This experiment is currently being set up at GSI. It focuses on precise measurements of magnetic moments and transition wavelength of forbidden transitions in highly charged ions. The measurement principle is a newly developed laser-microwave double-resonance technique which uses fluorescence light from an optical transition (in the fine or hyperfine structure) as a probe for microwave excitation between corresponding Zeeman sublevels. The energy of the microwave transitions directly yields the gF factor of the ion. A combined measurement of atomic gF factors on the ppb level of accuracy and of the HFS transition frequency allows a simultaneous and independent determination of the electronic gJ factor and the nuclear gI factor. For the first time, precise values for nuclear magnetic moments can be inferred without the use of diamagnetic corrections which currently limits the achievable accuracy substantially. At the same time, diamagnetic shielding effects can for the first time be quantified in a spectroscopic measurement. This information may be used to benchmark corresponding theories. The proposed measurements furthermore allow stringent tests of theoretical values for g factors as calculated in the framework of quantum electrodynamics of bound states.

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