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Molecular helium represents a benchmark system for testing $\textit{abinitio}$ calculations on few-electron molecules. We report on the determinationof the adiabatic ionization energy of the $a\,^3\Sigma_u^+$ state of He$_2$,corresponding to the energy interval between the $a\,^3\Sigma_u^+$ ($v''=0$,$N''=1$) state of He$_2$ and the $X^+\,^2\Sigma_u^+$ ($v^+=0$, $N^+=1$) stateof He${_2}^+$, and of the lowest rotational interval of He${_2}^+$. Thesemeasurements rely on the excitation of metastable He$_2$ molecules to highRydberg states using frequency-comb-calibrated continuous-wave UV radiation ina counter-propagating-laser-beam setup. The observed Rydberg states wereextrapolated to their series limit using multichannel quantum-defect theory.The ionization energy of He$_2$ ($a\,^3\Sigma_u^+$) and the lowest rotationalinterval of He${_2}^+$ ($X^+\,^2\Sigma_u^+$) are 34301.207002(23)$\pm0.000037_{\mathrm{sys}}$ cm$^{-1}$ and 70.937589(23)$\pm0.000060_{\mathrm{sys}}$ cm$^{-1}$, respectively.

physical review letters

Precision Measurements in Few-Electron Molecules: The Ionization Energy  of Metastable $\mathbf{^4}$He$\mathbf{{_2}}$ and the First Rotational  Interval of $\mathbf{^4}$He$\mathbf{{_2}^+}$