The two-electron QED contributions to the ground-state binding energy of ${\mathrm{Kr}}^{34+}$ ions have been determined in two independent experiments performed with electron beam ion traps (EBIT) in Heidelberg (HD) and Tokyo (BT, Belfast-Tokyo collaboration). X rays arising from radiative recombination (RR) of free electrons to the ground state of initially bare ${\mathrm{Kr}}^{36+}$ and hydrogenlike ${\mathrm{Kr}}^{35+}$ ions were observed as a function of the interacting electron energy. The $K$ edge absorption by thin Eu and W foils provided fixed photon energy references used to measure the difference in binding energy $\Delta E_{2e}$ between the H- and He-like Kr ions (${\mathrm{Kr}}^{35+}$ and ${\mathrm{Kr}}^{34+}$, respectively). The two values agree well, yielding a final result of $\Delta E_{2e}=641.8\pm 1.7\mathrm{eV}$, confirming recent results of rigorous QED calculations. This accuracy is just of the order required to access screened radiative QED contributions.

• Received 19 October 2007

DOI:https://doi.org/10.1103/PhysRevA.77.012506