Microwave-Optical Double Resonance of the Metastable4f65dLevel ofEu2+in the Fluorite Lattices

The results of an electron-spin-resonance investigation of the metastable $4f^{6}5d$ level of ${\mathrm{Eu}}^{2+}$ in Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, and Ba${\mathrm{F}}_2$ are presented. The experiments utilized the intense fluorescence from the lowest-lying ${\Gamma }_8\left(4\mathrm{}{f}^{6}5d\right)$ level of ${\mathrm{Eu}}^{2+}$ to the ${}_{}{}^8{}_{}{}^{}S_{\frac{7}{2}}^{}{}_{}{}^{}\left(4\mathrm{}{f}^7\right)$ ground state for the optical detection of resonance transitions in the excited state. The magnitudes of the Zeeman and hyperfine splittings for the excited state are consistent with the model of a large crystal-field splitting for the $5d$ electron and a considerably smaller coupling of this electron to the $4f^6$ configuration. The large average $g$ value of 3.80 for this level can be explained by an effective exchange coupling between the $5d$ spin and the $4f^6\left({}_{}{}^7{}_{}{}^{}F_J^{}{}_{}{}^{}\right)$ spin component if this coupling is comparable to the 400-${\mathrm{cm}}^{-1\mathrm{}}$ spin-orbit coupling parameter for the ${}_{}{}^7{}_{}{}^{}F_J^{}{}_{}{}^{}$ multiplets. The anomalous line shapes of the resonance spectrum can be interpreted on the basis of a dynamical Jahn-Teller distortion of the $e_g$ orbital of the $5d$ electron. A singlet tunneling level associated with the Jahn-Teller effect is estimated to be at about 10 ${\mathrm{cm}}^{-1\mathrm{}}$ above the orbital doublet ${\Gamma }_8$ level in both Sr${\mathrm{F}}_2$ and Ca${\mathrm{F}}_2$ on the basis of the line shapes observed in the resonance spectrum. Previously unidentified optical transitions to these levels have been observed by other workers, who also measured the splitting of the ${\Gamma }_8$ level by uniaxial strains and the strain coupling of the singlet tunneling level to the ${\Gamma }_8$ level. These piezo-optic data are in quantitative agreement with the model of a dynamical Jahn-Teller interaction for the observed levels. The nature of the fluorescence spectrum of ${\mathrm{Eu}}^{2+}$ in Ba