Electron-Electron Interaction in Strong Electromagnetic Fields: The Two-Electron Contribution to the Ground-State Energy in He-Like Uranium

Radiative recombination transitions into the ground state of cooled bare and hydrogenlike uranium ions were measured at the storage ring ESR. By comparing the corresponding x-ray centroid energies, this technique allows for a direct measurement of the electron-electron contribution to the ionization potential in the heaviest He-like ions. For the two-electron contribution to the ionization potential of He-like uranium we obtain a value of $2248\ifmmode\pm\else\textpm\fi{}9\text{ }\text{ }\mathrm{eV}$. This represents the most accurate determination of two-electron effects in the domain of high-$Z$ He-like ions, and the accuracy reaches already the size of the specific two-electron radiative QED corrections.