Precision spectroscopy of atomic helium

Abstract

Helium is a prototype three-body system and has long been a model system for developing quantum mechanics theory and computational methods. The fine structure splitting in the 23P state of helium is considered to be the most suitable for determining the fine structure constant α in atoms. After more than 50 years of efforts by many theorists and experimentalists, now we are working toward a determination of α with an accuracy of a few parts per billion, which can be compared to the results obtained by entirely different methods to verify the self-consistency of QED. Moreover, the precision spectroscopy of helium allows determination of the nuclear charge radius, and it is expected to help resolve the “proton radius puzzle”. In this review, we introduce the latest developments in the precision spectroscopy of the helium atom, especially the discrepancies among theoretical and experimental results, and give an outlook on future progress.

Toward XUV frequency comb spectroscopy of the 1 S–2 S transition in $$\hbox {He}^+$$

“…Therefore, the measurement will determine the charge radius difference Δr 2 = r( 3 He) 2 − r( 4 He) 2 with a smaller uncertainty than the radii themselves. The result can be compared to isotope shift measurements of neutral 3,4 He, where the problematic high-order three-body QED contributions are largely cancelled out [35][36][37][38][39][40][41]. Measurements of different transitions starting from the metastable 2 3 S state of neutral 3,4 He show statistically significant inconsistencies [35,36].…”

“…The result can be compared to isotope shift measurements of neutral 3,4 He, where the problematic high-order three-body QED contributions are largely cancelled out [35][36][37][38][39][40][41]. Measurements of different transitions starting from the metastable 2 3 S state of neutral 3,4 He show statistically significant inconsistencies [35,36]. The measurements on the 3,4 He + isotopes may provide new insight into this problem.…”

Line Profile Asymmetry in Precision Spectroscopy

“…The interest in the two-electron 3 He isotope system, as well as in helium-4, is primarily due to the evaluation of root-mean-square charge radii and the current discrepancy in experimental results and theory, see for example [86,87,88]. In this section we analyse the NR effects in application to the experiment [86], where transitions between different hyperfine sublevels were observed.…”

Calculation of the Fine Structure Constant derived from Principle Theory.

“…Figure 2 Theoretical predictions (from fit red line figure 1) of the FSC number (blue circle) depending on the 1 st and 2 nd ionisation energy of the emitting electron from different atoms including six experimental results (red square) presented in table 3 [4] (2018) Rb 4.1771 137.035999050 137.035999037(91) Bouchendira [6] [7] (2017) Rb (2) 27.2895 137.035999206 137.035999206(11) Morel [5] (2017) He (2) 54.417 137.035999390 137.035999550(640) Smiciklas [8] (2010) Notice: He(2) is not shown figure 2 Acknowledgement: I would like to thank Norbert Dahmen and Andrea Ehrmann for profound discussions and John Wilcoxen who encouraged me to continue the research 20 years ago.…”