Precision measurement of the rotational energy-level structure of the three-electron molecule He2+

The term values of all rotational levels of the ⁴He₂⁺ X⁺ ²Σu⁺ (ν⁺=0) ground vibronic state with rotational quantum number N⁺ ≤ 19 have been determined with an accuracy of 8 × 10⁻⁴ cm⁻¹ (∼25 MHz) by multichannel-quantum-defect-theory-assisted Rydberg spectroscopy of metastable He₂∗. Comparison of these term values with term values recently calculated ab initio by Tung et al. [J. Chem. Phys. 136, 104309 (2012)] reveals discrepancies that rapidly increase with increasing rotational quantum number and reach values of 0.07 cm⁻¹ (∼2.1 GHz) at N⁺ = 19.