Auger parameter and Wagner plot in the characterization of chemical states by X-ray photoelectron spectroscopy: a review

In this review we describe the Auger parameter concept and its relation to the final state relaxation energy. After a historical introduction we illustrate the several models proposed to calculate relaxation energies and Auger parameter shifts. It is stressed how the initial empirical approach of the Anger parameter concept developed by Wagner in the early 1970s is now changed thanks to the elaboration of theoretical models based on classical electrostatic and quantum chemistry. The Auger parameter permits to obtain information on the local chemical environment of core-ionized atoms. Even more information can be obtained arranging photoemission and Auger lines in the format of the so called Wagner plots (chemical state plots). These plots contain information an the initial state effects, i.e. the Madelung potential and the ground state valence atomic charge at the core-ionized atom, and on the final state relaxation energy. By means of the point-charge potential approximation, the Auger parameter and the Wagner plot, we describe a simple semiquantitative model useful to rationalize the dependence of the local electronic structure on the atomic environment. The Auger parameter is useful to study the screening mechanism in the final state of the photoemission process. In the case of the non-local screening mechanism (polarization of the charge from the nearest-neighbour ligands towards extended empty orbitals of the core-ionized atom) a simple electrostatic model permits the estimation of the Auger parameter shifts. It is shown that the Auger parameter shift is a function of the number, distance, electronic polarizability and local geometry of the nearest-neighbour ligands around the core-ionized atom. The electrostatic model has been applied successfully for interpreting Auger parameter shifts of core-ionized atoms in insulators and semiconductors compounds. On the other hand, when the electrostatic model cannot be applied, as in the case of the local screening mechanism (complete tranfer of one electron towards empty and localized orbitals of the core-ionized atom), the extended potential model of the Auger parameter is able to relate the potential in an atomic core to the atomic valence charge. According to this model the Auger parameter shift depends on atomic properties of the core-ionized atom, obtained from atomic structure calculations, on the valence charge in the initial state and on the screening charge in the final state. It has been applied successfully for interpreting metal-free atom and alloy-pure metal Auger parameter shifts. Other models of the Auger parameter based on electrostatics, quantum chemistry and thermochemistry are also reviewed. (C) 1998 Elsevier Science B.V. All rights reserved.