How XPS works

The principle in X-ray photoelectron spectroscopy experiments is to study the properties of electrons photo emitted from a sample by photons. It is a quantitative spectroscopic technique which utilises core level shift to obtain information on the chemical and electronic state of a sample's elemental components to a penetration depth of  ~ 10-100 Å.

When a photon of energy (hν) penetrates the surface of a solid it is absorbed by an electron. If the photon reaches a sufficient level of energy, the electron moves from its initial state to an excited state as a photoelectron. The energy required for this process is dependent upon the electron's energy in its unbound state. The energy of the photoelectrons is directly related to their atomic number and the molecular environment from which they originated. The number of photoelectrons emitted is related to the concentration of emitting atoms in a sample.