Metrology in a two-electron atom: The ionization energy of metastable triplet helium <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>2</mml:mn> <mml:mmultiscripts> <mml:mi mathvariant="normal">S</mml:mi> <mml:mn>1</mml:mn> <mml:none/> <mml:mprescripts/> <mml:none/> <mml:mn>3</mml:mn> </mml:mmultiscripts> </mml:mrow> </mml:math>

Abstract

Helium (He) is the ideal atom to perform tests of ab initio calculations in two-electron systems that consider all known effects, including quantum-electrodynamics and nuclear-size contributions. Recent state-of-the-art calculations and measurements of energy intervals involving the He $2{\phantom{\rule{0.16em}{0ex}}}^{3}{S}_{1}$ metastable state reveal discrepancies at the level of $7\phantom{\rule{0.16em}{0ex}}\ensuremath{\sigma}$ that require clarification both from the experimental and theoretical sides. We report on a new determination of the ionization energy ${E}_{\mathrm{I}}\phantom{\rule{0.16em}{0ex}}(2{\phantom{\rule{0.16em}{0ex}}}^{3}{S}_{1})$ of the $(1s)(2s){\phantom{\rule{0.16em}{0ex}}}^{3}{S}_{1}$ metastable state of He. The measurements rely on an approach combining interferometric laser-alignment control, SI-traceable frequency calibration, and imaging-assisted Doppler-free spectroscopy. With this approach we record spectra of the $np$ Rydberg series in a highly collimated cold supersonic beam of metastable He generated by a cryogenic valve and an electric discharge. Extrapolation of the Rydberg series yields a new value of the ionization energy [${E}_{\mathrm{I}}\phantom{\rule{0.16em}{0ex}}(2{\phantom{\rule{0.16em}{0ex}}}^{3}{S}_{1})/h=1\phantom{\rule{0.16em}{0ex}}152\phantom{\rule{0.16em}{0ex}}842\phantom{\rule{0.16em}{0ex}}742.7082{(55)}_{\mathrm{stat}}{(25)}_{\mathrm{sys}}\phantom{\rule{0.16em}{0ex}}\mathrm{MHz}$] that deviates by $9\phantom{\rule{0.16em}{0ex}}\ensuremath{\sigma}$ from the most precise theoretical result [$1\phantom{\rule{0.16em}{0ex}}152\phantom{\rule{0.16em}{0ex}}842\phantom{\rule{0.16em}{0ex}}742.231(52)\phantom{\rule{0.28em}{0ex}}\mathrm{MHz}$], reported by Patk\'o\ifmmode \check{s}\else \v{s}\fi{} et al. [Phys. Rev. A 103, 042809 (2021)], confirming earlier discrepancies between experiment and theory in this fundamental system.

Topics & Concepts