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$\mathbf{{_2}^+}$

Molecular helium represents a benchmark system for testing $\textit{ab initio}$ calculations on few-electron molecules. We report on the determination of the adiabatic ionization energy of the $a\,^3Σ_u^+$ state of He$_2$, corresponding to the energy interval between the $a\,^3Σ_u^+$ ($v''=0$, $N''=1$) state of He$_2$ and the $X^+\,^2Σ_u^+$ ($v^+=0$, $N^+=1$) state of He${_2}^+$, and of the lowest rotational interval of He${_2}^+$. These measurements rely on the excitation of metastable He$_2$ molecules to high Rydberg states using frequency-comb-calibrated continuous-wave UV radiation in a counter-propagating-laser-beam setup. The observed Rydberg states were extrapolated to their series limit using multichannel quantum-defect theory. The ionization energy of He$_2$ ($a\,^3Σ_u^+$) and the lowest rotational interval of He${_2}^+$ ($X^+\,^2Σ_u^+$) are 34301.207002(23)$\pm 0.000037_{\mathrm{sys}}$ cm$^{-1}$ and 70.937589(23)$\pm 0.000060_{\mathrm{sys}}$ cm$^{-1}$, respectively.