Effects of nonzero photon momentum in (\gamma,2e) processes
  We study the effects of nonzero photon momentum on the triply-differential cross section for (\gamma,2e) processes. Due to the low value of the photon momentum, these effects are weak and manifest only in special kinematical conditions like the back-to-back emission of the electrons with equal energy sharing. Helium and a few light helium-like ions are treated in detail. Quite unexpectedly, the magnitude of these effects is maximal for relatively small photon energies. However, although this effect on the TDCS remains rather small, of the order of a few mbarn eV^{-1} sr^{-2}, it is sufficient to be observed experimentally. 
Alexander G. Galstyan, Ochbadrakh Chuluunbaatar, Yuri V. Popov and   Bernard Piraux
physics.atom-ph