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 ab initio calculations on few-electron molecules. We report on the determination of 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) 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 counterpropagating 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 (3)Sigma(+)(u)) and the lowest rotational interval of He-2(+) (X+ (2)Sigma(+)(u)) are 34 301.207 002(23) +/- 0.000 037(syst) cm(-1) and 70.937 589(23) +/- 0.000 060(syst) cm(-1), respectively.