X-ray photoelectron spectroscopy
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In X-ray photoelectron spectrosopy (XPS) a sample is bombarded with X-rays in a high vacuum. Some of the electrons in a rather shallow region at the surface of the sample will be ejected from the sample. The kinetic energy of these electrons (their speed) is measured.

When using monochromatized 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

The energy of the photon is usually enough to eject not only valence electrons but also the core electrons of an atom. 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).

The original article can be found at: http://en.wikipedia.org/wiki/X-ray_photoelectron_spectroscopy.
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