The Wagner plot and the Auger parameter as tools to separate initial- and final-state contributions in X-ray photoemission spectroscopy

The application of the Wagner plot and Auger parameter concepts as tools for the identification of chemical states of atoms in molecules or solids is growing steadily in the surface science literature. In several publications, however, the chemical and physical insights provided by such conceptual tools are not fully investigated. In this contribution it is shown how easy is to use the Wagner plot to get information on the local valence charge and Madelung (molecular) potential (initial-state effects) and on the extra-atomic relaxation energy gained from the environment (final-state effects) for a core-ionized atom. To cover the gap with previous reviews on the applications of the Wagner plot and the Auger parameter concepts, a brief overview of the contributions which appeared in the literature in the last 15 years is presented.