Core-level binding energies (BE) are defined primarily by the number of protons within the nucleus of the emitting atom, and therefore has a dependence on Z.

The various contributions to BE values of bound atoms are shown in the figure below.

As show in the diagram, the observed binding energy is dependent on two primary factors:

Initial State Effects: These are effects on the BE which occur due to the bonding. Although binding occurs through valence electrons, all the electrons in the atom will experience an induced change in electron density and is responsible for XPS allowing differentiation of different chemical states and species. The figure below shows one such initial state effect (Chemical Shift) for ethyl trifluoroacetate, the so-called ESCA molecule.

Such initial state effects arising from bonding (ground state polarisation) can be further categorised as inter-atomic effects (from neighbouring atoms) and intra-atomic effects (from within the atom). These effects typically include:

• Oxidation state of the emitting atom
• Bond distance of the emitting atom with its neighbours
• Madulung potentials – applicable to ionic lattices
• Electronegativity of neighbouring atoms (see the ESCA Molecule above)

Final State Effects: These influence the effect caused by the perturbation of the electronic structure as a consequence of photoemission. As they also are dependent upon the initial electronic structure of the bonding state, these are also useful in revealing the speciation of the emitting atom.

Example of final state effects are:

• Multiplet splitting
• Shake-up and shake-off satellites
• Plasmon loss features (e.g. in metallic Al)
• Auger peaks
• Spin-orbit splitting

An important note

In the first diagram we show spin-orbit splitting to be a final state effect. Some texts (Such as the second link in further reading), show it to be an initial state effect. Whilst atomic theory indicates the electrons spin as it orbits causes coupling and hence splitting of the orbital energies, the spin orbit splitting in photoemission is typically defined as a final state effect. This definition arises since the emission of an electron from an initially fully occupied shell, leaves an unpaired electron with a spin which couples parallel or anti-parallel to the magnetic moment of the electrons orbit and leaves the atom in 2 different final energy states.

Further Reading

The ESCA molecule—Historical remarks and new results

X‐Ray Photoelectron Spectroscopy: An Introduction to Principles and Practices – Chapter 5.