The relativistic operator of interaction of two quasimolecular electrons as a third-order effect of quantum electrodynamics

We solve the problem of interaction two quasimolecular electrons located at an arbitrary separation near different atoms (nuclei). We consider third-order effects in quantum electrodynamics, which include the virtual photon exchange between electrons with emission (absorption) of a real photon. We obtain the general expression for matrix elements of the operator of the effective interaction energy of two quasimolecular electrons with the external radiation field, which allows calculating probabilities of inelastic processes with rearrangement at slow collisions of multicharge ions with relativistic atoms. We demonstrate that consistently taking the natural condition of the interaction symmetry with respect to the two electrons into account results in the appearance of additional terms in the operators of spin–orbit, spin–spin, and retarded interactions compared with the previously obtained expressions for these operators. We construct the operator of the dipole–dipole interaction of two neutral atoms located at an arbitrary separation.