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The term values of the rotational levels of the first excited vibrational state of the electronic ground state of He 2 + 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 He 2 + 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 He 2 + was determined to be 1628.3832(12) cm−1 by fitting effective molecular constants to the obtained term values. [ABSTRACT FROM AUTHOR]

The term values of all rotational levels of the 4He2+ X+ ³Su+ (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]

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]

Measuring spin-rotation intervals in molecular cations is challenging, particularly so when the ions do not have electric-dipole-allowed rovibrational transitions. We present a method, based on an angular-momentum basis transformation, to determine the spin-rotational fine structure of molecular ions from the fine structure of high Rydberg states. The method is illustrated by the determination of the so far unknown spin-rotation fine structure of the fundamentally important He2+ ion in the X ²Σ+u state. The fine-structure splittings of the v+=0, N+=1, 3, and 5 levels of He2+ are 7.96(14), 17.91(32), and 28.0(6) MHz, respectively. The experiment relies on the use of single-mode cw radiation to record spectra of high Rydberg states of He2 from the a ³Σ+u metastable state. [ABSTRACT FROM AUTHOR]