Project 2: Probing Molecular Geometry by Means of XANES Spectroscopy

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Project 2: Probing molecular geometry by means of XANES spectroscopy X-Ray Absorption Spectroscopy (XAS) is a powerful technique that allows to study with high accuracy the chemical environment of a selected atom, since it yields a spectrum that depends intimately on the geometry of the atoms in the surrounding of the photoabsorber. When an electron is supplied with an energy at least equal to its binding energy, it will be expelled with a residual kinetic energy EK=Eλ-E0, and it will be scattered by the surrounding atoms and finally re-scattered back to its parent ion. The backscattered waves will interfere with the outgoing waves, yielding an oscillating spectrum that strongly depends on the position of the back-scatterers. In particular, the low-energy region of the spectrum, called XANES (X-ray Absorption Near-Edge Structure), is highly sensitive to the coordination number and geometry of the atoms around the photoabsorber, while the high-energy region, called EXAFS (Extended X-ray Absorption Fine Structure) is highly sensitive to the bond distances, with an accuracy of hundredths of ångström. In this project, we will determine the coordination geometry of a metallic complex in solution by analyzing the XANES region of its X-ray absorption spectrum. The analysis is a multiparametric minimization of the residual function between the experimental spectrum and the theoretical one, calculated on a suggested model. You will need to handle all the parameters involved in the calculation of the theoretical spectrum, and this will help you to understand the very basics of this spectroscopic technique. Eventually, you will be asked to write a report, in which you will have to i) explain briefly the main concepts behind the XANES spectroscopy and ii) show your results and explain the physical meaning of the parameters you fitted..

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