Precision measurement of the rotational energy-level structure of the three-electron molecule He${}_2^{+}$

Abstract

The term values of all rotational levels of the ${}^4$He${{}_2}^{+}{X}^{+}{}^2{\Sigma }_u^{+}({\nu }^{+}=0)$ ground vibronic state with rotational quantum number $N^{+}\le 19$ have been determined with an accuracy of 8 x 10${}^{-4}$ cm${}^{-1}$ ($\sim 25$ MHz) by MQDT-assisted Rydberg spectroscopy of metastable He${}_2^{\ast }$. Comparison of these term values with term values recently calculated ab initio by Tung et al. [J. Chem. Phys. 136, 104309 (2012)] reveal discrepancies that rapidly increase with increasing rotational quantum number and reach values of 0.07 cm${}^{-1}$ ($\sim 2.1$ GHz) at $N^{+}=19$.

11 pages, 6 figures