Extended X-Ray Absorption Fine Structure
Encyclopedia
X-ray Absorption Spectroscopy (XAS) includes both Extended X-Ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES
). XAS is the measurement of the x-ray
absorption coefficient (
in the equations below) of a material as a function of energy. X-rays of a narrow energy resolution are shone on the sample and the incident and transmitted x-ray intensity is recorded as the incident x-ray energy is incremented. The number of x-ray photons that are transmitted through a sample (It) is equal to the number of x-ray photons shone on the sample (I0) multiplied by a decreasing exponential that depends of the type of atoms in the sample, the absorption coefficient 
, and the thickness of the sample 
.
The absorption coefficient is obtained by taking the log ratio of the incident x-ray intensity to the transmitted x-ray intensity.
When the incident x-ray energy matches the binding energy of an electron
of an atom within the sample, the number of x-rays absorbed by the sample increases dramatically, causing a drop in the transmitted x-ray intensity. This results in an absorption edge. Each element on the periodic table has a set of unique absorption edges corresponding to different binding energies of its electrons. This gives XAS element selectivity. XAS spectra are most often collected at synchrotron
s. Because X-rays are highly penetrating, XAS samples can be gases, solids or liquids. And because of the brilliance of Synchrotron X-ray sources the concentration of the absorbing element can be as low as a few ppm.
EXAFS spectra are displayed as graphs of the absorption coefficient of a given material versus energy
, typically in a 500 – 1000 eV range beginning before an absorption edge
of an element in the sample. The x-ray absorption coefficient is usually normalized to unit step height. This is done by regressing a line to the region before and after the absorption edge, subtracting the pre-edge line from the entire data set and dividing by the absorption step height, which is determined by the difference between the pre-edge and post-edge lines at the value of E0 (on the absorption edge).
The normalized absorption spectra are often called XANES
spectra. These spectra can be used to determine the average oxidation state of the element in the sample. The XANES spectra are also sensitive to the coordination environment of the absorbing atom in the sample. Finger printing methods have been used to match the XANES spectra of an unknown sample to those of known "standards". Linear combination fitting of several different standard spectra can give an estimate to the amount of each of the known standard spectra within an unknown sample.
X-ray absorption spectra are produced over the range of 200 – 35,000 eV. The dominant physical process is one where the absorbed photon ejects a core photoelectron from the absorbing atom, leaving behind a core hole. The atom with the core hole is now excited. The ejected photoelectron’s energy will be equal to that of the absorbed photon minus the binding energy
of the initial core state. The ejected photoelectron interacts with electrons in the surrounding non-excited atoms.
If the ejected photoelectron is taken to have a wave
-like nature and the surrounding atoms are described as point scatterers, it is possible to imagine the backscatter
ed electron waves interfering with the forward-propagating waves. The resulting interference pattern shows up as a modulation
of the measured absorption coefficient, thereby causing the oscillation in the EXAFS spectra. A simplified plane-wave single-scattering theory has been used for interpretation of EXAFS spectra for many years, although modern methods (like FEFF, GNXAS) have shown that curved-wave corrections and multiple-scattering effects can not be neglected. The photelectron scattering amplitude in the low energy range (5-200 eV) of the phoelectron kinetic energy become much larger so that multiple scattering events become dominant in the NEXAFS (or XANES
) spectra.
The wavelength
of the photoelectron is dependent on the energy and phase of the backscattered wave which exists at the central atom. The wavelength changes as a function of the energy of the incoming photon. The phase
and amplitude
of the backscattered wave are dependent on the type of atom doing the backscattering and the distance of the backscattering atom from the central atom. The dependence of the scattering on atomic species makes it possible to obtain information pertaining to the chemical coordination environment of the original absorbing (centrally excited) atom by analyzing these EXAFS data.
s, often at beamline
s which are especially optimized for the purpose. The utility of a particular synchrotron to study a particular solid depends on the brightness of the x-ray flux at the absorption edges of the relevant elements.
understanding the details of local structure in:
, a highly sensitive technique with elemental specificity. As such, EXAFS is an extremely useful way to determine the chemical state of practically important species which occur in very low abundance or concentration. Frequent use of EXAFS occurs in environmental chemistry
, where scientists try to understand the propagation of pollutant
s through an ecosystem
. EXAFS can be used along with accelerator mass spectrometry
in forensic examinations, particularly in nuclear
non-proliferation applications.
For an example of an EXAFS study of uranium
chemistry in glass
see http://www.osti.gov/bridge/servlets/purl/459339-8dNh9T/webviewable/459339.pdf, and for a general study of trivalent lanthanide
s and actinide
s in chloride
containing aqueous media can be read at http://www-ssrl.slac.stanford.edu/pubs/activity_rep/ar98/2525-edelstein.pdf
A more modern and accurate account of the history of XAFS (EXAFS and XANES) is given by the leader of the group that developed the modern version of XAFS in the award lecture "Musings about the development of the XAFS" by Edward A. Stern, J.Synchrotron Rad. (2001). 8, 49-54.
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....
). XAS is the measurement of the x-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
absorption coefficient (
in the equations below) of a material as a function of energy. X-rays of a narrow energy resolution are shone on the sample and the incident and transmitted x-ray intensity is recorded as the incident x-ray energy is incremented. The number of x-ray photons that are transmitted through a sample (It) is equal to the number of x-ray photons shone on the sample (I0) multiplied by a decreasing exponential that depends of the type of atoms in the sample, the absorption coefficient , and the thickness of the sample .The absorption coefficient is obtained by taking the log ratio of the incident x-ray intensity to the transmitted x-ray intensity.
When the incident x-ray energy matches the binding energy of an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
of an atom within the sample, the number of x-rays absorbed by the sample increases dramatically, causing a drop in the transmitted x-ray intensity. This results in an absorption edge. Each element on the periodic table has a set of unique absorption edges corresponding to different binding energies of its electrons. This gives XAS element selectivity. XAS spectra are most often collected at synchrotron
Synchrotron
A synchrotron is a particular type of cyclic particle accelerator in which the magnetic field and the electric field are carefully synchronised with the travelling particle beam. The proton synchrotron was originally conceived by Sir Marcus Oliphant...
s. Because X-rays are highly penetrating, XAS samples can be gases, solids or liquids. And because of the brilliance of Synchrotron X-ray sources the concentration of the absorbing element can be as low as a few ppm.
EXAFS spectra are displayed as graphs of the absorption coefficient of a given material versus energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
, typically in a 500 – 1000 eV range beginning before an absorption edge
Absorption edge
An absorption edge is a sharp discontinuity in the absorption spectrum of a material. These discontinuities occur at wavelengths at which the energy of an absorbed photon corresponds to an electronic transition or ionisation potential....
of an element in the sample. The x-ray absorption coefficient is usually normalized to unit step height. This is done by regressing a line to the region before and after the absorption edge, subtracting the pre-edge line from the entire data set and dividing by the absorption step height, which is determined by the difference between the pre-edge and post-edge lines at the value of E0 (on the absorption edge).
The normalized absorption spectra are often called 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....
spectra. These spectra can be used to determine the average oxidation state of the element in the sample. The XANES spectra are also sensitive to the coordination environment of the absorbing atom in the sample. Finger printing methods have been used to match the XANES spectra of an unknown sample to those of known "standards". Linear combination fitting of several different standard spectra can give an estimate to the amount of each of the known standard spectra within an unknown sample.
X-ray absorption spectra are produced over the range of 200 – 35,000 eV. The dominant physical process is one where the absorbed photon ejects a core photoelectron from the absorbing atom, leaving behind a core hole. The atom with the core hole is now excited. The ejected photoelectron’s energy will be equal to that of the absorbed photon minus the binding energy
Binding energy
Binding energy is the mechanical energy required to disassemble a whole into separate parts. A bound system typically has a lower potential energy than its constituent parts; this is what keeps the system together—often this means that energy is released upon the creation of a bound state...
of the initial core state. The ejected photoelectron interacts with electrons in the surrounding non-excited atoms.
If the ejected photoelectron is taken to have a wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
-like nature and the surrounding atoms are described as point scatterers, it is possible to imagine the backscatter
Backscatter
In physics, backscatter is the reflection of waves, particles, or signals back to the direction they came from. It is a diffuse reflection due to scattering, as opposed to specular reflection like a mirror...
ed electron waves interfering with the forward-propagating waves. The resulting interference pattern shows up as a modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
of the measured absorption coefficient, thereby causing the oscillation in the EXAFS spectra. A simplified plane-wave single-scattering theory has been used for interpretation of EXAFS spectra for many years, although modern methods (like FEFF, GNXAS) have shown that curved-wave corrections and multiple-scattering effects can not be neglected. The photelectron scattering amplitude in the low energy range (5-200 eV) of the phoelectron kinetic energy become much larger so that multiple scattering events become dominant in the NEXAFS (or 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....
) spectra.
The wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the photoelectron is dependent on the energy and phase of the backscattered wave which exists at the central atom. The wavelength changes as a function of the energy of the incoming photon. The phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
and amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
of the backscattered wave are dependent on the type of atom doing the backscattering and the distance of the backscattering atom from the central atom. The dependence of the scattering on atomic species makes it possible to obtain information pertaining to the chemical coordination environment of the original absorbing (centrally excited) atom by analyzing these EXAFS data.
Experimental considerations
Since EXAFS requires a tunable x-ray source, data are always collected at synchrotronSynchrotron
A synchrotron is a particular type of cyclic particle accelerator in which the magnetic field and the electric field are carefully synchronised with the travelling particle beam. The proton synchrotron was originally conceived by Sir Marcus Oliphant...
s, often at beamline
Beamline
In particle physics, a beamline is the line in a linear accelerator along which a beam of particles travels. It may also refer to the line of travel within a bending section such as a storage ring or cyclotron, or an external beam extracted from a cyclic accelerator.In materials science, physics,...
s which are especially optimized for the purpose. The utility of a particular synchrotron to study a particular solid depends on the brightness of the x-ray flux at the absorption edges of the relevant elements.
Applications
XAS is an interdisciplinary technique and its unique properties, as compared to x-ray diffraction, have been exploited forunderstanding the details of local structure in:
- glassGlassGlass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
, amorphous and liquidLiquidLiquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
systems - solid solutions
- Doping and ionic implantation materials for electronicsElectronicsElectronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
- local distortions of crystal lattices
- organometallic compoundsOrganometallic chemistryOrganometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...
- metalloproteins
- metal clusters
- vibrational dynamics
- ions in solutions
- speciationSpeciationSpeciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
of elements
Example of Significance
EXAFS is, like NEXAFS/XANESXANES
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....
, a highly sensitive technique with elemental specificity. As such, EXAFS is an extremely useful way to determine the chemical state of practically important species which occur in very low abundance or concentration. Frequent use of EXAFS occurs in environmental chemistry
Environmental chemistry
Environmental chemistry is the scientific study of the chemical and biochemical phenomena that occur in natural places. It should not be confused with green chemistry, which seeks to reduce potential pollution at its source...
, where scientists try to understand the propagation of pollutant
Pollutant
A pollutant is a waste material that pollutes air, water or soil, and is the cause of pollution.Three factors determine the severity of a pollutant: its chemical nature, its concentration and its persistence. Some pollutants are biodegradable and therefore will not persist in the environment in the...
s through an ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
. EXAFS can be used along with accelerator mass spectrometry
Accelerator mass spectrometry
Accelerator mass spectrometry differs from other forms of mass spectrometry in that it accelerates ions to extraordinarily high kinetic energies before mass analysis. The special strength of AMS among the mass spectrometric methods is its power to separate a rare isotope from an abundant...
in forensic examinations, particularly in nuclear
Nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...
non-proliferation applications.
For an example of an EXAFS study of uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
chemistry in glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
see http://www.osti.gov/bridge/servlets/purl/459339-8dNh9T/webviewable/459339.pdf, and for a general study of trivalent lanthanide
Lanthanide
The lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...
s and actinide
Actinide
The actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.The actinide series derives its name from the group 3 element actinium...
s in chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
containing aqueous media can be read at http://www-ssrl.slac.stanford.edu/pubs/activity_rep/ar98/2525-edelstein.pdf
History
A very detailed, balanced and informative account about the history of EXAFS (originally called Kossel's structures) is given in the paper "A History of the X-ray Absorption Fine Structure} by R. Stumm von Bordwehr", Ann. Phys. Fr. vol. 14, 377-466 (1989) (author's name is C. Brouder).A more modern and accurate account of the history of XAFS (EXAFS and XANES) is given by the leader of the group that developed the modern version of XAFS in the award lecture "Musings about the development of the XAFS" by Edward A. Stern, J.Synchrotron Rad. (2001). 8, 49-54.
Relevant Websites
- International XAFS Society
- FEFF Project, University of Washington, Seattle
- GNXAS project and XAS laboratory, Università di Camerino
- EXAFS theory Introduction
- Community web site for XAFS
Books
- G. Bunker, "Introduction to XAFS: A Practical Guide to X-ray Absorption Fine Structure Spectroscopy", Cambridge University Press, 2010
- 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, Wiley 1988
Book Chapters
- Kelly, S.D., Hesterberg, D., and Ravel, B., Analysis of Soils and Minerals Using X-ray Absorption Spectroscopy in Methods of Soil Analysis, Part 5 -Mineralogical Methods, (A.L. Ulery and L.R. Drees, Eds.) p. 367. Soil Science Society of America, Madison, WI, USA, 2008.
Papers
- Edward A. Stern, "Musing about the development of XAFS", J. Synchrotron Rad. (2001). 8, 49-54
- J.J. Rehr and R.C. Albers, "Theoretical approaches to X-ray absorption fine structure", Reviews of Modern PhysicsReviews of Modern PhysicsThe Reviews of Modern Physics is a journal of the American Physical Society. The journal started in paper form. All volumes are also online by subscription.Issue 1, Volume 1 consisted of the review by...
72 (2000), 621-654 - A. Filipponi, A. Di Cicco and C.R. Natoli, "X-ray absorption spectroscopy and n-body distribution functions in condensed matter", Physical ReviewPhysical ReviewPhysical Review is an American scientific journal founded in 1893 by Edward Nichols. It publishes original research and scientific and literature reviews on all aspects of physics. It is published by the American Physical Society. The journal is in its third series, and is split in several...
B 52/21 (1995) 15122-15148 - F. de Groot, "High-resolution X-ray emission and X-ray absorption spectroscopy", Chemical ReviewsChemical ReviewsChemical Reviews , is a peer-reviewed scientific journal, published since 1924 by the American Chemical Society. As the name indicates, it publishes comprehensive, critical reviews of an area rather than original research...
101 (2001) 1779-1808 - F.W. Lytle, "The EXAFS family tree: a personal history of the development of extended X-ray absorption fine structure",
- Dale E. Sayers, Edward A. Stern, and Farrel W. Lytle, New Technique for Investigating Noncrystalline Structures: Fourier Analysis of the Extended X-Ray—Absorption Fine Structure, Phys. Rev. Lett. 27, 1204–1207 (1971).
- A. Kodre, I. Arčon, Proceedings of 36th International Conference on Microelectronics, Devices and Materials, MIDEM, Postojna, Slovenia, October 28-20, (2000), p. 191-196