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

Abstract

The term values of all rotational levels of the 4 He 2 + X + 2 Σ u + ( 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 He 2 ∗ . 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.