Helium (He) is the ideal atom to perform tests of ab initio calculations in two-electron systems that consider all known effects, including quantum-electrodynamics and nuclear-size contributions. Recent state-of-the-art calculations and measurements of energy intervals involving the He $2{}^3{S}_1$ metastable state reveal discrepancies at the level of $7\sigma$ that require clarification both from the experimental and theoretical sides. We report on a new determination of the ionization energy $E_{\mathrm{I}}\left(2{}^3{S}_1\right)$ of the $\left(1s\right)\left(2s\right){}^3{S}_1$ metastable state of He. The measurements rely on an approach combining interferometric laser-alignment control, SI-traceable frequency calibration, and imaging-assisted Doppler-free spectroscopy. With this approach we record spectra of the $np$ Rydberg series in a highly collimated cold supersonic beam of metastable He generated by a cryogenic valve and an electric discharge. Extrapolation of the Rydberg series yields a new value of the ionization energy [$E_{\mathrm{I}}\left(2{}^3{S}_1\right)/h=1152842742.7082{\left(55\right)}_{\mathrm{stat}}{\left(25\right)}_{\mathrm{sys}}\mathrm{MHz}$] that deviates by $9\sigma$ from the most precise theoretical result [$1152842742.231\left(52\right)\mathrm{MHz}$], reported by Patkóš et al. [Phys. Rev. A 103, 042809 (2021)], confirming earlier discrepancies between experiment and theory in this fundamental system.

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• Atomic & molecular structure
• Atomic spectra
• Electronic transitions
• Atomic & molecular beams
• Atoms
• Rydberg atoms & molecules
• Imaging & optical processing
• Laser techniques
• Optical interferometry
• Optical spectroscopy
• Photoionization