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 Σ þ u state of He 2 , corresponding to the energy interval between the a 3 Σ þ u (v 00 ¼ 0, N 00 ¼ 1) state of He 2 and the X þ 2 Σ þ u (v þ ¼ 0, N þ ¼ 1) state of He þ 2 , and of the lowest rotational interval of He þ 2. These measurements rely on the excitation of metastable He 2 molecules to high Rydberg states using frequency-comb-calibrated continuouswave 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 Σ þ u) and the lowest rotational interval of He þ 2 (X þ 2 Σ þ u) are 34 301.207 002ð23Þ AE 0.000 037 syst cm −1 and 70.937 589ð23Þ AE 0.000 060 syst cm −1 , respectively.