x-ray absorption spectroscopy (xas) includes both extended x-ray absorption fine structure (exafs) and x-ray absorption near edge structure (xanes). xas is the measurement of the x-ray absorption coefficient ( \mu (e) in the equations below) of a material as a function of energy. x-rays of a narrow energy resolution are shone on the sample and the incident and transmitted x-ray intensity is recorded as the incident x-ray energy is incremented. the number of x-ray photons that are transmitted through a sample (it) is equal to the number of x-ray photons shone on the sample (i0) multiplied by a decreasing exponential that depends on the type of atoms in the sample, the absorption coefficient \mu, and the thickness of the sample x. i_t = i_0 e^{-\mu x} the absorption coefficient is obtained by taking the log ratio of the incident x-ray intensity to the transmitted x-ray intensity. \mu = \frac{-ln({i}_{t}/{i}_{0})}{x} when the incident x-ray energy matches the binding energy of an electron of an atom within the sample, the number of x-rays absorbed by the sample increases dramatically, causing a drop in the transmitted x-ray intensity. this results in an absorption edge. each element on the periodic table has a set of unique absorption edges corresponding to different binding energies of its electrons, giving xas element selectivity. xas spectra are most often collected at synchrotrons. because x-rays are highly penetrating, xas samples can be gases, solids or liquids. and because of the brilliance of synchrotron x-ray sources the concentration of the absorbing element can be as low as a few ppm. exafs spectra are displayed as graphs of the absorption coefficient of a given material versus energy, typically in a 500 â 1000 ev range beginning before an absorption edge of an element in the sample. the x-ray absorption coefficient is usually normalized to unit step height. this is done by regressing a line to the region before and after the absorption edge, subtracting the pre-edge line from the entire data set and dividing by the absorption step height, which is determined by the difference between the pre-edge and post-edge lines at the value of e0 (on the absorption edge). the normalized absorption spectra are often called xanes spectra. these spectra can be used to determine the average oxidation state of the element in the sample. the xanes spectra are also sensitive to the coordination environment of the absorbing atom in the sample. finger printing methods have been used to match the xanes spectra of an unknown sample to those of known "standards". linear combination fitting of several different standard spectra can give an estimate to the amount of each of the known standard spectra within an unknown sample. x-ray absorption spectra are produced over the range of 200 â 35,000 ev. the dominant physical pr
Condensed_matter_physics Environmental_chemistry Materials_science Scientific_techniques Synchrotron-related_techniques