Hyperfine-structure-resolved laser spectroscopy of many-electron highly charged ions

Hyperfine structures of highly charged ions (HCIs) are favourable spectroscopic targets for exploring fundamental physics along with nuclear properties. Recent proposals of HCI atomic clocks highlight their importance, especially for many-electron HCIs, and they have been theoretically investigated by refining atomic-structure calculations. However, developments in hyperfine spectroscopy of many-electron HCIs have not proceeded due to experimental difficulty. Here, we demonstrate hyperfine-structure-resolved laser spectroscopy of HCIs in an electron beam ion trap plasma, employing the magnetic-dipole transition in the 4d(9)5s state of I-127(7+). Ion-state manipulation by controlled electron collisions in the well-defined laboratory plasma enables laser-induced fluorescence spectroscopy of trapped HCIs. The observed spectrum of evaporatively cooled ions under low magnetic fields shows characteristic features reflecting the hyperfine structures. The present demonstration using combined optical and plasma approaches provides a benchmark for state-of-the-art atomic calculations of hyperfine structures in many-electron HCIs, and offers possibilities for a variety of unexploited experiments. Highly charged ions (HCIs) have opened up various research frontiers in fundamental physics, such as recent proposals for atomic clocks based on heavy HCIs with many electrons. This paper demonstrates a laser spectroscopic method using an electron beam ion trap plasma with a quasi-Zeeman-free low magnetic field to deepen understanding of their hyperfine structure.