Testing strong-field QED to second-order in the highly correlated atomic system berylliumlike Pb78+ by electron-ion recombination spectroscopy

TL;DR

This study measures electron-ion recombination in highly charged berylliumlike lead ions to test second-order strong-field QED predictions, achieving results that closely match advanced theoretical calculations and indicating potential for even higher precision.

Contribution

First experimental verification of second-order strong-field QED effects in a heavy, few-electron ion using high-resolution spectroscopy.

Findings

Measured excitation energy of Pb$^{78+}$ with high precision

Results agree with second-order strong-field QED calculations

Demonstrated potential for further experimental accuracy improvements

Abstract

A low-energy storage ring with an ultracold electron cooler has been coupled with a heavy-ion accelerator facilitating high-resolution electron-ion collision spectroscopy of the heaviest few-electron ions. In the present work resonant electron-ion recombination of berylliumlike Pb$^{78+}$ ions was measured in the collision-energy range 9.3-16.5eV and a value of 244.937(30) eV is derived for the Pb$^{78+}$($2s^2\;^1S_0 - 2s\,2p\;^3P_1$) excitation energy. This result agrees with the most recent (less accurate) theoretical value of 244.942(52) eV [Malyshev et al., Physical Review A 110, 062824 (2024)], which has been calculated by applying strong-field QED rigorously up to the second order. The present investigation suggests that further technical improvements can potentially increase the experimental accuracy by an order of magnitude.