High-Precision Penning Trap Spectroscopy of the Ground State Spin Structure of HD^{+}.

We present high-precision spectroscopy of the ground state hyperfine structure of HD^{+} at 4 T. We determine the bound-electron g factor, g_{e,bound}=-2.002 278 540 96(40), to a relative uncertainty of 2×10^{-10}, the most precise determination of a bound-electron g factor of a molecular ion to date. The experimental value agrees with recently developed ab initio theory that now includes quantum-electrodynamical effects up to order α^{5} and has reduced the theoretical uncertainty by three orders of magnitude [O. Kullie et al., Precision calculation of the bound-electron g factor in molecular hydrogen ions, Phys. Rev. A 112, 052813 (2025)PLRAAN2469-992610.1103/cb1q-fhk5]. In addition, we extract the scalar spin-spin interaction coefficients E_{4}=925 395.758(41)  kHz (electron-proton) and E_{5}=142 287.821(22)  kHz (electron-deuteron), which show a moderate tension with another state-of-the-art theoretical prediction [M. Haidar et al., Higher-order corrections to the spin-orbit and spin-spin tensor interactions in HD^{+}, Phys. Rev. A 106, 042815 (2022)PLRAAN2469-992610.1103/PhysRevA.106.042815].