X-ray absorption near edge structure (XANES), also known as near edge X-ray absorption fine structure (NEXAFS), is a type of
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 fie ...
that indicates the features in the X-ray absorption spectra (
XAS
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 facilities, which provide intense and tunabl ...
) of condensed matter due to the photoabsorption
cross section
Cross section may refer to:
* Cross section (geometry)
** Cross-sectional views in architecture & engineering 3D
*Cross section (geology)
* Cross section (electronics)
* Radar cross section, measure of detectability
* Cross section (physics)
**Abs ...
for electronic transitions from an
atomic core Core electrons are the electrons in an atom that are not valence electrons and do not participate in chemical bonding. The nucleus and the core electrons of an atom form the atomic core. Core electrons are tightly bound to the nucleus. Therefore, un ...
level to final states in the energy region of 50–100 eV above the selected atomic core level ionization energy, where the wavelength of the photoelectron is larger than the interatomic distance between the absorbing atom and its first neighbour atoms.
Terminology
Both XANES and NEXAFS are acceptable terms for the same technique. XANES name was invented in 1980 by Antonio Bianconi to indicate strong absorption peaks in X-ray absorption spectra in condensed matter due to multiple scattering resonances above the ionization energy.
The name NEXAFS was introduced in 1983 by Jo Stohr and is synonymous with XANES, but is generally used when applied to surface and molecular science.
Theory
The fundamental phenomenon underlying XANES is the absorption of an x-ray photon by condensed matter with the formation of many body excited states characterized by a core hole in a selected atomic core level (refer to the first Figure). In the single-particle theory approximation, the system is separated into one electron in the core levels of the selected atomic species of the system and N-1 passive electrons. In this approximation the final state is described by a core hole in the atomic core level and an excited photoelectron. The final state has a very short life time because of the short life-time of the core hole and the short mean free path of the excited photoelectron with kinetic energy in the range around 20-50 eV. The core hole is filled either via an
Auger
Auger may refer to:
Engineering
* Wood auger, a drill for making holes in wood (or in the ground)
** Auger bit, a drill bit
* Auger conveyor, a device for moving material by means of a rotating helical flighting
* Auger (platform), the world's f ...
process or by capture of an electron from another shell followed by emission of a
fluorescent
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
photon. The difference between NEXAFS and traditional
photoemission
The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid sta ...
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
An exciton is a bound state of an electron and an electron 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. The ...
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
The many-body problem is a general name for a vast category of physical problems pertaining to the properties of microscopic systems made of many interacting particles. ''Microscopic'' here implies that quantum mechanics has to be used to provid ...
effects than initial states, meaning that NEXAFS spectra are more easily calculable than photoemission spectra. Due to the summation over final states, various
sum rules 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 (also known as magnetobremsstrahlung radiation) is the electromagnetic radiation emitted when relativistic charged particles are subject to an acceleration perpendicular to their velocity (). It is produced artificially in ...
has a natural
polarization that can be utilized to great advantage in NEXAFS studies. The commonly studied molecular adsorbates have
sigma
Sigma (; uppercase Σ, lowercase σ, lowercase in word-final position ς; grc-gre, σίγμα) is the eighteenth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 200. In general mathematics, uppercase Σ is used as ...
and
pi bond
In chemistry, pi bonds (π bonds) are covalent chemical bonds, in each of which two lobes of an orbital on one atom overlap with two lobes of an orbital on another atom, and in which this overlap occurs laterally. Each of these atomic orbitals ...
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
In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways:
*An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system i ...
selection rule
In physics and chemistry, a selection rule, or transition rule, formally constrains the possible transitions of a system from one quantum state to another. Selection rules have been derived for electromagnetic transitions in molecules, in atoms, in ...
s.
Experimental considerations
Soft x-ray absorption 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
A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle beam travels around a fixed closed-loop path. The magnetic field which bends the particle beam into its closed p ...
s. Because 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.
Energy range
Edge energy range
In the absorption edge region of metals, the
photoelectron
The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid sta ...
is excited to the first unoccupied level above the
Fermi level
The Fermi level of a solid-state body is the thermodynamic work required to add one electron to the body. It is a thermodynamic quantity usually denoted by ''µ'' or ''E''F
for brevity. The Fermi level does not include the work required to remove ...
. Therefore, its
mean free path
In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...
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
In solid state physics and condensed matter physics, the density of states (DOS) of a system describes the number of modes per unit frequency range. The density of states is defined as D(E) = N(E)/V , where N(E)\delta E is the number of states i ...
and matrix elements in single electron excitations,
many-body
The many-body problem is a general name for a vast category of physical problems pertaining to the properties of microscopic systems made of many interacting particles. ''Microscopic'' here implies that quantum mechanics has to be used to provid ...
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
In thermodynamics, the chemical potential of a species is the energy that can be absorbed or released due to a change of the particle number of the given species, e.g. in a chemical reaction or phase transition. The chemical potential of a species ...
but the unscreened core hole forms a localized bound state called core
exciton
An exciton is a bound state of an electron and an electron 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. The ...
.
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
In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction from which they came. It is usually a diffuse reflection due to scattering, as opposed to specular reflection as from a mirror, a ...
events due to the low amplitude photoelectron scattering.
NEXAFS energy range
In the NEXAFS region, starting about 5 eV beyond the absorption threshold, because of the low kinetic energy range (5-150 eV) the photoelectron
backscattering
In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction from which they came. It is usually a diffuse reflection due to scattering, as opposed to specular reflection as from a mirror, a ...
amplitude by neighbor atoms is very large so that multiple
scattering
Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
events become dominant in the NEXAFS spectra.
The different energy range between NEXAFS and EXAFS can be also explained in a very simple manner by the comparison between the photoelectron
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
and the interatomic distance of the photoabsorber-backscatterer pair. The photoelectron kinetic energy is connected with the wavelength
by the following relation:
:
which 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
Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
regime.
Final states
The absorption peaks of NEXAFS spectra are determined by multiple scattering
resonance
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
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
In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...
, that is strongly reduced (down to about 0.3 nm at 50 eV) in this energy range because of
inelastic scattering In chemistry, nuclear physics, and particle physics, inelastic scattering is a fundamental scattering process in which the kinetic energy of an incident particle is not conserved (in contrast to elastic scattering). In an inelastic scattering proces ...
of the photoelectron by electron-hole excitations (
excitons
An exciton is a bound state of an electron and an electron 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. The ...
) and collective electronic oscillations of the valence electrons called
plasmons
In physics, a plasmon is a quantum of plasma oscillation. Just as light (an optical oscillation) consists of photons, the plasma oscillation consists of plasmons. The plasmon can be considered as a quasiparticle since it arises from the quantiz ...
.
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 media buried beneath a surface lubricant or dopants below an electrode in an
integrated circuit
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
. 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 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. It can be defined as t ...
and
geochemistry
Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...
. 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 NEXAFS 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
The Fermi level of a solid-state body is the thermodynamic work required to add one electron to the body. It is a thermodynamic quantity usually denoted by ''µ'' or ''E''F
for brevity. The Fermi level does not include the work required to remove ...
can be seen. Thus NEXAFS 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 NEXAFS 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
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
of the
plutonium
Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibi ...
in the
soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
at
Rocky Flats
The Rocky Flats Plant was a U.S. manufacturing complex that produced nuclear weapons parts in the western United States, near Denver, Colorado. The facility's primary mission was the fabrication of plutonium pits, which were shipped to ot ...
.
History
The acronym XANES was first used in 1980 during interpretation of multiple scattering resonances spectra measured at the
Stanford Synchrotron Radiation Laboratory
The Stanford Synchrotron Radiation Lightsource (formerly Stanford Synchrotron Radiation Laboratory), a division of SLAC National Accelerator Laboratory, is operated by Stanford University for the Department of Energy. SSRL is a National User Fac ...
(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 adsorbed on surfaces appeared. The first XAFS paper, describing the intermediate region between EXAFS and XANES, appeared in 1987.
Software for NEXAFS analysis
*
ADF Calculation of NEXAFS using spin-orbit coupling TDDFT or the Slater-TS method.
*
FDMNES 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 XANE ...
Calculation of NEXAFS using finite difference method and full multiple scattering theory.
*
FEFF8 FEFF is a software program used in x-ray absorption spectroscopy. It contains self-consistent space, real space multiple-scattering code for simultaneous calculations of x-ray-Absorption spectrum, absorption spectra and electronic structure. Output ...
Calculation of NEXAFS using full multiple scattering theory.
*
MXAN NEXAFS fitting using full multiple scattering theory.
*
FitIt FitIt is graphical software to fit X-ray absorption near edge structure (XANES). 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 ...
NEXAFS fitting using multidimensional interpolation approximation.
*
PARATEC
PARATEC (PARAllel Total Energy Code) is a package that performs ab initio quantum mechanical total energy calculations using pseudopotentials and a plane wave basis set. PARATEC is designed primarily for a massively parallel computing platform, a ...
NEXAFS calculation using plane-wave pseudopotential approach
*
WIEN2k
The WIEN2k package is a computer program written in Fortran which performs Computational chemical methods in solid state physics, quantum mechanical calculations on periodic solids. It uses the full-potential Linearized augmented-plane-wave method ...
NEXAFS calculation on the basis of full-potential (linearized) augmented plane-wave approach.
References
*
*
*
Bibliography
* "X-ray Absorption Near-Edge Structure (XANES) Spectroscopy", G. S. Henderson, F. M. F. de Groot, B. J. A. Moulton in Spectroscopic Methods in Mineralogy and Materials Sciences, (G.S. Henderson, D. R. Neuville, R. T. Downs, Eds) ''Reviews in Mineralogy & Geochemistry'' vol. 78, p 75, 2014
DOI:10.2138/rmg.2014.78.3
* "X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS, and XANES", D. C. Koningsberger, R. Prins; A. Bianconi, P.J. Durham Chapters, Chemical Analysis 92, 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.
* ''NEXAFS Spectroscopy'' by J. Stöhr, Springer 1992, .
External links
* M. Newville,
Fundamentals of XAFS'
* S. Bare,
XANES measurements and interpretation'
* B. Ravel,
A practical introduction to multiple scattering'
{{DEFAULTSORT:Xanes
X-ray absorption spectroscopy