An apparatus has been constructed for laser–radio-frequency double-resonance studies of very-low-energy-ion beams with collinear laser-ion geometry. The ion energy of only 1–2 keV leads to extremely narrow linewidths (∼60 kHz). This paper describes measurements of the hyperfine structure of ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}_{}^{+}{}_{}{}^{}$ and ${}_{}{}^{153}{}_{}{}^{}\mathrm{Eu}_{}^{+}{}_{}{}^{}$ in the level 4$f^7$(${}^8$$S^o$)5d ${}^9$$D_4^o$. The hyperfine constants A and B have been determined with a much higher accuracy than before. The present values of these constants (in MHz) are &=113.159(1), &=-215.356(10), &=-50.415(1), & =-548.357(10). .AE

• Received 10 November 1986

DOI:https://doi.org/10.1103/PhysRevA.35.3145