Small-atom approximation in forward- and back-scattering photoelectron spectroscopies

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Abstract
The small-atom (or plane-wave) approximation has been used in many works to evaluate the final-state wave function of a photoelectron scattered by a neighboring atom. In this work, we show that while this approximation is valid for back-scattered electrons, in techniques such as extended x-ray-absorption fine structure or angle-resolved x-ray photoelectron spectroscopy, it fails in techniques which measure scattered electrons in the forward direction (e.g., in forward-focusing angle-resolved x-ray photoelectron spectroscopy).