Charge distribution and local and non‐local screening effects studied by means of the Auger parameter and chemical state plots

Abstract

AbstractCopper‐ and zinc‐containing compounds, as well as other 3dn transition metal compounds, are characterized by making use of two concepts introduced first by Wagner: the Auger parameter, α = EB (photoelectron) + EK (Auger), and the so‐called chemical state plots (EK vs. EB, with the binding energy in the negative direction on the abscissa and the Auger parameter on a diagonal grid with slope equal to +1). These are useful tools to obtain information on: (1) the relative initial state charge distributions; and on (2) the relative extra‐atomic relaxation energy in the final state.In the present paper, for the first time, we show that the Auger parameter can also be helpful in the analysis of the screening mechanism in transition metal compounds.The position in the chemical state plots of different classes of compounds is discussed. The constancy of the Auger parameter for Cu(II) compounds is interpreted as further evidence of the electronic nature of the 2p3/2 final state. It can be represented by a 2p53d10L electronic structure, a local screended final state, where a charge transfer from the ligand valence orbitals toward localized d‐orbitals allows screening of the core‐hole. For copper, this relaxation energy is similar to the screening energy in the metallic state.For Cu(I) compounds, as in the case of Zn(II) compounds (d10 electronic configuration), a large dependence of the Auger parameter on the chemical nature of the ligands suggests a non‐local screening mechanism in the final state, i.e. a strong influence of electronic polarizability of the ligands on the relaxation energy. In this case, the extended s‐p cation orbitals are involved in the screening process.The charge distribution in the initial state can be studied analytically or by using chemical state plots. For example, Cu(II) compounds have about a constant Auger parameter (they fall on the same diagonal line with slope equal to +1), with the more ionic compounds at higher 2p3/2 binding energy and lower (L3M45M45, 1G) Auger kinetic energy.It appears that the Auger parameter and the chemical state plots permit an accurate characterization of the ground‐state electronic structure of cations in transition element compounds and of the screening mechanism in the final state.