Coulomb-gauge electrodynamics analysis of two-photon exchange in electron-atom scattering. II. Interaction at all distances beyond atomic dimensions

We follow up on our previous analysis of the two-photon exchange potential in the scattering of a light point-charged particle by a neutral system (electron-atom scattering) by examining all the time-ordered Feynman diagrams involved. In a close parallel with the work of J. F. Babb and L. Spruch [Phys. Rev. A 36, 456 (1987)] on Rydberg states of helium, the present analysis on scattering is valid for all distances larger than the atomic radius (R>a). In the dipole approximation, we reproduce the potential obtained by G. Feinberg and J. Sucher [Phys. Rev. A 27, 1958 (1983)] using a more powerful covariant field-dispersion-theory technique. We give simple expressions for the additional nonadiabatic and multipole corrections in the two-Coulomb-photon exchange channel and also give the lowest nonadiabatic and energy-dependent corrections to the retardation potentials. These potentials are valid for all R>a. The implication of these additional correction potentials in the study of heliumlike Rydberg states is briefly discussed