Finite Mass Effect on Two Photon Processes in Hydrogenic Systems:   Effective Scalar Photon Interaction
  We consider a hydrogenic system with a nucleus of finite mass. The coupling of the radiation field to the center of mass motion gives rise to an effective scalar type coupling. This induced scalar photon interaction emerges as a correction in competition with the usual multipole interactions. This effect is particularly important in positronium where the electric quadrupole interaction is totally suppressed. We illustrate this effect with the two-photon decay of metastable hydrogenic systems. 
C.K. Au and C.S. Chu