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⁺ ³S_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 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. [ABSTRACT FROM AUTHOR]