Core-level binding-energy shifts at surfaces and in solids

This review presents an overview of the theory and of various successful approaches to the interpretation of core-level binding-energy shifts observed in photoelectron spectroscopy. The review concentrates specifically on shifts since most of the chemical and physical insights provided by core levels are derived not from the core-level binding energies themselves but from shifts they exhibit. The theoretical background is presented at a level readily accessible to the general reader. Particular attention is paid to relative merits of the two basically different conceptual frameworks for interpreting core-level binding-energy shifts, the initial-state-final-state approach and the equivalent-core approach.