X-ray absorption fine structure
Encyclopedia
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 electronic states.
s and attenuates gradually with the X-ray energy. The XAS spectra are usually divided in three energy regions: 1) the edge region, 2) the X-ray Absorption Near Edge Structure (XANES
); 3) the extended X-ray absorption fine structure (EXAFS).
The absorption peaks at the absorption edges in a range of about 5 eV were first explained by Walther Kossel as due to electronic transitions to first unoccupied molecular levels above the chemical potential
, and for many years was referred to as the “Kossel structure” now it is known as absorption edge region. The oscillatory structure extending for hundreds of electron volts past the edges was called the “Kronig structure” after the scientist, Ralph Kronig
, who assigned this structure to the single scattering
of the excited photoelectron by neighbouring atoms. This is now called EXAFS. The energy region of XANES
(extending over a range of about 100 eV) between the edge region and the EXAFS region has been assigned to multiple scattering resonances. In the case of organic molecules this energy region has been later called near-edge X-ray absorption fine structure (NEXAFS), but NEXAFS is synonymous with XANES.
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.
Spectra
The X-ray absorption spectra show a steep rise at the core-level binding energy of X-ray-absorbing atomAtom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s and attenuates gradually with the X-ray energy. The XAS spectra are usually divided in three energy regions: 1) the edge region, 2) the X-ray Absorption Near Edge Structure (XANES
XANES
X-ray Absorption Near Edge Structure , also known as Near edge X-ray absorption fine structure is a type of absorption spectroscopy. NEXAFS also at times used the abbreviation EXAFS....
); 3) the extended X-ray absorption fine structure (EXAFS).
The absorption peaks at the absorption edges in a range of about 5 eV were first explained by Walther Kossel as due to electronic transitions to first unoccupied molecular levels above the chemical potential
Chemical potential
Chemical potential, symbolized by μ, is a measure first described by the American engineer, chemist and mathematical physicist Josiah Willard Gibbs. It is the potential that a substance has to produce in order to alter a system...
, and for many years was referred to as the “Kossel structure” now it is known as absorption edge region. The oscillatory structure extending for hundreds of electron volts past the edges was called the “Kronig structure” after the scientist, Ralph Kronig
Ralph Kronig
Ralph Kronig was a German-American physicist . He is noted for the discovery of particle spin and for his theory of x-ray absorption spectroscopy...
, who assigned this structure to the single scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
of the excited photoelectron by neighbouring atoms. This is now called EXAFS. The energy region of XANES
XANES
X-ray Absorption Near Edge Structure , also known as Near edge X-ray absorption fine structure is a type of absorption spectroscopy. NEXAFS also at times used the abbreviation EXAFS....
(extending over a range of about 100 eV) between the edge region and the EXAFS region has been assigned to multiple scattering resonances. In the case of organic molecules this energy region has been later called near-edge X-ray absorption fine structure (NEXAFS), but NEXAFS is synonymous with XANES.
Edge spectroscopy
X-ray absorption edge spectroscopy corresponds to the transition from a core-level to an unoccupied orbitalAtomic orbital
An atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus...
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
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
peaks in the XANES spectra. The XAFS acronym has been later introduced to indicate the sum of the XANES and EXAFS spectra.
Books
- 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
- X-ray Absorption: principles, applications and techniques of EXAFS, SEXAFS and XANES, a cura di D.C. Koeningsberger, R. Prins, John Wiley & Sons 1988
External Links
- M. Newville, Fundamentals of XAFS
- S. Bare, XANES measurements and interpretation
- B. Ravel, A practical introduction to multiple scattering