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We have recently reported on the first direct measurement of the 2s hyperfine transition in lithium-like bismuth (209Bi80+) at the GSI Helmholtz Centrefor Heavy Ion Research (GmbH) in Darmstadt, Germany. Combined with a newmeasurement of the 1s hyperfine splitting in hydrogen-like (209Bi82+) the so-called specific difference Δ’E = -61.37(36) meV could be determined and was found to bein good agreement with its prediction from strong-field bound-state QED. Here wereport on additional investigations performed to estimate systematic uncertainties of these results and on details of the experimental setup. We show that the dominating uncertainty arises from insufficient knowledge of the ion beam velocity which is determined by the electron-cooler (voltage). Two routes to obtain a cooler-voltage calibration are discussed and it is shown that agreement can be reached either of the experimental Δ’E with the theoretical result, or between the two measurements of the hyperfine splitting in hydrogen-like bismuth, but not both at the same time.

We performed a laser spectroscopic determination of the 2s hyperfine splitting (HFS) of Li-like 209Bi80+ and repeated the measurement of the 1s HFS of H-like 209Bi82+. Both ion species were subsequently stored in the Experimental Storage Ring at the GSI Helmholtzzentrum für Schwerionenforschung Darmstadt and cooled with an electron cooler at a velocity of ≈0.71c. Pulsed laser excitation of the M1 hyperfine transition was performed in anticollinear and collinear geometry for Bi82+ and Bi80+, respectively, and observed by fluorescence detection. We obtain ΔE(1s)=5086.3(11)meV for Bi82+, different from the literature value, and ΔE(2s)=797.50(18)meV for Bi80+. These values provide experimental evidence that a specific difference between the two splitting energies can be used to test QED calculations in the strongest static magnetic fields available in the laboratory independent of nuclear structure effects. The experimental result is in excellent agreement with the theoretical prediction and confirms the sum of the Dirac term and the relativistic interelectronic-interaction correction at a level of 0.5%, confirming the importance of accounting for the Breit interaction.

The long sought after ground-state hyperfine transition in lithium-like bismuth 209Bi80+ was observed for the first time using laser spectroscopy on relativistic ions in the experimental storage ring at the GSI Helmholtz Centre in Darmstadt. Combined with the transition in the corresponding hydrogen-like ion 209Bi82+, it will allow extraction of the specific difference between the two transitions that is unaffected by the magnetic moment distribution in the nucleus and can therefore provide a better test of bound-state QED in extremely strong magnetic fields.