Absorption and Magnetic-Circular-Dichroism Spectra of Octahedral ${\mathrm{Ce}}^{3+}$ in ${\mathrm{Cs}}_2$NaY${\mathrm{Cl}}_6$

The $4f^1\to 5d^1$ transitions of ${\mathrm{Ce}}^{3+}$, $E_u^{\prime \prime }\left({}_{}{}^2{}_{}{}^{}F_{\frac{5}{2}}^{}{}_{}{}^{}\right)\to U_g^{\prime }\left({}_{}{}^2{}_{}{}^{}T_{2g}^{}{}_{}{}^{}\right)$ and $E_u^{\prime \prime }\left({}_{}{}^2{}_{}{}^{}F_{\frac{5}{2}}^{}{}_{}{}^{}\right)\to E_g^{\prime \prime }\left({}_{}{}^2{}_{}{}^{}T_{2g}^{}{}_{}{}^{}\right)$ have been studied at precisely octahedral sites using absorption and magnetic-circular-dichroism (MCD) techniques at liquid-helium temperature. The no-phonon lines of the two transitions are located at 28 196 and 29435 ${\mathrm{cm}}^{-1\mathrm{}}$, respectively. Considerable vibronic structure is resolved in both bands characterized by long progressions in ${\nu }_1\left(a_{1g}\right)$ of the Ce${\mathrm{Cl}}_6^{3-}$ moiety with low-energy lattice progressions superimposed. The $U_g^{\prime }$ and $E_g^{\prime \prime }$ excited states show a spin-orbit splitting of ∼ 1240 ${\mathrm{cm}}^{-1\mathrm{}}$ versus a predicted free-ion value of $\frac{3{\zeta }_d}{2}\approx 1500$ ${\mathrm{cm}}^{-1\mathrm{}}$. The MCD spectrum strongly supports the proposed assignments. There are no indications of a significant Jahn-Teller effect in the $U_g^{\prime }\left({}_{}{}^2{}_{}{}^{}T_{2g}^{}{}_{}{}^{}\right)$ excited state. Since no further ${\mathrm{Ce}}^{3+}$ absorption is detected out to 50 000 ${\mathrm{cm}}^{-1\mathrm{}}$, $\Delta \left(\equiv 10Dq\right)\ge 20000$ ${\mathrm{cm}}^{-1\mathrm{}}$ in the excited state.