Access to Two-Photon QED Contributions via 2s 2S12?2p 2P12 Transitions in Heavy Li-Like Ions

Wavelengths of the 2s 2S1/2–2p 2P1/2 fine structure transition in lithium-like ions in the medium heavy Z range have been measured using beam foil excitation and grazing incidence spectroscopy. The achieved precision of 70 ppm allows for a 0.1% QED test which establishes a new benchmark. It provides sensitivity to two-photon QED contributions. Very good agreement with the newest calculations based on hydrogenic wave functions and applying a strict QED formalism is observed. All accurate published data along the isoelectronic Li-like series for Z > 20 are summarized. Their values and their accuracies are compared to the calculations and to the relative QED contributions, respectively.