The Chemical Significance of XPS BE Shifts: A Perspective

A recent publication from Paul Bagus (University of North Texas), Connie Nelin, and Dick Brundle [1] discusses the chemical significance of XPS BE shifts. Paul, Connie and Dick have made many outstanding contributions to the field of XPS, in particular by using computational chemistry approaches to model various XPS phenomena and spectral shapes - especially of transition metal species with complex multiplet splitting and satellite structures. 

Dr. Bagus describes this perspective below. This will be a good starting point for researchers interested in applying MO theory to XPS measurements. 

An all too common interpretation of the shifts of XPS BEs, Delta BE, for a given element in different compounds and in different environments is to relate the sign of the BE shift to the change in the effective charge, Q, of the core ionized atom. Thus, a shift to lower BE from sample 1 to sample 2 is interpreted as meaning that the atom in sample 2 has a smaller positive Q or a larger negative Q than the same atom in sample 1. Similarly, a shift to a larger BE is taken to mean that the atom in sample 2 has a larger Q. This paper shows that this simple interpretation of BE shifts is incomplete and that it is likely to be misleading.

While the effective charge Q does contribute to BE shifts, it is not the only physical or chemical mechanism that can contribute to XPS BE shifts. Two other mechanisms are the environment of the ionized atom that can lead to electrostatic potential that are different at different sites in a given sample and are different for different samples. Another mechanism is the degree of hybridization of an atom again at different sites and different compounds. An important objective of this perspective is to examine the mechanisms that lead to BE shifts. The chemical and physical content of these different mechanisms is first examined for a model system. With this model system, the different mechanisms can be separated and the magnitudes of the XPS BE shifts due to the different mechanisms can be understood directly in terms of the electronic charge distribution. Then five specific examples of XPS BEs measured for real systems are discussed and the observed BEs related to the physical and chemical mechanisms which are the origin of the BE shifts. The paper also considers the initial and final state contributions to the BE shifts and identifies when it is likely that initial state effects will dominate.

An important goal of the paper is that the principles and mechanisms for BE shifts can be applied, not only to the specific systems discussed in the paper but also to the understanding of the Delta BE for systems in general. It can lead readers to make suggestions for theoretical studies to help explain specific observations of BE shifts.

[1] P.S. Bagus, C.J. Nelin, C.R. Brundle, J. Vac. Sci. Technol. A 41 068501 (2023).