Hyperfine-structure measurements of the ${}_{}{}^{151,153}{}_{}{}^{}\mathrm{Eu}_{}^{+}{}_{}{}^{}$ ground state

The laser rf double-resonance technique in a Paul trap has been used to make high-precision measurements of the hyperfine structure (hfs) of the 4${\mathit{f}}^7$${(}^8$${\mathit{S}}_{7/2}$)6s ${}_{}{}^9{}_{}{}^{}$${\mathit{S}}_4$ ground state in ${}_{}{}^{151,153}{}_{}{}^{}\mathrm{Eu}_{}^{+}{}_{}{}^{}$. We have obtained linewidths below 1 kHz for the magnetic-field-dependent rf transitions at frequencies between 1.7 and 10 GHz. After corrections for the second-order hfs we obtain values for the magnetic-dipole, electric-quadrupole, magnetic-octupole, and electric-hexadecapole interaction constants.