X-ray photoelectron spectroscopy (XPS)

Any technique in which the sample is bombarded with X-rays and photoelectrons produced by the sample are detected as a function of energy. ESCA (Electron Spectroscopy for Chemical Analysis) refers to the use of this technique to identify elements, their concentrations, and their chemical state within the sample.
Orange Book, 2nd ed., p. 250 (http://media.iupac.org/publications/analytical_compendium/)