XPS is a surface spectroscopic technique for quantitative measurements of the elemental composition or stoichiometry and the chemical state of the present elements, like their oxidation state and chemical bonds. Due to the limited free path-length of the excited photoelectrons within the material, XPS is highly surface sensitive, giving chemical and binding energy information from the narrow region close to the surface.
Angular resolved (AR) XPS permits to obtain a depth profile of the elemental composition and chemical state in this narrow surface region, and permits to determine the relative concentrations of surface elements versus buried ones, as, e.g., between head and tail groups of adsorbed alkanes. In combination with sputtering, a larger depth profile is possible. The X-ray spot-size on the sample surface is typically in the range of 5-100 µm.