Since 1967, photo-electron spectroscopy has extended X-ray absorption data by allowing the ionization energy I of inner shells and penultimate orbitals of atoms in the surface of a solid sample to be measured. The chemical shift dI from one compound to another indicates the variation of the Hartree potential. dl is an increasing function of the fractional charge of the atom considered, but it is decreased by the Madelung potential produced by adjacent atoms.