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

The ab initio approach is used to evaluate the excitation energies of the 2s2p{0. 16em{0ex}}^2S+1P₉ states from the ground state as well as the 2s2p0. 16em{0ex}^3P₁2s2p0. 16em{0ex}^3P₀ and 2s2p0. 16em{0ex}^3P₂2s2p0. 16em{0ex}^3P₁ transition energies for selected Be-like highly charged ions over a wide range: from Ar^14+ to U^88+. The issue of a strong level mixing due to the proximity of states with the same symmetry is addressed by applying the QED perturbation theory for quasidegenerate levels. The employed approach combines a rigorous perturbative QED treatment up to the second order with electron-electron correlation contributions of the third and higher orders calculated in the Breit approximation. The higher-order QED effects are estimated using the model-QED-operator approach. The nuclear-recoil and nuclear-polarization effects are taken into account as well. The performed calculations are accompanied with a thorough analysis of uncertainties due to uncalculated effects. The most accurate theoretical predictions for the excitation and transition energies in Be-like ions are obtained, which, in general, are in perfect agreement with the available experimental data.