Speaker
Description
The metastable He ((1s)$^1$(2s)$^1$) atom in its singlet ($^1$S$_0$) or triplet ($^3$S$_1$) states is an ideal system to perform tests of ab-initio calculations of two-electron systems that include quantum-electrodynamics and nuclear finite-size effects. The recent determination of the ionization energy of the metastable
$2\,^1$S$_0$ state of $^4$He [1] confirmed a discrepancy between the latest theoretical values of the Lamb shifts in low-lying electronic states of triplet helium [2] and the measured $3\,^3$D ← 2 $^3$S [3] and $3\,^3$D ← 2 $^3$P [4] transition frequencies. This discrepancy could not be resolved in the latest calculations [5,6].
Currently, we focus on the development of a new experimental method for the determination of the ionization energy of the
$2\,^3$S$_1$ state of $^4$He via the measurement of transitions from the $2\,^3$S$_1$ state to $n$p Rydberg states. Extrapolation of the $n$p series yields the ionization energy with sub-MHz accuracy.
In this talk, we present the progress in the development of our experimental setup, which involves (i) the preparation of a cold, supersonic expansion of helium atoms in the $2\,^3$S$_1$ state, (ii) the development and characterization of a laser system for driving the transitions to the $n$p Rydberg states, and (iii) the implementation of a new sub-Doppler, background-free detection method. We present this new spectroscopic method, with which we cancel the $^1$st-order Doppler shift and illustrate its power with a new determination of the ionisation energy of $2\,^3$S$_1$ metastable He.
[1] G. Clausen et al., Phys. Rev. Lett. 127, 093001 (2021).
[2] V. Patkóš et al., Phys. Rev. A. 103, 042809 (2021).
[3] C. Dorrer et al., Phys. Rev. Lett. 78, 3658 (1997).
[4] P.-L. Luo et al., Phys. Rev. A. 94, 062507 (2016).
[5] V. A. Yerokhin et al., Eur. Phys. J. D. 76, 142 (2022).
[6] V. A. Yerokhin et al., Phys. Rev. A. 107, 012810 (2023).
