Energy shifts and electronic structure changes in alloys: an unfulfilled promise?

The sensitivity of XPS and Auger core level energies to the chemical environment promises a direct method for the determination of valence electronic changes. In alloys involving noble metals, the seemingly simple relations between the energy shifts and the d- and sp-electron occupations is complicated by a number of factors: they involve the relative Fermi energy, a bulk quantity not accessible to direct measurement; they involve potential parameters whose values in the solid are not well known; and relaxation, or screening, of the final state must take into account the difference of the d- and sp-electrons. In short, we seem to have too many unknown quantities for the two shifts to yield the desired electronic changes. In recent years, considerable progress has been achieved in overcoming these difficulties. The relative Fermi energy has been deter mined in a number of alloys, through consideration of experimental systematics and the different d- and sp-screening. Determination of the atomic potential parameters indicates an important dependence upon the valence electron configuration, indicating a previously unanticipated, non-linear, dependence of the energy shifts upon the valence occupation changes. Finally, the average charge transfer in the vicinity of the ionized atom has been derived in a linear coupling model. Here, we present a critical analysis of the XPS and Auger shifts for these alloys in view of these developments and indicate to what extent the resulting occupation changes reflect an internal consistency. (C) 1998 Elsevier Science B.V.