Dependence of photon-atom scattering on energy resolution and target angular momentum - art. no. 042704

We consider a more correct treatment of photon scattering from randomly ori ented atoms, going beyond the level of description used in currently availa ble results. We focus on cross sections which include an elastic scattering component. The most sophisticated results available to describe high-energ y elastic scattering are relativistic coherent elastic S-matrix calculation s within independent-particle approximation, which, however, perform an ave raging over magnetic substates at the level of the amplitude (averaged-ampl itude approach), exact only for fully filled subshells. The present S-matri x calculations also do not consider incoherent elastic scattering (in which an electron makes a transition to a different magnetic substate in the sam e subshell), which can occur when there are partially filled subshells. A m ore proper treatment of these situations involves an averaging over the cro ss sections for all possible orientations of the target. Here we consider t he total elastic scattering (both coherent and incoherent), and we also inc lude the unresolved contributions of inelastic (Raman and Compton) scatteri ng. In particular we consider inelastic Raman scattering between relativist ic subshells that are nearly energy degenerate, which may not be resolved, given finite experimental resolution, and which may be degenerate in nonrel ativistic theory (e.g., Coulombic 2p(1/2) and 2p(3/2) subshells). Thus, for example, the nonrelativistic result for elastic scattering (coherent and i ncoherent) from excited hydrogen in the 2p state corresponds to the result obtained by summing relativistic elastic scattering (coherent and incoheren t) together with the relativistic inelastic scattering for transitions betw een the 2p(1/2) and 2P(3/2) subshells. The averaged-amplitude approach does poorly in this case. However, results for scattering from ground-state bor on indicate that the averaged-amplitude approach generally works well for m any-electron ground-state atoms, due to the large coherent contribution fro m electrons in fully filled subshells.