Doppler-free two-photon spectroscopy has been applied to investigate the hyperfine structures and the isotope shifts of 27 fine-structure levels of ${}_{}{}^{151,153}{}_{}{}^{}\mathrm{Eu}$ in the energy region between 34 400 and 36700 ${\mathrm{cm}}^{\mathrm{-}1}$. With the experimental data the fine-structure calculations of Wyart have been widely confirmed, and two levels of the ${}_{}{}^8{}_{}{}^{}$D term of the configuration 4${\mathit{f}}^7$6s6d have been identified as e ${}_{}{}^8{}_{}{}^{}$${\mathit{D}}_{3/2}$ (35 282.22 ${\mathrm{cm}}^{\mathrm{-}1}$) and e ${}_{}{}^8{}_{}{}^{}$${\mathit{D}}_{7/2}$ (35 377.94 ${\mathrm{cm}}^{\mathrm{-}1}$).

With effective one-electron parameters ${\mathit{a}}^{\mathit{k}\mathit{l}}$ and ${\mathit{b}}^{\mathit{k}\mathit{l}}$ an analysis of the magnetic and electric hyperfine structure for the terms ${\mathit{e}}^6$D, ${\mathit{e}}^{10}$D, and ${}_{}{}^8{}_{}{}^{}\mathrm{D}$ of the configuration 4${\mathit{f}}^7$6s6d was performed: ${\mathit{a}}_{4\mathit{f}}^{10}$=-50(13) MHz, ${\mathit{a}}_{6\mathit{s}}^{10}$=12 590 (90) MHz, ${\mathit{a}}_{6\mathit{d}}^{12}$=25(20) MHz, ${\mathit{b}}_{4\mathit{f}}^{11}$=0(2) MHz, ${\mathit{b}}_{6\mathit{d}}^{11}$=-4(2) MHz, ${\mathit{b}}_{6\mathit{d}}^{13}$=-30(20) MHz, and ${\mathit{b}}_{6\mathit{d}}^{02}$=+81(5) MHz. From the quadrupole moment ${}_{}{}^{151}{}_{}{}^{}$Q(6d)=2.07(30) barn a Sternheimer quadrupole shielding factor was found to be ${}_{}{}^{151}{}_{}{}^{}$R(6d)=-1.29(35). In addition, higher-order effects of the isotope shift could be explained in the same terms of 4${\mathit{f}}^7$6s6d with the use of a parametric description. We obtained the following parameters: ${\mathit{d}}_{6\mathit{s}6\mathit{d}}$=-3140(80) MHz, ${\mathit{d}}_{5\mathit{d}}^2$=-5270(130) MHz, ${\mathit{g}}_1$(4f,6d)=+1(3) MHz, ${\mathit{g}}_2$(6d,6s)=-290(50) MHz, ${\mathit{g}}_3$(4f,6s)=-36(2) MHz, and ${\mathit{z}}_{6\mathit{d}}$=6(4) MHz.

• Received 23 October 1990

DOI:https://doi.org/10.1103/PhysRevA.43.4837