Ionization Energy of the Metastable 2 ^{1}S_{0} State of ^{4}He from Rydberg-Series Extrapolation

Area of Science:

• Atomic Physics
• Quantum Electrodynamics
• Spectroscopy

Background:

• First-principles calculations of helium's Lamb shift have shown discrepancies with experimental values.
• Previous theoretical work by Patkóś, Yerokhin, and Pachucki identified a significant discrepancy for specific transitions.

Purpose of the Study:

• To experimentally determine ionization energies for helium's 2³S₁ and 2³P states.
• To investigate the discrepancies between theoretical predictions and experimental values for helium's Lamb shift.
• To provide data that can help resolve the unknown theoretical contribution to helium's Lamb shifts.

Main Methods:

• Measured the ionization energy of the 2¹S₀ state of helium.
• Combined this measurement with previously reported experimental data for 2³S₁←2¹S₀ and 2³P←2³S₁ intervals.
• Derived experimental ionization energies for the 2³S₁ state and 2³P centroid energy.

Main Results:

• The experimental ionization energy of the 2¹S₀ state was measured as 960332040.491(32) MHz.
• Derived experimental ionization energies for the 2³S₁ state and 2³P centroid energy are 1152842742.640(32) MHz and 876106247.025(39) MHz, respectively.
• These experimental values show significant disagreements (6.5σ and 10σ) with the α⁷m Lamb shift predictions.

Conclusions:

• The experimental results support the existence of an unknown theoretical contribution to the Lamb shifts of helium's 2³S and 2³P states.
• The findings highlight the need for further theoretical advancements in calculating helium's Lamb shift.
• This work contributes to resolving the discrepancy hindering the determination of the He²⁺ charge radius from atomic spectroscopy.