Polarization effects in electron scattering from ion cores in solids

In current theories of x-ray absorption fine structure (EXAFS) and of low-energy electron diffraction, the elastic scattering of the photoelectrons from the ion cores in a solid is treated as the scattering from a static potential. We consider the effects of virtual excitation processes and discuss under which circumstances the polarization of the ion cores by the scattered electron are important. A complex energy-dependent optical potential is derived and a set of complex phase shifts are calculated with parameters appropriate for copper metal. Implications for the theory of EXAFS are discussed.