W8 - Ch24aHandout;lectures1416 ZeNanChang,ChangyiLi (providedbya synchrotron. time Measure

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Ch24a Handout; lectures 14 16 ZeNan Chang, Changyi Li X ray Absorption Spectroscopy Probes the environment of metals in proteins with a tunable source of X ray photons (provided by a synchrotron). X ray energy is tuned to coincide with the absorption edges of just one type of atom at a time. Measures the X ray absorption coefficient as a function of energy. o X ray absorption coefficient decreases smoothly with increasing photon energy, except for when the photon energy matches electron transition energies of the atom of interest. At these points, an absorption edge is observed (usually labeled K, L, M depending on the energy shell in which the excited electron originally resided). Figure 1. A graph of [absorption coefficient]/density for X rays. (Fundamental of XAFS by M Newville) Figure 2. An absorption edge. ( http://www.physto.se/~larsa/X ray%20Abs%20spec.html ) o Element specificity : The X ray photon can excite an electron to an unoccupied valence state. Here, the energy of the photon is below the ionization threshold, E o . The sum of the absorptions of such a transition gives rise to the absorption edge peak (the peak to the left of E o in Figure 2). The core electron energy levels depend on the element itself. Conventionally, the X ray absorption spectrum is separated into two regimes. They have the same physical origin, but are distinguished by nature of how they can be interpreted:
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o Extended X ray Absorption Fine Structure (EXAFS) : The plotted fine structures seen ~50 1000eV after the absorption edge. The energy of excited electron is such that it can be assumed that only backscattering involving a single neighboring atom occurs (right panel of Figure 3). This results in constructive or destructive interference of the outgoing photoelectron
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This note was uploaded on 10/27/2010 for the course BI 110 taught by Professor Richards,j during the Winter '08 term at Caltech.

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W8 - Ch24aHandout;lectures1416 ZeNanChang,ChangyiLi (providedbya synchrotron. time Measure

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