Auger parameters and wagner plots

A brief review on the use of Auger parameters and Wagner plots (chemical-state plots) in the characterization of surfaces by XPS is presented with particular reference on their applications to barium compounds. Computational (Tinker) and computer graphic (Molden) programs, available free of charge from the web permit, respectively, the calculation of Auger parameter shifts at the electrostatical level of theory and the graphical display of the atomic structures involved. An application of these methods is reported for a BaCl2 crystal, considering the Auger parameter shift of Ba2+ ions in different localizations. The effects of distant cations and anions from the ion with the core hole may be easily investigated together with the effect of decreasing the coordination number of an ion because of its localization at the surface of the material. For such an ion, the effect of adsorbed molecules (e.g. water and hydrogen sulfide molecules) on the Auger parameter shift can also be calculated. The initial- and final-state effects determine the localization of a barium compound in the Wagner plot which is a very useful tool in the chemical-state analysis.