A laser-microwave double-resonance experiment on electrodynamically trapped ${}_{}{}^{173}{}_{}{}^{}\mathrm{Yb}_{}^{+}{}_{}{}^{}$ ions has been performed and a value of Δ${\nu }_{\mathrm{h}\mathrm{f}\mathrm{s}=10491720239.55}$±0.09 Hz for the ground-state hyperfine separation has been determined. This value is corrected for small Zeeman and second-order Doppler shifts. Combined with a previous similar measurement on ${}_{}{}^{171}{}_{}{}^{}\mathrm{Yb}_{}^{+}{}_{}{}^{}$ and with the known $g_I$ values of both isotopes, we obtain a value of -0.004 25(6) for the differential hyperfine anomaly ${}_{}{}^{171}{}_{}{}^{}\mathrm{\Delta }_{}^{173}{}_{}{}^{}$.

• Received 19 November 1986

DOI:https://doi.org/10.1103/PhysRevA.35.4147