The atomic-beam laser-rf double-resonance technique has been used to obtain precise values for the hyperfine structure (hfs) splittings in ten 4${\mathit{f}}^7$5d6s ${}_{}{}^{10}{}_{}{}^{}$${\mathit{D}}_{\mathit{J}}$ and ${}_{}{}^8{}_{}{}^{}$${\mathit{D}}_{\mathit{J}}$ levels of ${}_{}{}^{151,153}{}_{}{}^{}\mathrm{i}$. Detailed studies of the second-order hfs lead to the isotopic ratios of the hfs constants, and reveal surprisingly strong hfs interactions between the two multiplets. The dipole ratios, when compared with the known ratio of the nuclear dipole moments, reveal striking J- and term-dependent hyperfine anomalies. The quadrupole ratios (after corrections for the second-order hfs) are consistent with the atomic ground-state value and self-consistent among the ten levels studied. The precision of the measurements allows evaluation of the ratio of the magnetic octupole hfs interaction constants between the two isotopes. The result, which is taken to be the ratio of the nuclear ground-state magnetic octupole moments Ω, is found to be Ω${(}^{151}$Eu)/Ω${(}^{153}$Eu)=0.87(6).

• Received 10 December 1990

DOI:https://doi.org/10.1103/PhysRevA.44.1523