Ab initio Calculations of Energy Levels in Be-Like Xenon: Strong Interference between Electron-Correlation and QED Effects

The strong mixing of close levels with two valence electrons in Be-like xenon greatly complicates ab initio QED calculations beyond the fast-order approximation. Because of a strong interplay between the electron-electron correlation and QED effects, the standard single-level perturbative QED approach may fail, even if it takes into account the second-order screened QED diagrams. In the present Letter, the corresponding obstacles are overcome by working out the QED perturbation theory for quasidegenerate states. The contributions of all the Feynman diagrams up to the second order are taken into account. The many-electron QED effects are rigorously evaluated in the framework of the extended Funy picture to all orders in the nuclear-strength parameter alpha Z. The higher-order electron-correlation effects are considered within the Breit approximation. The nuclear recoil effect is accounted for as well. The developed approach is applied to high-precision QED calculations of the ground and singly excited energy levels in Be-like xenon. The most accurate theoretical predictions for the binding and excitation energies are obtained. These results deviate from the most precise experimental value by 3 sigma but perfectly agree with a more recent measurement.

Automated summary

This study overcomes the obstacles in calculations for Be-like xenon by working on QED perturbation theory for quasidegenerate states, resulting in the most accurate theoretical predictions for binding and excitation energies. The approach takes into account the contributions of all Feynman diagrams up to the second order and evaluates many-electron QED effects rigorously within the extended Funy picture framework.