| Near Edge X-ray Absorption Fine Structure |
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NEXAFS (Near Edge X-ray Absorption Fine Structure) is an element-specific Electron Spectroscopic technique which is highly sensitive to Bond Angle s, bond lengths and the presence of adsorbates. It is widely used in surface science and has also been used to study Polymer s and magnetic materials. NEXAFS is synonymous with ''' XANES ''' (X-ray Absorption Near Edge Structure) but NEXAFS by convention is usually reserved for soft X-ray spectroscopy (photon energy less than 1000 Electron Volt s). NEXAFS is distinguished from the closely related EXAFS method in that NEXAFS concentrates on fine structure within about 30 EV of the absorption edge while EXAFS considers the ''extended'' spectrum out to much higher electron Kinetic Energies . THEORY
Synchrotron Radiation has a natural Polarization that can be utilized to great advantage in NEXAFS studies. The commonly studied molecular adsorbates have Sigma and Pi Bond s that may have a particular orientation on a surface. The angle dependence of the x-ray absorption tracks the orientation of resonant bonds due to Dipole Selection Rule s. EXPERIMENTAL CONSIDERATIONS NEXAFS spectra are usually measured either through the ''fluorescent yield,'' in which emitted photons are monitored, or ''total electron yield,'' in which the sample is connected to ground through an ammeter and the neutralization current is monitored. Because NEXAFS measurements require an intense tunable source of soft x-rays, they are performed at Synchrotron s like the CLS , ALS , NSLS , ELETTRA , BESSY , ANKA , the Daresbury SRS , SSRL , ESRF , Spring 8 or SOLEIL . Because NEXAFS soft x-rays are absorbed by air, the synchrotron radiation travels from the ring in an evacuated beam-line to the end-station where the specimen to be studied is mounted. Specialized beam-lines intended for NEXAFS studies often have additional capabilities such as heating a sample or exposing it to a dose of reactive gas. SIGNIFICANCE The great power of NEXAFS derives from its elemental specificity. Because the various elements have different core level energies, NEXAFS permits extraction of the signal from a surface monolayer or even a single buried layer in the presence of a huge background signal. Buried layers are very important in engineering applications, such as Magnetic Recording Media buried beneath a surface lubricant or dopants below an electrode in an Integrated Circuit . Because NEXAFS can also determine the chemical state of elements which are present in bulk in minute quantities, it has found widespread use in Environmental Chemistry and Geochemistry . The ability of NEXAFS to study buried atoms is due to its integration over all final states including inelastically scattered electrons, as opposed to photoemission and Auger spectroscopy, which study atoms only with a layer or two of the surface. REFERENCES ''NEXAFS Spectroscopy'' by J. Stöhr, Springer 1992, ISBN 3-540-54422-4. |
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