Precision determination of the ground-state hyperfine splitting inBa+137using the ion-storage technique

The ground-state hyperfine splitting of the ${}_{}{}^{137}{}_{}{}^{}\mathrm{Ba}_{}^{+}{}_{}{}^{}$ ion has been measured with a precision of 4.6 × ${10}^{-11\mathrm{}}$. About ${10}^5$ ions were confined in a rf quadrupole trap for many hours. Hyperfine pumping was achieved by a pulsed tunable dye laser and transitions between the Zeeman sublevels were induced by microwaves, locked to a primary frequency standard. The transitions were detected via changes in the ionic fluorescence perpendicular to the laser beam. The final result is $W_{\mathrm{hfs}}=8\mathrm{}037741667.69\left(0.36\right)\mathrm{}$ Hz. The uncertainty stems in about equal parts from the extrapolation to zero electric and magnetic field strength.