X-ray absorption fine structure
X-ray absorption fine structure (XAFS) is a specific structure observed in X-ray absorption spectroscopy (XAS). By analyzing the XAFS, information can be acquired on the local structure and on the unoccupied local electronic states.
Atomic spectra
The atomic X-ray absorption spectrum (XAS) of a
Spectra of molecules and condensed matter
The XAS spectra of condensed matter are usually divided in three energy regions:
Edge region
The edge region usually extends in a range of few eV around the absorption edge. The spectral features in the edge region i) in good metals are excitations to final delocalized states above the Fermi level; ii) in insulators are core excitons below the ionization potential; iii) in molecules are electronic transitions to the first unoccupied molecular levels above the chemical potential in the initial states which are shifted into the discrete part of the core absorption spectrum by the Coulomb interaction with the core hole. Multi-electron excitations and configuration interaction between many body final states dominate the edge region in strongly correlated metals and insulators. For many years the edge region was referred to as the “Kossel structure” but now it is known as "absorption edge region" since the Kossel structure refers only to unoccupied molecular final states which is a correct description only for few particular cases: molecules and strongly disordered systems.
X-ray Absorption Near Edge Structure
The XANES energy region[3] 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
Intermediate region
There is an intermediate region between the XANES and EXAFS regions where low n-body distribution functions play a key role.[8][9][10]
Extended X-ray absorption fine structure
The oscillatory structure extending for hundreds of
This regime was calledApplications of x-ray absorption spectroscopy
X-ray absorption edge spectroscopy corresponds to the transition from a core-level to an unoccupied orbital or band and mainly reflects the electronic unoccupied states. EXAFS, resulting from the interference in the single scattering process of the photoelectron scattered by surrounding atoms, provides information on the local structure. Information on the geometry of the local structure is provided by the analysis of the multiple scattering peaks in the XANES spectra. The XAFS acronym has been later introduced to indicate the sum of the XANES and EXAFS spectra.
See also
- SEXAFS
- EXAFS
- XANES
References
- S2CID 119640917.
- ISSN 0034-6861.
- ISBN 978-0-471875475
- ISSN 0378-5963.
- ISSN 0038-1098.
- ISSN 0031-9007.
- ISSN 0010-8545.
- PMID 9940417.
- PMID 9980866.
- PMID 9980867.
- ISSN 0034-6861.
- S2CID 44020569.
- ^ X-ray Absorption: principles, applications and techniques of EXAFS, SEXAFS and XANES, edited by D.C. Koeningsberger, R. Prins, John Wiley & Sons 1988.
- ^ Principles and Applications of EXAFS, Chapter 10 in Handbook of Synchrotron Radiation, pp 995–1014. E. A. Stern and S. M. Heald, E. E. Koch, ed., North-Holland, 1983.
- ^ B.-K. Teo, EXAFS: basic principles and data analysis, Springer 1986
- ISSN 0031-9007.
- PMID 15263225.
External links
- M. Newville, Fundamentals of XAFS
- S. Bare, XANES measurements and interpretation
- B. Ravel, A practical introduction to multiple scattering