XAS
Encyclopedia
X-ray absorption spectroscopy
(XAS) is a widely-used technique for determining the local geometric and/or electronic structure of matter. The experiment is usually performed at synchrotron radiation
sources, which provide intense and tunable X-ray
beams. Samples can be in the gas-phase, solution, or condensed matter (ie. solids).
XAS data are obtained by tuning the photon
energy using a crystalline monochromator to a range where core electrons can be excited
(0.1-100 keV photon energy). The "name" of the edge depends upon the core electron which is excited: the principal quantum numbers
n=1, 2, and 3, correspond to the K-, L-, and M-edges, respectively. For instance, excitation of a 1s electron occurs at the K-edge
, while excitation of a 2s or 2p electron occurs at an L-edge
(Figure 1).
There are three main regions found on a spectrum generated by XAS data (Figure 2). The dominant feature is called the "rising edge", and is sometimes referred to as XANES
(X-ray Absorption Near-Edge Structure) or NEXAFS (Near-edge X-ray Absorption Fine Structure). The pre-edge region is at energies lower than the rising edge. The EXAFS (Extended X-ray Absorption Fine Structure) region is at energies above the rising edge, and corresponds to the scattering of the ejected photoelectron off neighboring atoms. The combination of XANES
and EXAFS is referred to as XAFS.
Since XAS is a type of absorption spectroscopy
, it follows the same quantum mechanical selection rules. The most intense features are due to electric-dipole allowed transitions (ie. Δ l = ± 1) to unfilled orbitals. For example, the most intense features of a K-edge are due to 1s → np transitions, while the most intense features of the L3-edge are due to 2p → nd transitions.
XAS methodology can be broadly divided into four experimental categories that can give complementary results to each other: Metal K-edge
, metal L-edge
, ligand K-edge
, and EXAFS.
and condensed matter physics
, materials science and engineering, chemistry
, earth science
, and biology
. In particular, its unique sensitivity to the local structure, as compared to x-ray diffraction, have been exploited for studying:
Absorption spectroscopy
Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a...
(XAS) is a widely-used technique for determining the local geometric and/or electronic structure of matter. The experiment is usually performed at synchrotron radiation
Synchrotron radiation
The electromagnetic radiation emitted when charged particles are accelerated radially is called synchrotron radiation. It is produced in synchrotrons using bending magnets, undulators and/or wigglers...
sources, which provide intense and tunable 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...
beams. Samples can be in the gas-phase, solution, or condensed matter (ie. solids).
XAS data are obtained by tuning the photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
energy using a crystalline monochromator to a range where core electrons can be excited
(0.1-100 keV photon energy). The "name" of the edge depends upon the core electron which is excited: the principal quantum numbers
Principal quantum number
In atomic physics, the principal quantum symbolized as n is the firstof a set of quantum numbers of an atomic orbital. The principal quantum number can only have positive integer values...
n=1, 2, and 3, correspond to the K-, L-, and M-edges, respectively. For instance, excitation of a 1s electron occurs at the K-edge
Metal K-edge
Metal K-edge is the excitation of a 1s electron to valence bound states localized on the metal. The K-edge can be divided into the pre-edge region, which comprises the pre-edge and rising edge transitions and the near-edge region, which comprises the intense edge transition and ~150 eV above...
, while excitation of a 2s or 2p electron occurs at an L-edge
Metal L-edge
Metal L-edge XAS is an experimental technique that involves the excitation of a metal 2p electron to unfilled metal d orbitals...
(Figure 1).
There are three main regions found on a spectrum generated by XAS data (Figure 2). The dominant feature is called the "rising edge", and is sometimes referred to as 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....
(X-ray Absorption Near-Edge Structure) or NEXAFS (Near-edge X-ray Absorption Fine Structure). The pre-edge region is at energies lower than the rising edge. The EXAFS (Extended X-ray Absorption Fine Structure) region is at energies above the rising edge, and corresponds to the scattering of the ejected photoelectron off neighboring atoms. The combination 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....
and EXAFS is referred to as XAFS.
Since XAS is a type of absorption spectroscopy
Absorption spectroscopy
Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a...
, it follows the same quantum mechanical selection rules. The most intense features are due to electric-dipole allowed transitions (ie. Δ l = ± 1) to unfilled orbitals. For example, the most intense features of a K-edge are due to 1s → np transitions, while the most intense features of the L3-edge are due to 2p → nd transitions.
XAS methodology can be broadly divided into four experimental categories that can give complementary results to each other: Metal K-edge
Metal K-edge
Metal K-edge is the excitation of a 1s electron to valence bound states localized on the metal. The K-edge can be divided into the pre-edge region, which comprises the pre-edge and rising edge transitions and the near-edge region, which comprises the intense edge transition and ~150 eV above...
, metal L-edge
Metal L-edge
Metal L-edge XAS is an experimental technique that involves the excitation of a metal 2p electron to unfilled metal d orbitals...
, ligand K-edge
Ligand K-edge
Ligand K-edge XAS is a spectroscopic technique that allows the direct study of metal-ligand bonding. In a XAS experiment, electrons in ligand 1s orbitals are excited to unfilled p Ligand K-edge XAS is a spectroscopic technique that allows the direct study of metal-ligand bonding. In a XAS...
, and EXAFS.
Applications
XAS is an experimental technique used in different scientific fields including molecularMolecular physics
Molecular physics is the study of the physical properties of molecules, the chemical bonds between atoms as well as the molecular dynamics. Its most important experimental techniques are the various types of spectroscopy...
and condensed matter physics
Condensed matter physics
Condensed matter physics deals with the physical properties of condensed phases of matter. These properties appear when a number of atoms at the supramolecular and macromolecular scale interact strongly and adhere to each other or are otherwise highly concentrated in a system. The most familiar...
, materials science and engineering, chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, earth science
Earth science
Earth science is an all-embracing term for the sciences related to the planet Earth. It is arguably a special case in planetary science, the Earth being the only known life-bearing planet. There are both reductionist and holistic approaches to Earth sciences...
, and biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
. In particular, its unique sensitivity to the local structure, as compared to x-ray diffraction, have been exploited for studying:
- Amorphous solidAmorphous solidIn condensed matter physics, an amorphous or non-crystalline solid is a solid that lacks the long-range order characteristic of a crystal....
s 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 solutionSolid solutionA solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...
s - DopingDoping (semiconductor)In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...
and ion implantationIon implantationIon implantation is a materials engineering process by which ions of a material are accelerated in an electrical field and impacted into another solid. This process is used to change the physical, chemical, or electrical properties of the solid...
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 latticesCrystal structureIn mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
- 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...
- MetalloproteinsProteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
- Metal clustersCluster chemistryIn chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. Clusters exist of diverse stoichiometries and nuclearities. For example, carbon and boron atoms form fullerene and borane clusters, respectively. Transition metals and main group...
- CatalysisCatalysisCatalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
- Vibrational dynamics
- IonIonAn ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s in solutionSolutionIn chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
s - Speciation of elements
- Liquid water and aqueous solutions