The interest of precision measurements on few-electron molecules results from the fact that the properties of these molecules can be calculated extremely precisely using ab initio quantum-chemical methods which include the evaluation of relativistic and quantum-electrodynamics (QED) contributions. The comparison of experimental and theoretical results in these systems thus enables rigorous tests of the theory, potentially only limited by the uncertainties in fundamental constants [1-3]. The talk will focus on recent unpublished precision spectroscopic measurements of the Rydberg spectrum of He2 using a cold, slow beam of metastable He2 produced by multistage Zeeman deceleration [4] Rydberg-series extrapolation using multichannel quantum defect theory and parameters determined in Ref. [5] enabled the determination of the energy level structure of He2+ with unprecedented precision. The comparison with the latest ab initio calculations [6] provides information on the magnitude of relativistic and QED contributions to rovibrational energies in He2+. The advantage of using multistage Zeeman deceleration does not only result from the longer transit times of the decelerated molecules through the radiation field. The spin-rotational state selectivity of the Zeeman deceleration process can also be exploited to reduce the spectral congestion, minimize residual Doppler shifts, resolve the Rydberg series beyond n = 200, and assign their fine structures.
[1] Theoretical transition frequencies beyond 0.1 ppb accuracy in H2+, HD+, and antiprotonic helium, V. I. Korobov, L. Hilico, and J.-Ph. Karr, Phys. Rev. A 89, 032511 (2014)
[2] Theoretical determination of the dissociation energy of molecular hydrogen, K. Piszczatowski, G.Lach, M. Przybytek, J. Komasa, K. Pachucki, and B. Jeziorski, J. Chem. Theory Comput. 5, 3039 (2009)
[3] Towards measuring the ionisation and dissociation energies of molecular hydrogen with sub-MHz accuracy, D. Sprecher and Ch. Jungen and W. Ubachs and F. Merkt, Faraday Disc. 150, 51 (2011)
[4]Slow and velocity-tunable beams of metastable He2 by multistage Zeeman deceleration, M. Motsch, P. Jansen, J. A. Agner, H. Schmutz, and F. Merkt, Phys. Rev. A 89, 043420 (2014)
[5] High-resolution spectroscopy and quantum-defect model for the gerade triplet np and nf Rydberg states of He2, D. Sprecher, J. Liu, T. Krähenmann, M. Schäfer, and F. Merkt, J. Chem. Phys. 140, 064304 (2014)
[6] Very accurate potential energy curve of the He2+ ion, W.-C. Tung, M. Pavanello and L. Adamowicz, J. Chem. Phys. 136, 104309 (2012)