Hyperfine structure of the 2s state of ³He⁺

Using an electrostatic ion-storage device and a microwave state-selection technique, we have measured the hyperfine structure Δν₂ of the 2s state of ³He⁺. Our result is Δν₂=1083.354 980 7(88) MHz (the equivalent uncertainty is 0.009 ppm). When combined with the 1s-state hyperfine structure Δν₁ to make the quantity D₂₁≡8Δν₂-Δν₁, we obtain D₂₁(experiment)=1.189 979(71) MHz. Current theory produces D₂₁(theory)=1.189 801(1) MHz. The 178(71) Hz difference may arise from a combination of quantum-electrodynamic and nuclear structure effects which remain to be treated theoretically.