Non-relativistic momentum transfer in free-free absorption processes

We consider the non-relativistic free-free absorption of a photon in the presence of a Coulomb field and calculate the momentum transferred to the resulting ion for polarized photons and isotropic initial electron angular distributions. Our calculations, carried to the quadrupolar order, end in an integral which is solved numerically. We show that the final ion always recoils in the opposite direction to the incident photon and that the transferred momentum may be much greater than the photon momentum. Our results are consistent with those of bound-free absorption processes in the limit where the initial bound state tends to the ionization threshold.