QED Calculations of the Interelectron Interaction in Li-like Ions

Investigations of heavy few-electron atoms provide an excellent opportunity for testing QED in strong Coulomb fields. A remarkable high experimental precision has been reached in Lamb-shift measurements of Li-like ions. The uncertainty obtained e.g. for the 2p1/2 2s transition energy in Li-like Ni and Zn [1], Ag and Xe [2, 3] and U [4] is by an order of magnitude smaller than the QED contribution of second order in the fine structure constant α. To achieve that level of accuracy in theoretical predictions rigorous calculations of all Feynman diagrams of order α had to be carried out [5, 6, 7]. Traditional approaches for calculating energy levels in high-Z Li-like ions such as relativistic many-body perturbation theory, multiconfigurational Dirac-Fock, and the relativistic configuration-interaction method account for the correlation and the relativistic corrections but do not provide a rigorous treatment of the QED effects, which is required in high-Z systems. Since correlation effects cannot be separated from the QED radiative corrections ab initio calculations valid up to order α must be carried out starting from the complete set of QED-Feynman diagrams. Two-electron QED corrections of order α for a Li-like ion consist of three sets of Feynman diagrams, each of which is separately gauge invariant: the self-energy screening diagrams, the vacuum-polarization screening diagrams and the two-photon exchange diagrams. The two-photon ex-