First observed by Pierre V. Auger in the 1920s, the Auger effect is a process where an atom that has been ionized with the emission of a core level electron undergoes a transition in which a second electron, the Auger electron, is emitted. Auger electron spectroscopy, AES, is a surface analytical technique that utilizes the Auger effect to measure the elemental composition of surfaces. A beam of energetic electrons, 3 to 25 keV, is used to eject a core level electron from surface atoms. To release energy, those atoms may emit Auger electrons from their induced excited state. The energy of the Auger electron, specific to the atom from which it originated, is measured and the quantity of Auger electrons is proportional to the concentration of the atoms on the surface. Auger electron spectroscopy can measure two dimensional maps of elements on a surface and elemental depth profiles when accompanied by ion sputtering.