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Using the experimental technique of electron-ion collision spectroscopy at the electron cooler of the heavy-ion storage ring CRYRING@ESR [1] we have measured merged-beams rate coefficients for electron-ion recombination of berylliumlike Pb$^{78+}$ ions.In the electron-ion collision energy range 12-16.5 eV, we have observed $2s2p\;(^3P_1)\,19l_j$ dielectronic recombination (DR) resonances associated with a $2s^2\;^1S_0 \to 2p2p\;^3P_1$ core excitation and a simultaneous capture of the initially free electron into the $n=19$ shell of the boronlike Pb$^{77+}$ product ion. A careful analysis of the systematic experimental errors reveals that the uncertainty of the measured resonance positions amounts to $\pm30$~meV. This uncertainty is lower than the theoretical uncertainty of the calculation of the $2s^2\;^1S_0 \to 2p2p\;^3P_1$ excitation energy by Malyshev et al. [2]. By considering second order QED effects and electron-electron correlation effects, which are particularly strong in berylliumlike ions, these authors obtained $E_\mathrm{exci}=244.942(52)$ eV. Within the uncertainties this agrees with our experimental finding [3], which thus constrains second order strong-field QED.
This research was supported by the ErUM-FSP T05 "APPA" (BMBF grant nos.05P24RG2, 05P24SJA, 05P21PMFA1, 05P21RGFA1, 05P21SJFAA, https://fsp-appa.fair-center.eu).
[1] C. Brandau et al., Chin. Phys. C (2025), https://doi.org/10.1088/1674-1137/adbf81.
[2] A. Malyshev et al. Phys. Rev. A. 110, 062824 (2024), https://doi.org/10.1103/PhysRevA.110.062824.
[3] S. Schippers et al., Phys. Rev. Lett. submitted, https://arxiv.org/abs/2502.15433.
