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

Abstract

The \textit{ab initio} approach is used to evaluate the excitation energies of the $2s2p{}^{2S+1}{P}_J$ states from the ground state as well as the $2s2p{}^3{P}_1\to 2s2p{}^3{P}_0$ and $2s2p{}^3{P}_2\to 2s2p{}^3{P}_1$ transition energies for selected Be-like highly-charged ions over a wide range: from ${\mathrm{Ar}}^{14+}$ to ${\mathrm{U}}^{88+}$. 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.

15 pages, 1 figure, 5 tables