Hyperfine structure measurements in the ⁷S₃ metastable finestructure level in stable and unstable Eu⁺ isotopes

Clouds of stable and unstable Eu⁺ isotopes have been confined in a Paul trap, each containing about 10⁵ particles. In a microwave-optical double resonance experiment several hyperfine separations in the 4f⁷ 6s ⁷S₃ exited level have been measured with the experimental uncertainties ranging between 10^-8 and 3 × 10^-6. These experiments have confirmed that also in the case of an excited level with a large number of hyperfine or Zeeman sublevels the microwave-optical double resonance technique in a Paul trap can be useful for precise hyperfine structure investigation. The hyperfine coupling constants A and B have been determined for the isotopes ¹⁵³Eu⁺, ¹⁵¹Eu⁺, ¹⁵⁰Eu⁺ and ¹⁴⁸Eu⁺. The results complement earlier measurements on the ground state of Eu⁺ and give additional information on the hyperfine anomaly in the Eu isotopic chain.