Ionization Energy of Metastable $^3$He (2 $^3$S$_1$) and the Alpha- and Helion-Particle Charge-Radius Difference from Precision Spectroscopy of the np Rydberg Series
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2025-06-06
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Journal Article
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Abstract
The current experimental and theoretical values of the ionization energy of metastable $^4$He [(1s)(2s) $^3$S$_1$] differ by 9σ, which prevents the determination of the alpha-particle charge radius from spectroscopic measurements in $^4$He. To help clarify the origin of this discrepancy, we report on a precision measurement of hyperfine-resolved transitions from the (1s)(2s) $^3$S$_1$ metastable state of $^3$He to high np Rydberg states converging on the F$^+$ = 0, 1 hyperfine levels of the $^3$He$^+$ (1s) $^2$S$_{1/2}$ ground state. Rydberg-series extrapolation using multichannel quantum-defect theory (MQDT) enabled the determination of the ionization energy of the (1s)(2s) $^3$S$_1$ state of $^3$He [EI($^3$He)/h = 1 152 788 844.6154(77)$_{stat}$(25)$_{sys}$ MHz] and of the corresponding isotopic shift [(E$_1$($^4$He)-E$_1$($^3$He))/h = 53 898.093(9) MHz]. The MQDT analysis also permitted the quantification of singlet-triplet mixing in the np series induced by the hyperfine interaction. From the isotopic shift of the ionization energy of He, the difference δr$^2$ between the squared charge radii of the helion and alpha particles is determined to be 1.060(10) fm$^2$.
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published
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134 (22)
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223001
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American Physical Society
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03449 - Merkt, Frédéric / Merkt, Frédéric
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236716 - 10005304 - Precision Rydberg spectroscopy in few-electron atoms and molecules (SNF)