Photon-energy dependence of the core-level peak intensity in photoelectron spectra: A study of the extended fine structure

We have measured the intensity of several $d$ core-level photoemission peaks as a function of the photon energy up to 100 eV above threshold. The measurements have been carried out in single-crystal layer compounds and with limited angular integration. A strong modulation has been observed in the intensity versus photon-energy curves superimposed upon a slowly varying background of atomic origin. Our analysis indicates that the main cause of this modulation is the interference of the outgoing photoelectron wave function with the backscattered wave functions by the surrounding atoms, i.e., the same mechanism responsible for the extended x-ray absorption fine structure (EXAFS). This conclusion is supported by the positions of the maxima in the observed modulation and by a self-consistent analysis of the scattering phase shifts. The possibility is therefore raised of using photoemission intensity measurements to expand the applications of the surface-EXAFS technique.