Understanding the importance of dynamic, correlation, and exchange effects in new low-energy photon scattering experiments

Abstract

We consider the current status of theory in photon scattering, and new approaches to both elastic and inelastic scattering, in the light of recent x-ray scattering measurements on neon and helium in the range 4–22 keV, obtained using the APS third-generation synchrotron. The precision of these experiments requires a detailed consideration of the effects of nonlocal exchange, electron correlations, and dynamic effects in making predictions. These effects can often be regarded as perturbations, though this assumption fails as threshold regions are approached. As well as considering procedures for making composite predictions including all significant effects, we also consider the approximations that are often employed in describing elastic (form-factor approximation) and inelastic (incoherent-scattering-factor and impulse approximations) scattering. Dynamic effects and nonlocal-exchange effects can be significant in elastic scattering. The incoherent-scattering-factor approximation works best for inelastic scattering at lower energies, and less so at higher energies. The converse occurs for impulse approximation.