Editing X-ray absorption fine structure (section)

=== X-ray Absorption Near Edge Structure ===

The XANES energy region<ref name="xanes1">D. C. Koningsberger, R. Prins (eds) A. Bianconi "X-ray absorption: Principles, applications, techniques of EXAFS, SEXAFS and XANES" (Chemical Analysis 92), Wiley, New York (1988)  pp 573-662 {{ISBN|978-0-471875475}}</ref>  extends between the '''edge region''' and the '''EXAFS region''' over a 50-100 eV energy range around the core level x-ray absorption threshold.
Before 1980 the XANES region was wrongly assigned to different final states: a) unoccupied total density of states, or b) unoccupied molecular orbitals (kossel structure) or c) unoccupied atomic orbitals or d) low energy EXAFS oscillations.
In the seventies, using synchrotron radiation in Frascati and Stanford synchrotron sources, it was experimentally shown that the features in this energy region are due to multiple scattering [[resonances]] of the photoelectron in a nanocluster of variable size. Antonio Bianconi in 1980 invented the acronym XANES to indicate the spectral region dominated by multiple scattering [[resonances]] of the photoelectron in the soft x-ray range<ref name="xanes2">{{cite journal | last=Bianconi | first=Antonio | title=Surface X-ray absorption spectroscopy: Surface EXAFS and surface XANES | journal=Applications of Surface Science | publisher=Elsevier BV | volume=6 | issue=3–4 | year=1980 | issn=0378-5963 | doi=10.1016/0378-5963(80)90024-0 | pages=392–418}}</ref>
and in the hard X-ray range.<ref name="xanes3">{{cite journal | last1=Belli | first1=M. | last2=Scafati | first2=A. | last3=Bianconi | first3=A. | last4=Mobilio | first4=S. | last5=Palladino | first5=L. | last6=Reale | first6=A. | last7=Burattini | first7=E. | title=X-ray absorption near edge structures (XANES) in simple and complex Mn compounds | journal=Solid State Communications | publisher=Elsevier BV | volume=35 | issue=4 | year=1980 | issn=0038-1098 | doi=10.1016/0038-1098(80)90515-3 | pages=355–361}}</ref>
In the XANES energy range the kinetic energy of the photoelectron in the final state is between few eV and 50-100 eV. In this regime the photoelectron has a strong scattering amplitude by neighboring atoms in molecules and condensed matter, its wavelength is larger than interatomic distances, its mean free path could be smaller than one nanometer and finally the lifetime of the excited state is in the order of femtoseconds.
The XANES spectral features are described by full multiple scattering theory proposed in the early seventies.<ref name="msr">{{cite journal | last1=Dehmer | first1=J. L. | last2=Dill | first2=Dan | title=Shape Resonances in K-Shell Photoionization of Diatomic Molecules | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=35 | issue=4 | date=1975-07-28 | issn=0031-9007 | doi=10.1103/physrevlett.35.213 | pages=213–215}}</ref>
Therefore, the key step for XANES interpretation is the determination of the size of the atomic cluster of neighbor atoms, where the final states are confined, which could range from 0.2&nbsp;nm to 2&nbsp;nm in different systems.
This energy region has been called later (in 1982) also near-edge X-ray absorption fine structure ([[NEXAFS]]), which is synonymous with XANES.
During more than 20 years the XANES interpretation has been object of discussion but recently there is agreement that the final states are  "multiple scattering resonances" and many body final states play an important role.<ref name="msr2">{{cite journal | last1=Rehr | first1=J.J. | last2=Ankudinov | first2=A.L. | title=Progress in the theory and interpretation of XANES | journal=Coordination Chemistry Reviews | publisher=Elsevier BV | volume=249 | issue=1–2 | year=2005 | issn=0010-8545 | doi=10.1016/j.ccr.2004.02.014 | pages=131–140}}</ref>