm{Lu}}^{3+}$ outside (≳80 000 ${\mathrm{cm}}^{-1\mathrm{}}$) of the transparent region of Ca${\mathrm{F}}_2$ crystal. Compared with the data available for the free ions ${\mathrm{Ce}}^{3+}$, ${\mathrm{Pr}}^{3+}$, and ${\mathrm{Yb}}^{3+}$, the location of the lowest $4f\to 5d$ transition of ${\mathrm{}\left(\mathrm{RE}\right)\mathrm{}}^{3+}$ ions in Ca${\mathrm{F}}_2$ is found to be lowered ∼18 000 ${\mathrm{cm}}^{-1\mathrm{}}$ in the crystal environment. The energy of the lowest $4f\to 5d$ transition is described satisfactorily by a formula due to Jørgensen for $4f^n\to 4f^{n-1\mathrm{}} 5d$ transitions in the free ion, decreased by 18 000 ${\mathrm{cm}}^{-1\mathrm{}}$.

• Received 20 January 1966

DOI:https://doi.org/10.1103/PhysRev.147.332