The photoelectrons of interest have relatively low kinetic energy. XPS is a surface sensitive technique because only those electrons generated near the surface escape and are detected. The peak shape and precise position indicates the chemical state for the element. The area under a peak in the spectrum is a measure of the relative amount of the element represented by that peak. The energy corresponding to each peak is characteristic of an element present in the sampled volume. By counting the number of photoelectrons and Auger electrons as a function of their energy, a spectrum representing the surface composition is obtained. The photoelectrons and Auger electrons emitted from the sample are detected by an electron energy analyzer, and their energy is determined as a function of their velocity entering the detector. Analysis of Auger electrons can be used in XPS, in addition to emitted photoelectrons. The energy of this transition is balanced by the emission of an Auger electron or a characteristic x-ray. When the core electron is ejected by the incident x-ray, an outer electron fills the core hole. Energy analysis of the emitted photoelectrons is the primary data used for XPS. The energy of a photoemitted core electron is a function of its binding energy and is characteristic of the element from which it was emitted. The incident x-rays cause the ejection of core-level electrons from sample atoms. The sample is placed in an ultrahigh vacuum environment and exposed to a low-energy, monochromatic x-ray source. Insulators and conductors can easily be analyzed in surface areas from a few microns to a few millimeters across.
Both composition and the chemical state of surface constituents can be determined by XPS. X-Ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA), is an analysis technique used to obtain chemical information about the surfaces of solid materials.
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