The theory of quantum electrodynamics (QED) has achieved great success under weak electromagnetic fields in few-electron quantum systems. QED calculations are sensitive to high-order corrections with the increasing nuclear charge Z, in particular within many-electron systems where electron-electron correlations are complicated. 

Atomic energy levels of highly charged ions are ideal systems for testing the QED and relativistic effects. The resonant character of dielectronic recombination (DR) provides an opportunity to precisely determine the transition energies in the ions.  

Recently, the researchers from the University of Science and Technology of China, the Institute of Modern Physics of the Chinese Academy of Sciences (CAS) and the University of Giessen have performed the precision DR spectroscopy measurement with fluorine-like nickel ions by employing the merged-beams method. The measurement builds a bridge which enables comparisons between different theories. 

Heavy-ion storage rings with an electron cooler are ideal platforms for performing merged-beams electron-ion recombination experiments and obtaining high precision transition energy. The experiment was conducted at the main cooler storage ring of the heavy ion research facility in Lanzhou (HIRFL-CSRm). The measurement covered all the DR resonances for ΔN = 0 within the energy range of 0-160 eV.  

The researchers from Fudan University carried out theoretical calculations to identify the measured resonances. The researchers recognized the intermediate state of the first resonance at 86 meV, which enables them to determine the 2s²2p⁵[²P_3/2] → 2s2p⁶[²S_1/2] transition energy by calculating the 6s binding energy in cooperation with theoreticians from the Northwest Normal University, Hebei University, St. Petersburg State University and ITMO University.