Significance of Röntgen current in quantum optics: Spontaneous emission of moving atoms

We argue that for moving atoms, the conventional dipole interaction must be supplemented by a so-called R\"ontgen term in order to obtain results which are trustworthy to order V/c in the atomic velocity. As an illustration, we include the R\"ontgen interaction in a Hamiltonian model to derive the angular distribution of photons spontaneously emitted by a freely moving atom. We find agreement with an independent calculation which is based on the transformation properties of such distribution under Lorentz boosts. This resolves the previously reported inconsistency of the conventional dipole interaction for moving atoms [Phys. Rev. A 47, 671 (1993)].