QED effects in lithiumlike krypton

The ${(2s}_{1/2}{-2p}_{1/2})$ Lamb shift in lithiumlike krypton was obtained through a recombination measurement. The Lamb shift was extracted from the recombination spectrum by comparison with a calculation of the dielectronic-recombination resonance positions. The energy calibration used in this work is based on an accurate energy determination of one arbitrary point in the spectrum. Our value for the ${2s}_{1/2}{-2p}_{1/2}$ energy difference is 71.243(8) eV. The relativistic many-body contributions to the Lamb shift can be calculated very precisely, which enabled an extraction of an “experimental” quantum-electrodynamical contribution of 1.539(8) eV. This precision is beyond what can be achieved with conventional estimates of the screened quantum-electrodynamical effects. The main error originates from the uncertainty in the length of the ion-beam orbit. Improved knowledge of this value can improve our result significantly.