XANES
Encyclopedia
X-ray Absorption Near Edge Structure (XANES), also known as Near edge X-ray absorption fine structure (NEXAFS) is a type of absorption spectroscopy
. NEXAFS also at times used the abbreviation EXAFS.
XANES data indicate the absorption peaks due to the photoabsorption cross section
in the X-ray Absorption Spectra (XAS
) observed in the energy region. Here the XANES spectroscopy and its applications in these last 30 years is described.
The fundamental phenomenon underlying NEXAFS is the absorption of an x-ray photon by a core level of an atom in a solid and the consequent emission of a photoelectron (see the first Figure). The resulting core hole is filled either via an Auger
process or by capture of an electron from another shell followed by emission of a fluorescent
photon. The difference between NEXAFS and traditional photoemission experiments is that in photoemission, the initial photoelectron itself is measured, while in NEXAFS the fluorescent photon or Auger electron or an inelastically scattered photoelectron may also be measured. The distinction sounds trivial but is actually significant: in photoemission the final state of the emitted electron captured in the detector must be an extended, free-electron state. By contrast in NEXAFS the final state of the photoelectron may be a bound state such as an exciton
since the photoelectron itself need not be detected. The effect of measuring fluorescent photons, Auger electrons, and directly emitted electrons is to sum over all possible final states of the photoelectrons, meaning that what NEXAFS measures is the total joint density of states of the initial core level with all final states, consistent with conservation rules. The distinction is critical because in spectroscopy final states are more susceptible to many-body
effects than initial states, meaning that NEXAFS spectra are more easily calculable than photoemission spectra. Due to the summation over final states, various sum rule
s are helpful in the interpretation of NEXAFS spectra. When the x-ray photon energy resonantly connects a core level with a narrow final state in a solid, such as an exciton, readily identifiable characteristic peaks will appear in the spectrum. These narrow characteristic spectral peaks give the NEXAFS technique a lot of its analytical power as illustrated by the B 1s π* exciton shown in the second Figure.
Synchrotron radiation
has a natural polarization that can be utilized to great advantage in NEXAFS studies. The commonly studied molecular adsorbates have sigma
and pi bond
s that may have a particular orientation on a surface. The angle dependence of the x-ray absorption tracks the orientation of resonant bonds due to dipole
selection rule
s.
NEXAFS spectra are usually measured either through the fluorescent yield, in which emitted photons are monitored, or total electron yield, in which the sample is connected to ground through an ammeter and the neutralization current is monitored. Because NEXAFS measurements require an intense tunable source of soft x-rays, they are performed at synchrotron
s. Because NEXAFS soft x-rays are absorbed by air, the synchrotron radiation travels from the ring in an evacuated beam-line to the end-station where the specimen to be studied is mounted. Specialized beam-lines intended for NEXAFS studies often have additional capabilities such as heating a sample or exposing it to a dose of reactive gas.
. Therefore its mean free path
in a pure single crystal at zero temperature is as large as infinite, and it remains very large, increasing the energy of the final state up to about 5 eV above the Fermi level. Beyond the role of the unoccupied density of states
and matrix elements in single electron excitations, many-body effects appear as an "infrared singularity" at the absorption threshold in metals.
In the absorption edge region of insulators the photoelectron is excited to the first unoccupied level above the chemical potential
but the unscreened core hole forms a localized bound state called core exciton
.
In fact the final state of the excited photoelectron in the high kinetic energy range (150-2000 eV ) is determined only by single backscattering events due to the low amplitude photoelectron scattering.
events become dominant in the XANES spectra.
The different energy range between XANES and EXAFS can be also explained in a very simple manner by the comparison between the photoelectron wavelength
and the interatomic distance of the photoabsorber-backscatterer pair. The photoelectron kinetic energy is connected with the wavelength 
by the following relation:
that means that for high energy the wavelength is shorter than interatomic distances and hence the EXAFS region corresponds to a single scattering regime; while for lower E,
is larger than interatomic distances and the XANES region is associated with a multiple scattering
regime.
s of the photoelectron excited at the atomic absorption site and scattered by neighbor atoms.
The local character of the final states is determined by the short photoelectron mean free path
, that is strongly reduced (down to about 0.3 nm at 50 eV) in this energy range because of inelastic scattering
of the photoelectron by electron-hole excitations (excitons) and collective electronic oscillations of the valence electrons called plasmons.
buried beneath a surface lubricant or dopants below an electrode in an integrated circuit
. Because NEXAFS can also determine the chemical state of elements which are present in bulk in minute quantities, it has found widespread use in environmental chemistry
and geochemistry
. The ability of NEXAFS to study buried atoms is due to its integration over all final states including inelastically scattered electrons, as opposed to photoemission and Auger spectroscopy, which study atoms only with a layer or two of the surface.
Much chemical information can be extracted from the XANES region: formal valence
(very difficult to experimentally determine in a nondestructive way); coordination environment (e.g., octahedral, tetrahedral coordination) and subtle geometrical distortions of it.
Transitions to bound vacant states just above the Fermi level
can be seen. Thus XANES spectra can be used as a probe of the unoccupied band structure of a material.
The near-edge structure is characteristic of an environment and valence state hence one of its more common uses is in fingerprinting: if you have a mixture of sites/compounds in a sample you can fit the measured spectra with a linear combinations of XANES spectra of known species and determine the proportion of each site/compound in the sample. One example of such a use is the determination of the oxidation state
of the plutonium
in the soil
at Rocky Flats
.
(SSRL) by A. Bianconi. In 1982 the first paper on the application of XANES for determination of local structural geometrical distortions using multiple scattering theory was published by A. Bianconi, P. J. Durham and J. B. Pendry. In 1983 the first NEXAFS paper examining molecules absorbed on surfaces appeared. The first XAFS paper, describing the intermediate region between EXAFS and XANES, appeared in 1987.
Calculation of XANES using finite difference method and full multiple scattering theory.
FEFF8
Calculation of XANES using full multiple scattering theory.
MXAN XANES fitting using full multiple scattering theory.
FitIt
XANES fitting using multidimensional interpolation approximation.
PARATEC
XANES calculation using plane-wave pseudopotential approach
WIEN2k
XANES calculation on the basis of full-potential (linearized) augmented plane-wave approach.
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...
. NEXAFS also at times used the abbreviation EXAFS.
XANES data indicate the absorption peaks due to the photoabsorption cross section
Cross section (physics)
A cross section is the effective area which governs the probability of some scattering or absorption event. Together with particle density and path length, it can be used to predict the total scattering probability via the Beer-Lambert law....
in the X-ray Absorption Spectra (XAS
XAS
X-ray absorption spectroscopy 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...
) observed in the energy region. Here the XANES spectroscopy and its applications in these last 30 years is described.
Terminology
Both XANES and NEXAFS are acceptable terms for the same technique. The difference in usage is the energy range above the absorption edge. NEXAFS is synonymous with XANES, but NEXAFS by convention is usually reserved for soft X-ray spectroscopy (photon energy less than 1000 electron volts). NEXAFS is generally used when applied to surface and molecular science, while XANES is used in most other fields.Theory
Auger electron spectroscopy
Auger electron spectroscopy is a common analytical technique used specifically in the study of surfaces and, more generally, in the area of materials science...
process or by capture of an electron from another shell followed by emission of a fluorescent
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
photon. The difference between NEXAFS and traditional photoemission experiments is that in photoemission, the initial photoelectron itself is measured, while in NEXAFS the fluorescent photon or Auger electron or an inelastically scattered photoelectron may also be measured. The distinction sounds trivial but is actually significant: in photoemission the final state of the emitted electron captured in the detector must be an extended, free-electron state. By contrast in NEXAFS the final state of the photoelectron may be a bound state such as an exciton
Exciton
An exciton is a bound state of an electron and hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids...
since the photoelectron itself need not be detected. The effect of measuring fluorescent photons, Auger electrons, and directly emitted electrons is to sum over all possible final states of the photoelectrons, meaning that what NEXAFS measures is the total joint density of states of the initial core level with all final states, consistent with conservation rules. The distinction is critical because in spectroscopy final states are more susceptible to many-body
Many-body theory
The many-body theory is an area of physics which provides the framework for understanding the collective behavior of vast assemblies of interacting particles. In general terms, the many-body theory deals with effects that manifest themselves only in systems containing large numbers of constituents...
effects than initial states, meaning that NEXAFS spectra are more easily calculable than photoemission spectra. Due to the summation over final states, various sum rule
Sum rule
Sum rule may refer to:*Sum rule in differentiation*Sum rule in integration*Rule of sum, a counting principle in combinatorics*Sum rule in quantum mechanics...
s are helpful in the interpretation of NEXAFS spectra. When the x-ray photon energy resonantly connects a core level with a narrow final state in a solid, such as an exciton, readily identifiable characteristic peaks will appear in the spectrum. These narrow characteristic spectral peaks give the NEXAFS technique a lot of its analytical power as illustrated by the B 1s π* exciton shown in the second Figure.
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...
has a natural polarization that can be utilized to great advantage in NEXAFS studies. The commonly studied molecular adsorbates have sigma
Sigma bond
In chemistry, sigma bonds are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is...
and pi bond
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...
s that may have a particular orientation on a surface. The angle dependence of the x-ray absorption tracks the orientation of resonant bonds due to dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
selection rule
Selection rule
In physics and chemistry a selection rule, or transition rule, formally constrains the possible transitions of a system from one state to another. Selection rules have been derived for electronic, vibrational, and rotational transitions...
s.
Experimental considerations
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 NEXAFS soft x-rays are absorbed by air, the synchrotron radiation travels from the ring in an evacuated beam-line to the end-station where the specimen to be studied is mounted. Specialized beam-lines intended for NEXAFS studies often have additional capabilities such as heating a sample or exposing it to a dose of reactive gas.
Edge energy range
In the absorption edge region of metals, the photoelectron is excited to the first unoccupied level above the Fermi levelFermi level
The Fermi level is a hypothetical level of potential energy for an electron inside a crystalline solid. Occupying such a level would give an electron a potential energy \epsilon equal to its chemical potential \mu as they both appear in the Fermi-Dirac distribution function,which...
. Therefore its mean free path
Mean free path
In physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.-Derivation:...
in a pure single crystal at zero temperature is as large as infinite, and it remains very large, increasing the energy of the final state up to about 5 eV above the Fermi level. Beyond the role of the unoccupied density of states
Density of states
In solid-state and condensed matter physics, the density of states of a system describes the number of states per interval of energy at each energy level that are available to be occupied by electrons. Unlike isolated systems, like atoms or molecules in gas phase, the density distributions are not...
and matrix elements in single electron excitations, many-body effects appear as an "infrared singularity" at the absorption threshold in metals.
In the absorption edge region of insulators the photoelectron is excited to the first unoccupied level 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...
but the unscreened core hole forms a localized bound state called core exciton
Exciton
An exciton is a bound state of an electron and hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids...
.
EXAFS energy range
The fine structure in the x-ray absorption spectra in the high energy range extending from about 150 eV beyond the ionization potential is a powerful tool to determine the atomic pair distribution (i.e. interatomic distances) with a time scale of about 10−15 s.In fact the final state of the excited photoelectron in the high kinetic energy range (150-2000 eV ) is determined only by single backscattering events due to the low amplitude photoelectron scattering.
XANES energy range
In the XANES region, starting about 5 eV beyond the absorption threshold, because of the low kinetic energy range (5-150 eV) the photoelectron backscattering amplitude by neighbor atoms is very large so that multiple scatteringScattering
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...
events become dominant in the XANES spectra.
The different energy range between XANES and EXAFS can be also explained in a very simple manner by the comparison between the photoelectron 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...
and the interatomic distance of the photoabsorber-backscatterer pair. The photoelectron kinetic energy is connected with the wavelength by the following relation:that means that for high energy the wavelength is shorter than interatomic distances and hence the EXAFS region corresponds to a single scattering regime; while for lower E,
is larger than interatomic distances and the XANES region is associated with a multiple scatteringScattering
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...
regime.
Final States
The absorption peaks of XANES spectra are determined by multiple scattering resonanceResonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
s of the photoelectron excited at the atomic absorption site and scattered by neighbor atoms.
The local character of the final states is determined by the short photoelectron mean free path
Mean free path
In physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.-Derivation:...
, that is strongly reduced (down to about 0.3 nm at 50 eV) in this energy range because of inelastic scattering
Inelastic scattering
In particle physics and chemistry, inelastic scattering is a fundamental scattering process in which the kinetic energy of an incident particle is not conserved . In an inelastic scattering process, some of the energy of the incident particle is lost or gained...
of the photoelectron by electron-hole excitations (excitons) and collective electronic oscillations of the valence electrons called plasmons.
Applications
The great power of NEXAFS derives from its elemental specificity. Because the various elements have different core level energies, NEXAFS permits extraction of the signal from a surface monolayer or even a single buried layer in the presence of a huge background signal. Buried layers are very important in engineering applications, such as magnetic recording mediaHard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
buried beneath a surface lubricant or dopants below an electrode in an integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
. Because NEXAFS can also determine the chemical state of elements which are present in bulk in minute quantities, it has found widespread use 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...
and geochemistry
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...
. The ability of NEXAFS to study buried atoms is due to its integration over all final states including inelastically scattered electrons, as opposed to photoemission and Auger spectroscopy, which study atoms only with a layer or two of the surface.
Much chemical information can be extracted from the XANES region: formal valence
Valence (chemistry)
In chemistry, valence, also known as valency or valence number, is a measure of the number of bonds formed by an atom of a given element. "Valence" can be defined as the number of valence bonds...
(very difficult to experimentally determine in a nondestructive way); coordination environment (e.g., octahedral, tetrahedral coordination) and subtle geometrical distortions of it.
Transitions to bound vacant states just above the Fermi level
Fermi level
The Fermi level is a hypothetical level of potential energy for an electron inside a crystalline solid. Occupying such a level would give an electron a potential energy \epsilon equal to its chemical potential \mu as they both appear in the Fermi-Dirac distribution function,which...
can be seen. Thus XANES spectra can be used as a probe of the unoccupied band structure of a material.
The near-edge structure is characteristic of an environment and valence state hence one of its more common uses is in fingerprinting: if you have a mixture of sites/compounds in a sample you can fit the measured spectra with a linear combinations of XANES spectra of known species and determine the proportion of each site/compound in the sample. One example of such a use is the determination of the oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
of the plutonium
Plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
in the soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
at Rocky Flats
Rocky Flats Plant
The Rocky Flats Plant was a United States nuclear weapons production facility near Denver, Colorado that operated from 1952 to 1992. It was under the control of the United States Atomic Energy Commission until 1977, when it was replaced by the Department of Energy .-1950s:Following World War II,...
.
History
The acronym XANES was first used in 1980 during interpretation of multiple scattering resonances spectra measured at the Stanford Synchrotron Radiation LaboratoryStanford Synchrotron Radiation Laboratory
The Stanford Synchrotron Radiation Lightsource , a division of SLAC National Accelerator Laboratory, is operated by Stanford University for the Department of Energy...
(SSRL) by A. Bianconi. In 1982 the first paper on the application of XANES for determination of local structural geometrical distortions using multiple scattering theory was published by A. Bianconi, P. J. Durham and J. B. Pendry. In 1983 the first NEXAFS paper examining molecules absorbed on surfaces appeared. The first XAFS paper, describing the intermediate region between EXAFS and XANES, appeared in 1987.
Software for XANES analysis
FDMNESFDMNES
The FDMNES program calculates the spectra of different spectroscopies related to the real or virtual absorption of x-ray in material. It gives the absorption cross sections of photons around the ionization edge, that is in the energy range of XANES. The calculation is performed with all conditions...
Calculation of XANES using finite difference method and full multiple scattering theory.
FEFF8
FEFF8
FEFF8 is self-consistent real space multiple-scattering code for simultaneous calculations ofx-ray-absorption spectra and electronic structure. Output includes extended x-ray-absorption...
Calculation of XANES using full multiple scattering theory.
MXAN XANES fitting using full multiple scattering theory.
FitIt
FitIt
FitIt is user-friendly graphical software to fit X-ray absorption near edge structure . It can be used to determine the values of local atomic structure parameters on the basis of minimization between theoretical and experimental spectra. It is the program for the fitting and therefore it always...
XANES fitting using multidimensional interpolation approximation.
PARATEC
PARATEC
PARATEC is a package that performs ab-initio quantum-mechanical total energy calculations using pseudopotentials and a plane wave basis set...
XANES calculation using plane-wave pseudopotential approach
WIEN2k
WIEN2k
The WIEN2k package is a computer program written in Fortran which performs quantum mechanical calculations on periodic solids. It uses the full-potential augmented plane-wave and local-orbitals [FP-APW+lo] basis set to solve the Kohn–Sham equations of density functional theory.WIEN2k was...
XANES calculation on the basis of full-potential (linearized) augmented plane-wave approach.
External links
- M. Newville, The fundamentals of XAFS
- S. Bare, XANES measurements and interpretation
- B. Ravel, A practical introduction to multiple scattering