Precision Laser Spectroscopy of the Ground State Hyperfine Splitting of HydrogenlikeBi82+209

The first direct observation of a hyperfine splitting in the optical regime is reported. The wavelength of the $M1\mathrm{}$ transition between the $F=4\mathrm{}$ and $F=5\mathrm{}$ hyperfine levels of the ground state of hydrogenlike ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}_{}^{82+}{}_{}{}^{}$ was measured to be ${\lambda }_0=243.87\mathrm{}\left(4\right)\mathrm{}$ nm by detection of laser induced fluorescence at the heavy-ion storage ring ESR at GSI. In addition, the lifetime of the laser excited $F=5\mathrm{}$ sublevel was determined to be ${\tau }_0=0.351\mathrm{}\left(16\right)\mathrm{}$ ms. The method can be applied to a number of other nuclei and should allow a novel test of QED corrections in the previously unexplored combination of strong magnetic and electric fields in highly charged ions.