Retarded boson-fermion interaction in atomic systems.

The retarded interaction between an electron and a spin-0 nucleus is derived from electrodynamical perturbation theory. The contribution of retardation at order v(2)c(2) mimics the Breit interaction [Phys. Rev. 34, 553 (1929); 36, 388 (1930); 39, 616 (1932)] with the Dirac matrix alpha(2) being replaced by p(2)m(2)c where p(2) is the linear momentum operator for the nucleus. An effective one-electron retardation operator is obtained in relative coordinates, and this can be used through all orders in perturbation theory without any problem of infinite degeneracy. A few steps of unitary transformation lead to the nonrelativistic limit. The leading terms in retardation corrections to energy are of order (m(e)m(n))alpha(2)Z(4)(alpha(2)m(e)c(2)). The implications for atomic systems are discussed.