Retardation effects in radiative electron capture

Three-particle radiative electron-capture cross sections with retardation are calculated within the nonrelativistic treatment. The formalism is carried out in the laboratory frame. It is found that the center of mass genuinely radiates, and it plays an important role in restoring the backward-forward symmetry for angular distributions. Comparison with experiments are made. The formalism also describes the nonrelativistic Doppler effect which shifts and spreads the photon energy distributions. Total, single-, and double-differential cross sections are reported, and comparisons with the dipole approximation are presented. The range of validity of the results obtained is discussed.