TOWARD IMPROVED PHOTON-ATOM SCATTERING PREDICTIONS

Photon-atom scattering is important in a variety of applications, but scattering from a composite system depends on the accurate characteriz ation of the scattering from an isolated atom or ion. We have been exa mining the validity of simpler approximations of elastic scattering in the light of second-order S-matrix theory. Partitioning the many-body amplitude into Rayleigh and Delbruck components, processes beyond pho toionization contribute. Subtracted cross sections for bound-bound ato mic transitions and bound pair annihilation are required in anomalous scattering factors for: (1) convergence of the dispersion integral; (2 ) agreement with predictions of the more sophisticated S-matrix approa ch; (3) satisfying the Thomas-Reiche-Kuhn sum rule. New accurate tabul ations of anomalous scattering factors have been prepared for all Z, f or energies 0-10000 keV, within the independent particle approximation (IPA) using a Dirac-Slater model of the atom. Separately, experimenta l atomic photoabsorption threshold information has been used to modify these IPA predictions for improved comparison with experiment.