The influence of relativistic, Breit interaction, and QED effects on the 1s²2p² and 2s2p³ energy levels of Be-like (4=Z=74) isoelectronic sequence

The excitation energies of the Be-like ions (4= Z =74) doubly excited state 1s(2)2p(2) and doubly core-excited state 2s2p3 are calculated using the multi-configuration Dirac-Hartree-Fock method. The contributions of relativistic effects, Breit interaction, and QED effects to the excitation energies are analyzed and discussed in detail. A reasonable electron correlation model has been constructed using the active set method. The electron correlation and the finite size of the atomic nucleus correction have also been appropriately considered. The calculated results agree with the available theoretical and experimental data. It is found that the contribution of relativistic effects, Breit interaction, and QED effects to the excitation energy becomes significant with increasing Z. The contribution of the relativistic effects reaches about 8.5% at Z =74. The P-3(0) level of the doubly excited state 1s(2)2p(2) is sensitive to the Breit interaction and the QED effects, but these two contributions cancel each other out. For the doubly core-excited state 2s2p(3), the Breit interaction and the QED effects are almost equally important, and their maximum contribution is about 0.5%. These results are expected to help future experimental and theoretical work.