Abstract
A simple type of expansion is presented for the x-ray photoemission spectroscopy (XPS) excitation distribution of the Mahan-Nozières—de Dominicis x-ray model. No assumption about any special form of the potential is used. The exact threshold behavior for the XPS spectrum is correctly given by the lowest-order term in the expansion. The results are compared with exact numerical solutions in case of a contact potential. The agreement is good for one of the approximations presented.