Theoretical study of Doniach and Šunjić (DS) on X-ray photoemission spectra (XPS) is extended beyond the behaviour precisely at threshold. A perturbational calculation where the static hole potential is present only after photoemission is performed to the fourth-order term based on Mizuno and Ishikawa's model. The anomalous exponent α in DS theory is found to have a correction term which is linear in ω F -ω. Here, \(ω\) is the energy of an emitted electron and \(ωF\) the threshold energy. This correction term is appreciably large at finite distances from threshold. Exponents experimentally obtained away from threshold are different by nature from those given by DS theory.
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Ohmura et al. (1980) studied this question.