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X-ray Photoelectron SpectroscopyX-ray photoelectron spectroscopy (XPS) is a surface sensitive analytic tool to study the surface composition and electronic state of a sample. A sample maintained under ultra high vacuum is bombarded with X-rays . The X-rays penetrate a substantial distance into the sample (~mm) and excites electrons (referred to as photoelectrons). A small fraction of these electrons from the top ~ 5 nanometres make it outside the sample and are detected. The kinetic energy of these electrons is measured by the analyzer. When using monochromatic X-rays, the energy that one photon imparts on an electron is a known quantity. The binding energy of the ejected electron can then be determined from: Ebinding= Ephoton - Ekinetic In XPS the photon is absorbed by an atom leading to ionization and the emission of a core electron (inner shell electron). This is in contrast to a complementary technique called UPS (Ultraviolet Photoelectron Spectroscopy) in which the ultraviolet photon, with much lower energy than the X-ray interacts with valence levels of the atom, leading to ionization by removal of one of the valence electrons. In XPS, because the energy of core electrons is very specific for the element that the atom belongs to, the spectrum gives information on the elemental composition of the shallow surface region. From small shifts in the binding energies additional chemical information can be derived (e.g. the oxidation state of the element). XPS is also often referred to as ESCA, short for Electron Spectroscopy for Chemical Analysis. See also
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