XPS Binding Energies and Oxidation States

In order to appreciate the meaning of a binding energy, it is necessary to consider final state effects. In practice, we use XPS data as if they were characteristic for the atoms as they are before the photoemission event takes place. We must realize that this is not correct photoemission data represent a state from which an electron has just left. Thus, we must analyze the event in more detail. 

We start with an atom containing N electrons, with a total energy E in the initial state, denoted with the superscript i. The atom absorbs a photon of energy hv, the absorption event taking less than 10 17 s. Some 10-14 s later, the atom has emitted the photoelectron with kinetic energy E, and is itself in the final state with one electron less and a hole in one of the core levels. The energy balance of the event is 

E is the total energy of the atom with N electrons in the initial state, i.e., before the photoemission has taken place hv is the energy of the photon  

A rearrangement of terms shows that the binding energy with respect to the Fermi level equals  

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