Microwave-Optical Double-Resonance Spectroscopy Experiment of ¹⁹⁹Hg⁺ Ground State Hyperfine Splitting in a Linear Ion Trap

We report a spectroscopy experiment of the ¹⁹⁹Hg⁺ ground state hyperfine splitting in a linear ion trap. The ions are optically pumped by a discharge lamp and cooled by helium buffer gas. The ground state hyperfine splitting is measured to be 40507347996.8(0.1) Hz by the microwave-optical double-resonance method. A narrow line width as 30 mHz is also observed. This progress builds the foundation for the realization of trapped ¹⁹⁹Hg⁺ ion frequency standards.