Hyperfine separations in unstable ${\mathrm{Eu}}^{+}$ ions of mass 148, 149, and 150 have been measured in laser-microwave double-resonance experiments in a Paul ion trap. In spite of the small available quantities of the isotopes, the experimental uncertainties are of the order of ${10}^{-8}$ or below, which is of the same order as in earlier measurements on stable isotopes of ${\mathrm{Eu}}^{+}.$ Extensive second-order perturbation calculation is required to obtain coupling constants for magnetic-dipole $\mathrm{}\left(A\right)\mathrm{}$ and electric-quadrupole $\mathrm{}\left(B\right)\mathrm{}$ interactions. The uncertainties are a few times ${10}^{-7}$ for $A$ and ${10}^{-3}$ for $B.\mathrm{}$ The experiments are a step in an attempt to determine the differential hyperfine anomaly (Bohr-Weisskopf effect) in a long chain of isotopes.

• Received 25 October 1996

DOI:https://doi.org/10.1103/PhysRevA.56.265