RELATIVISTIC CALCULATION OF 2-PHOTON BOUND-BOUND TRANSITION AMPLITUDES IN HYDROGENIC ATOMS

Using a representation of the (first-order) Dirac Coulomb Green functi on in terms of an expansion over a Sturmian basis, we have computed tw o-photon probability amplitudes for transitions in hydrogenic atoms fr om the ground state towards states in the L and M shells. The treatmen t takes care of the relativistic structure of the atoms according to t he Dirac theory and of the fully retarded interaction with the electro magnetic field. We present in detail the Sturmian-like expansion for t he Dirac Coulomb Green function and we discuss the relative importance of relativistic and retardation effects occurring for higher nuclear charge Z. Comparison is made also with the nonrelativistic Schrodinger treatment, retardation included or not.