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Recently, considerable progress has been made in the calculation of the energy level structures of the two-electron
systems He and H2. Patkóš, Yerokhin, and Pachucki have performed the first complete calculation of the Lamb shift of the helium 2 3S1 and 2 3PJ triplet states up to the term in a7m [1]. Whereas their theoretical result for the frequency of the 2 3P - 2 3S transition perfectly agrees with the experimental value, a more than 10sigma discrepancy was identified for the 3 3D - 2 3S and 3 3D - 2 3P transitions, which hinders the determination of the He2+ charge radius from atomic spectroscopy that would complement the recent determination using muonic helium [2]. We report on the determination of the ionization energy of the metastable 2 1S0 state of helium by Rydberg-series extrapolation through the measurement of the frequencies of np - 2 1S0 transitions in the range of principal quantum number n between 24 to 102 [3]. The experiments are carried out by single-photon excitation in a doubly-skimmed supersonic beam of metastable helium atoms. Combining the ionization energy of the 2 1S0 state of helium with earlier measurements of the 2 3S1 - 2 1S0 [4] and the 2 3P - 2 3S1 interval [5,6], we derive new experimental values for the ionization energies of the 2 3S1 state (1 152 842 742.640(32) MHz) and the 2 3P centroid energy (876 106 247.025(39) MHz). These values reveal disagreements with the a7m Lamb shift predictions by 6.5sigma and 11sigma, respectively, and support the suggestion by Patkóš et al. [1] of an unknown theoretical contribution to the Lamb shifts of the 2 3S1 and 2 3P states of He. A new experiment aiming at the direct determination of the ionization energy of 2 3S1 state will also be presented, in which multistage
Zeeman deceleration and transverse laser cooling is used to generate a slow, highly collimated beam of metastable 2 3S1 He. If time permits, new measurements of the ionization energy of H2 and of the level structure of H2+ will be presented. Starting from the GK 1Sg+ (v=0, N=0) level of H2, we record transitions to Rydberg-Stark states belonging to series converging on different rotational and vibrational levels of H2+ in the presence of weak electric fields. From the analysis of the Stark effect, we determine the positions of zero quantum defect for different series, which we then use to extract rotational and vibrational energy intervals in H2+.
[1] V. Patkóš, V. A. Yerokhin, and K. Pachucki, Phys. Rev. A 103, 042809 (2021).
[2] J. J. Krauth et al., Nature (London) 589, 527 (2021).
[3] G. Clausen, P. Jansen, S. Scheidegger, J. A. Agner, H. Schmutz and F. Merkt, Phys. Rev. Lett. 127, 093001
(2021)
[4] R. J. Rengelink, Y. van der Werf, R. P. M. J.W. Notermans, R. Jannin, K. S. E. Eikema, M. D. Hoogerland, and W. Vassen, Nat. Phys. 14, 1132 (2018).
[5] P. Cancio Pastor, G. Giusfredi, P. De Natale, G. Hagel, C. de Mauro, and M. Inguscio, Phys. Rev. Lett. 92, 023001 (2004); 97, 139903(E) (2006).
[6] X. Zheng, Y. R. Sun, J.-J. Chen, W. Jiang, K. Pachucki, and S.-M. Hu, Phys. Rev. Lett. 119, 263002 (2017)
