Precision Spectroscopy in Cold Molecules: The Lowest Rotational Interval of He2+ and Metastable He2.

Multistage Zeeman deceleration was used to generate a slow, dense beam of translationally cold He2 molecules in the metastable a 3Σ+u state. Precision measurements of the Rydberg spectrum of these molecules at high values of the principal quantum number n have been carried out. The spin-rotational state selectivity of the Zeeman-deceleration process was exploited to reduce the spectral congestion, minimize residual Doppler shifts, resolve the Rydberg series around n = 200 and assign their fine structure. The ionization energy of metastable He2 and the lowest rotational interval of the X+2Σ+u (ν+ = 0) ground state of 4He2+ have been determined with unprecedented precision and accuracy by Rydberg-series extrapolation. Comparison with ab initio predictions of the rotational energy level structure of 4He2+ [W.-C. Tung, M. Pavanello, and L. Adamowicz, J. Chem. Phys. 136,104309 (2012)] enabled us to quantify the magnitude of relativistic and quantum-electrodynamics contributions to the fundamental rotational interval of He2+. [ABSTRACT FROM AUTHOR]