FINITE MASS EFFECT ON 2-PHOTON PROCESSES IN HYDROGENIC SYSTEMS - EFFECTIVE SCALAR PHOTON INTERACTION

We consider a hydrogenic system with a nucleus of finite mass. The cou pling of the radiation field to the center of mass motion gives rise t o an effective scalar type coupling. This induced scalar photon intera ction emerges as a correction in competition with the usual multipole interactions, This effect is particularly important in positronium whe re the electric quadrupole interaction is totally suppressed. We illus trate this effect with the two-photon decay of metastable hydrogenic s ystems. (C) 1998 Elsevier Science B.V.