X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA), is a highly specialized surface analytical method reaching the first 5-10 nm of surface layer. Samples are excited with a very intense beam of low-energy X-rays.  The photoelectric effect causes electrons to be ejected from outer orbitals of target atoms. These electrons have a characteristic energy, equivalent to the energy of the incident photon less the ionization energy of the appropriate electron orbital. From the data, it is possible to determine the binding energy that varies from compound to compound and is a function of both oxidation state and coordination number of that element.