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 He-2 molecules in the metastable a(3)Sigma(+)(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 He-2 and the lowest rotational interval of the X+ (2)Sigma(+)(u) (nu(+) = 0) ground state of He-4(2)+ have been determined with unprecedented precision and accuracy by Rydberg-series extrapolation. Comparison with ab initio predictions of the rotational energy level structure of He-4(2)+ [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 He-2(+).