Vibronic Transitions ofSm2+andEu2+in SrF2and Their Zeeman Effect

The vibronic emission spectra of ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Eu}}^{2+}$ in Sr${\mathrm{F}}_2$ have been studied at low temperatures. The Zeeman effect of the transitions ${}_{}{}^5{}_{}{}^{}D_0^{}{}_{}{}^{}\left({{\Gamma }_1}^{+}\right)\to {}_{}{}^7{}_{}{}^{}F_1^{}{}_{}{}^{}\left({{\Gamma }_4}^{+}\right)$ and ${}_{}{}^7{}_{}{}^{}F_2^{}{}_{}{}^{}\left({{\Gamma }_5}^{+}\right)$ of ${\mathrm{Sm}}^{2+}$ was observed with fields up to 70 kOe applied along the [100] axis. The vibronic spectra of ${\mathrm{Sm}}^{2+}$ associated with electronic transitions within the $4f^6$ configuration cut off sharply at phonon energy of 310 ${\mathrm{cm}}^{-1\mathrm{}}$. Several sharp peaks are observed within this range. The vibronic spectrum accompanying the transition $4f^{6}5d\left({{\Gamma }_8}^{+}\right)\to 4f^7{}_{}{}^8{}_{}{}^{}S_{\frac{7}{2}}^{}{}_{}{}^{}({{\Gamma }_6}^{-}+{{\Gamma }_7}^{-}+{{\Gamma }_8}^{-})$ of ${\mathrm{Eu}}^{2+}$ spreads up to eight-phonon excitations, with sharp peaks appearing in the region of single-phonon excitations. The vibronic transitions of both ions are shown to have electric dipole character. Then a comparison between phonon energies derived from the vibronic spectra of ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Eu}}^{2+}$ determines the parity of the vibrations at the rare-earth-ion site. By using selection rules for vibronic transitions associated with the ${{\Gamma }_1}^{+}\to {{\Gamma }_i}^{+} (i=1,\cdots ,5)$ transitions of ${\mathrm{Sm}}^{2+}$, the number of possible irreducible representations to which the vibration belongs is reduced. Further reduction of this number is obtained by examining the polarization of the vibronic Zeeman components. It is found that only one of the potential components set up by the phonon at the rare-earth site has the main contribution to the vibronic intensity. In most cases this is the component which transforms like ${{\Gamma }_4}^{-}$, presumably because of the electric field, but components transforming like ${{\Gamma }_1}^{-}$ and ${{\Gamma }_3}^{-}$ are also observed.