QED calculations of intra-$L$-shell doubly excited states in Be-like ions

Abstract

The rigorous QED approach is employed to calculate the energies of the $2p2p{}^3{P}_{0,1,2}$, $2p2p{}^1{D}_2$, and $2p2p{}^1{S}_0$ states of selected Be-like highly charged ions over a wide range of nuclear-charge numbers, $18⩽Z⩽92$. Combined with the previously reported energies of the $2s2p{}^3{P}_{0,1,2}$ and $2s2p{}^1{P}_1$ states [A. V. Malyshev et al., Phys. Rev. A 110, 062824 (2024)], the obtained results are used to study various intra-$L$-shell transition energies. Strong level mixing, caused by the proximity of states with the same symmetry, is overcome by means of the QED perturbation theory for quasi-degenerate levels. The applied approach merges a rigorous perturbative QED treatment up to the second order with the consideration of electron-electron correlation contributions of the third and higher orders evaluated within the Breit approximation. The higher-order screened QED effects are estimated using the model-QED-operator approach. The nuclear-recoil and nuclear-polarization effects are also taken into account. The obtained predictions represent the most accurate theoretical description of the electronic structure of Be-like ions to date and demonstrate good agreement with available experimental data.

13 pages, 8 tables