Fundamental vibration frequency and rotational structure of the first excited vibrational level of the molecular helium ion (He2+)

The term values of the rotational levels of the first excited vibrational state of the electronic ground state of He2+ with a rotational quantum number N+ ≤ 13 have been determined with an accuracy of 1.2 × 10−3 cm−1 (∼35 MHz) by multichannel-quantum-defect-theory-assisted Rydberg spectroscopy of metastable He2. Comparison of the experimental term values with the most accurate ab initio results for He2+ available in the literature [W.-C. Tung, M. Pavanello, and L. Adamowicz, J. Chem. Phys. 136, 104309 (2012)] reveals inconsistencies between the theoretical and experimental results that increase with increasing rotational quantum numbers. The fundamental vibrational wavenumber of He2+ was determined to be 1628.3832(12) cm−1 by fitting effective molecular constants to the obtained term values.