Access to two-photon QED contributions via 2s 2S1/2-2p 2P1/2 transitions in heavy Li-like ions

Wavelengths of the 2s S-2(1/2) - 2p(2)P(1/2) fine structure transition in lithium-Re 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.