We calculate the energy shifts, in Rydberg states of helium, due to retardation corrections to the two-photon exchange potential. The correction to the level spacings is at the accuracy of existing measurements in the $n=10\mathrm{}$ states of neutral He. It may soon be possible for the first time to detect the effect of these corrections in atomic systems. Estimates of the energy shifts for high-$Z$ He-like ions yield effects of several percent of the total spacing.

• Received 7 June 1984

DOI:https://doi.org/10.1103/PhysRevLett.53.1145