![MössbauerSpectrum57Fe](https://upload.wikimedia.org/wikipedia/commons/6/66/M%C3%B6ssbauerSpectrum57Fe.svg)
Mössbauer spectroscopy is a
spectroscopic
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
technique based on the
Mössbauer effect
The Mössbauer effect, or recoilless nuclear resonance fluorescence, is a physical phenomenon discovered by Rudolf Mössbauer in 1958. It involves the resonant and recoil-free emission and absorption of gamma radiation by atomic nuclei bound in ...
. This effect, discovered by
Rudolf Mössbauer
Rudolf Ludwig Mössbauer (German spelling: ''Mößbauer''; ; 31 January 1929 – 14 September 2011) was a German physicist best known for his 1957 discovery of ''recoilless nuclear resonance fluorescence'' for which he was awarded the 1961 Nobe ...
(sometimes written "Moessbauer", German: "Mößbauer") in 1958, consists of the nearly
recoil
Recoil (often called knockback, kickback or simply kick) is the rearward thrust generated when a gun is being discharged. In technical terms, the recoil is a result of conservation of momentum, as according to Newton's third law the force requ ...
-free emission and absorption of nuclear
gamma ray
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...
s in
solid
Solid is one of the State of matter#Four fundamental states, four fundamental states of matter (the others being liquid, gas, and Plasma (physics), plasma). The molecules in a solid are closely packed together and contain the least amount o ...
s. The consequent
nuclear spectroscopy Nuclear spectroscopy is a superordinate concept of methods that uses properties of a nucleus to probe material properties. By emission or absorption of radiation from the nucleus information of the local structure is obtained, as an interaction of ...
method is exquisitely sensitive to small changes in the chemical environment of certain nuclei.
Typically, three types of
nuclear interactions may be observed: the
isomer shift due to differences in nearby electron densities (also called the
chemical shift
In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of an atomic nucleus relative to a standard in a magnetic field. Often the position and number of chemical shifts are diagnostic of the structure of ...
in older literature),
quadrupole splitting
Quadrupole splitting is an example of a hyperfine structure, hyperfine interaction found in gamma-ray spectroscopy, in the circumstance where nuclei with a non-radially-symmetric shape (that is, with a spin quantum number greater than 1/2) are foun ...
due to atomic-scale electric field gradients; and magnetic
Zeeman splitting due to non-nuclear magnetic fields. Due to the high energy and extremely
narrow line widths of nuclear gamma rays, Mössbauer spectroscopy is a highly sensitive technique in terms of energy (and hence frequency) resolution, capable of detecting changes of just a few parts in 10
11. It is a method completely unrelated to
nuclear magnetic resonance spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fiel ...
.
Basic principle
Just as a gun recoils when a bullet is fired,
conservation of momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...
requires a nucleus (such as in a gas) to recoil during emission or absorption of a gamma ray. If a nucleus at rest ''emits'' a gamma ray, the energy of the gamma ray is slightly ''less'' than the natural energy of the transition, but in order for a nucleus at rest to ''absorb'' a gamma ray, the gamma ray's energy must be slightly ''greater'' than the natural energy, because in both cases energy is lost to recoil. This means that nuclear resonance (emission and absorption of the same gamma ray by identical nuclei) is unobservable with free nuclei, because the shift in energy is too great and the emission and absorption spectra have no significant overlap.
Nuclei in a solid
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
, however, are not free to recoil because they are bound in place in the crystal lattice. When a nucleus in a solid emits or absorbs a gamma ray, some energy can still be lost as recoil energy, but in this case it always occurs in discrete packets called
phonon
In physics, a phonon is a collective excitation in a periodic, Elasticity (physics), elastic arrangement of atoms or molecules in condensed matter physics, condensed matter, specifically in solids and some liquids. A type of quasiparticle, a phon ...
s (quantized vibrations of the crystal lattice). Any whole number of phonons can be emitted, including zero, which is known as a "recoil-free" event. In this case conservation of momentum is satisfied by the momentum of the crystal as a whole, so practically no energy is lost.
[International Board on the Applications of the Mössbauer Effect (IBAME) and Mössbauer Effect Data Center (MEDC)]
Mössbauer Effect website
Accessed June 3, 2010.
Mössbauer found that a significant fraction of emission and absorption events will be recoil-free, which is quantified using the
Lamb–Mössbauer factor In physics, the Lamb–Mössbauer factor (LMF, after Willis Lamb and Rudolf Mössbauer) or elastic incoherent structure factor (EISF) is the ratio of elastic to total incoherent neutron scattering, or the ratio of recoil-free to total nuclear resona ...
.
[Gütlich, J. M.]
The Principle of the Mössbauer Effect and Basic Concepts of Mössbauer Spectrometry
. This fact is what makes Mössbauer spectroscopy possible, because it means that gamma rays emitted by one nucleus can be resonantly absorbed by a sample containing nuclei of the same isotope, and this absorption can be measured.
The recoil fraction of the Mössbauer absorption is analyzed by
nuclear resonance vibrational spectroscopy
Nuclear resonance vibrational spectroscopy is a synchrotron-based technique that probes vibrational energy levels. The technique, often called NRVS, is specific for samples that contain nuclei that respond to Mössbauer spectroscopy, most commonl ...
.
Typical method
In its most common form, Mössbauer absorption spectroscopy, a solid sample is exposed to a beam of
gamma radiation
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically s ...
, and a detector measures the intensity of the beam transmitted through the sample. The atoms in the source emitting the gamma rays must be of the same isotope as the atoms in the sample absorbing them.
If the emitting and absorbing nuclei were in identical chemical environments, the nuclear transition energies would be exactly equal and resonant absorption would be observed with both materials at rest. The difference in chemical environments, however, causes the nuclear energy levels to shift in a few different ways, as described below. Although these energy shifts are tiny (often less than a micro-
electronvolt
In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defi ...
), the extremely narrow
spectral linewidths of gamma rays for some radionuclides make the small energy shifts correspond to large changes in
absorbance
Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative lo ...
. To bring the two nuclei back into resonance it is necessary to change the energy of the gamma ray slightly, and in practice this is always done using the
Doppler shift.
During Mössbauer absorption spectroscopy, the source is accelerated through a range of velocities using a
linear motor to produce a Doppler effect and scan the gamma ray energy through a given range. A typical range of velocities for
57Fe, for example, may be ± ().
[Mössbauer Spectroscopy Group, ]Royal Society of Chemistry
The Royal Society of Chemistry (RSC) is a learned society (professional association) in the United Kingdom with the goal of "advancing the chemistry, chemical sciences". It was formed in 1980 from the amalgamation of the Chemical Society, the Ro ...
(RSC) website
Introduction to Mössbauer Spectroscopy Part 1
Accessed June 3, 2010
In the resulting spectra, gamma ray intensity is plotted as a function of the source velocity. At velocities corresponding to the resonant energy levels of the sample, a fraction of the gamma rays are absorbed, resulting in a drop in the measured intensity and a corresponding dip in the spectrum. The number, positions, and intensities of the dips (also called peaks; dips in transmitted intensity are peaks in absorbance) provide information about the chemical environment of the absorbing nuclei and can be used to characterize the sample.
Selecting a suitable source
Suitable gamma-ray sources consist of a radioactive parent that decays to the desired isotope. For example, the source for
57Fe consists of
57Co, which decays by
electron capture
Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom absorbs an inner atomic electron, usually from the K or L electron shells. Thi ...
to an
excited state of
57Fe, which in turn decays to a
ground state
The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state. ...
via a series of gamma-ray emissions that include the one exhibiting the Mössbauer effect. The radioactive cobalt is prepared on a foil, often of rhodium. Ideally the
parent
A parent is a caregiver of the offspring in their own species. In humans, a parent is the caretaker of a child (where "child" refers to offspring, not necessarily age). A ''biological parent'' is a person whose gamete resulted in a child, a male t ...
isotope will have a convenient half-life. Also, the gamma-ray energy should be relatively low, otherwise the system will have a low recoil-free fraction resulting in a poor
signal-to-noise ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...
and requiring long collection times. The periodic table below indicates those elements having an isotope suitable for Mössbauer spectroscopy. Of these,
57Fe is by far the most common element studied using the technique, although
129I,
119Sn, and
121Sb are also frequently studied.
Analysis of Mössbauer spectra
As described above, Mössbauer spectroscopy has an extremely fine energy resolution and can detect even subtle changes in the nuclear environment of the relevant atoms. Typically, there are three types of
nuclear interactions that are observed:
isomeric shift
The isomeric shift (also called isomer shift) is the shift on atomic spectral lines and gamma spectral lines, which occurs as a consequence of replacement of one nuclear isomer by another. It is usually called isomeric shift on atomic spectral line ...
,
quadrupole splitting
Quadrupole splitting is an example of a hyperfine structure, hyperfine interaction found in gamma-ray spectroscopy, in the circumstance where nuclei with a non-radially-symmetric shape (that is, with a spin quantum number greater than 1/2) are foun ...
, and
hyperfine
In atomic physics, hyperfine structure is defined by small shifts in otherwise degenerate energy levels and the resulting splittings in those energy levels of atoms, molecules, and ions, due to electromagnetic multipole interaction between the nuc ...
magnetic splitting.
[Mössbauer Spectroscopy Group, ]Royal Society of Chemistry
The Royal Society of Chemistry (RSC) is a learned society (professional association) in the United Kingdom with the goal of "advancing the chemistry, chemical sciences". It was formed in 1980 from the amalgamation of the Chemical Society, the Ro ...
(RSC) website
Introduction to Mössbauer Spectroscopy Part 2
Accessed June 3, 2010.[P. Gütlich, J. M. Greneche, F. J. Berry]
Mössbauer Spectroscopy: A Powerful Tool in Scientific Research
Accessed June 3, 2010.
Isomer shift
![Mossbauer Isomer Shift and Quadrupole Splitting for 57Fe](https://upload.wikimedia.org/wikipedia/commons/6/66/Mossbauer_Isomer_Shift_and_Quadrupole_Splitting_for_57Fe.svg)
Isomer shift (δ) (also sometimes called
chemical shift
In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of an atomic nucleus relative to a standard in a magnetic field. Often the position and number of chemical shifts are diagnostic of the structure of ...
, especially in the older literature) is a relative measure describing a shift in the resonance energy of a nucleus (see Fig. 2) due to the transition of electrons within its ''s'' orbitals. The whole spectrum is shifted in either a positive or negative direction depending upon the ''s'' electron charge density in the nucleus. This change arises due to alterations in the electrostatic response between the non-zero probability ''s'' orbital electrons and the non-zero volume nucleus they orbit.
Only electrons in ''s'' orbitals have a non-zero probability of being found in the nucleus (see
atomic orbitals
In atomic theory and quantum mechanics, an atomic orbital is a function describing the location and wave-like behavior of an electron in an atom. This function can be used to calculate the probability of finding any electron of an atom in any sp ...
). However, ''p'', ''d'', and ''f'' electrons may influence the ''s'' electron density through a
screening effect
In physics, screening is the damping of electric fields caused by the presence of mobile charge carriers. It is an important part of the behavior of charge-carrying fluids, such as ionized gases (classical plasmas), electrolytes, and charge c ...
.
Isomer shift can be expressed using the formula below, where ''K'' is a nuclear constant, the difference between ''R''
e2 and ''R''
g2 is the effective nuclear charge radius difference between excited state and the ground state, and the difference between
s2(0)">¨s2(0)sub>a and
s2(0)">¨s2(0)sub>b is the electron density difference in the nucleus (a = source, b = sample). The Chemical Isomer shift as described here does not change with temperature, however, Mössbauer spectra do have a temperature sensitivity due to a relativistic effect known as the second-order Doppler effect. Generally, the impact of this effect is small, and the
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
standard allows the Isomer Shift to be reported without correcting for it.
[International Board on the Applications of the Mössbauer Effect (IBAME) and Mössbauer Effect Data Center (MEDC)]
Mössbauer Effect website
Accessed December 20, 2017
:
The physical meaning of this equation can be clarified using examples:
# While an increase in ''s''-electron density in
57Fe spectrum gives a negative shift because the change in the effective nuclear charge is negative (owing to ''R''
e < ''R''
g), an increase in ''s''-electron density in
119Sn gives a positive shift due to a positive change in overall nuclear charge (owing to ''R''
e > ''R''
g).
# Oxidised
ferric
In chemistry, iron(III) refers to the element iron in its +3 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe3+.
The adjective ferric or the prefix ferri- is often used to sp ...
ions (Fe
3+) have lower isomer shifts than
ferrous
In chemistry, the adjective Ferrous indicates a compound that contains iron(II), meaning iron in its +2 oxidation state, possibly as the divalent cation Fe2+. It is opposed to "ferric" or iron(III), meaning iron in its +3 oxidation state, such a ...
ions (Fe
2+) because ''s''-electron density at the nucleus of ferric ions is greater due to a weaker screening effect by ''d'' electrons.
The isomer shift is useful for determining oxidation state, valency states, electron shielding and the electron-drawing power of electronegative groups.
Quadrupole splitting
Quadrupole splitting
Quadrupole splitting is an example of a hyperfine structure, hyperfine interaction found in gamma-ray spectroscopy, in the circumstance where nuclei with a non-radially-symmetric shape (that is, with a spin quantum number greater than 1/2) are foun ...
reflects the interaction between the nuclear energy levels and the surrounding
electric field gradient In atomic, molecular, and solid-state physics, the electric field gradient (EFG) measures the rate of change of the electric field at an atomic nucleus generated by the electronic charge distribution and the other nuclei. The EFG couples with the ...
(EFG). Nuclei in states with non-spherical charge distributions, i.e. all those with spin quantum number (''I'') greater than 1/2, may have a nuclear quadrupole moment. In this case an asymmetrical electric field (produced by an asymmetric electronic charge distribution or ligand arrangement) splits the nuclear energy levels.
In the case of an isotope with a ''I'' = 3/2 excited state, such as
57Fe or
119Sn, the excited state is split into two substates ''m''
''I'' = ±1/2 and ''m''
''I'' = ±3/2. The ground to excited state transitions appear as two specific peaks in a spectrum, sometimes referred to as a "doublet". Quadrupole splitting is measured as the separation between these two peaks and reflects the character of the electric field at the nucleus.
The quadrupole splitting can be used for determining oxidation state, spin state, site symmetry, and the arrangement of ligands.
Magnetic hyperfine splitting
Magnetic
hyperfine splitting
In atomic physics, hyperfine structure is defined by small shifts in otherwise degenerate energy levels and the resulting splittings in those energy levels of atoms, molecules, and ions, due to electromagnetic multipole interaction between the nuc ...
is a result of the interaction between the nucleus and any surrounding magnetic field, as described by the
Zeeman Effect
The Zeeman effect (; ) is the effect of splitting of a spectral line into several components in the presence of a static magnetic field. It is named after the Dutch physicist Pieter Zeeman, who discovered it in 1896 and received a Nobel priz ...
. A nucleus with spin ''I'' splits into 2''I'' + 1 sub-energy levels in the presence of a magnetic field. For example, the first excited state of the
57Fe nucleus with spin state ''I'' = 3/2 will split into 4 non-degenerate sub-states with ''m''
''I'' values of +3/2, +1/2, −1/2 and −3/2. The equally-spaced splits are said to be hyperfine, being on the order of 10
−7 eV. The
selection rule for magnetic dipole transitions means that transitions between the excited state and ground state can only occur where ''m''
''I'' changes by 0 or 1 or −1. This gives 6 possible for a 3/2 to 1/2 transition.
The extent of splitting is proportional to the magnetic field strength at the nucleus, which in turn depends on the electron distribution ("chemical environment") of the nucleus. The splitting can be measured, for instance, with a sample foil placed between an oscillating source and a photon detector (see Fig. 5), resulting in an absorption spectrum, as illustrated in Fig. 4. The magnetic field can be determined from the spacing between the peaks if the quantum "g-factors" of the nuclear states are known. In ferromagnetic materials, including many iron compounds, the natural internal magnetic fields are quite strong and their effects dominate the spectra.
Combination of all
The three Mössbauer parameters: isomer shift, quadrupole splitting, and hyperfine splitting can often be used to identify a particular compound by comparison to spectra for standards. In some cases, a compound may have more than one possible position for the Mössbauer active atom. For example, the crystal structure of
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
(Fe
3O
4) supports two different sites for the iron atoms. Its spectrum has 12 peaks, a sextet for each potential atomic site, corresponding to two sets of Mössbauer parameters.
Many times all effects are observed: isomer shift, quadrupole splitting, and magnetic Zeeman effect. In such cases the isomer shift is given by the average of all lines. The quadrupole splitting when all the four excited substates are equally shifted (two substates are lifted and other two are lowered) is given by the shift of the outer two lines relative to the inner four lines (all inner four lines shift in opposition to the outermost two lines). Usually fitting software is used for accurate values.
In addition, the relative intensities of the various peaks reflect the relative concentrations of compounds in a sample and can be used for semi-quantitative analysis. Also, since ferromagnetic phenomena are size-dependent, in some cases spectra can provide insight into the crystallite size and grain structure of a material.
Mössbauer emission spectroscopy
Mössbauer emission spectroscopy is a specialized variant of Mössbauer spectroscopy where the emitting element is in the probed sample, and the absorbing element is in the reference. Most commonly, the technique is applied to the
57Co/
57Fe pair. A typical application is the characterization of the cobalt sites in amorphous Co-Mo catalysts used in
hydrodesulfurization. In such a case, the sample is doped with
57Co.
Applications
Among the drawbacks of the technique are the limited number of gamma ray sources and the requirement that samples be solid in order to eliminate the recoil of the nucleus. Mössbauer spectroscopy is unique in its sensitivity to subtle changes in the chemical environment of the nucleus including oxidation state changes, the effect of different
ligand
In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
s on a particular atom, and the magnetic environment of the sample.
As an analytical tool Mössbauer spectroscopy has been especially useful in the field of geology for identifying the composition of iron-containing specimens including meteorites and moon rocks. ''In situ'' data collection of Mössbauer spectra has also been carried out on iron rich rocks on Mars.
In another application, Mössbauer spectroscopy is used to characterize phase transformations in iron catalysts, e.g., those used for
Fischer–Tropsch synthesis. While initially consisting of hematite (Fe
2O
3), these catalysts transform into a mixture of
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
(Fe
3O
4) and several
iron carbides. The formation of carbides appears to improve catalytic activity, but it can also lead to the mechanical break-up and attrition of the catalyst particles, which can cause difficulties in the final separation of catalyst from reaction products.
Mössbauer spectroscopy has also been used to determine the relative concentration change in the oxidation state of antimony (
Sb) during the selective oxidation of
olefins. During
calcination
Calcination refers to thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), genera ...
, all the Sb ions in an antimony-containing
tin dioxide
Tin(IV) oxide, also known as stannic oxide, is the inorganic compound with the formula SnO2. The mineral form of SnO2 is called cassiterite, and this is the main ore of tin. With many other names, this oxide of tin is an important material in tin ...
catalyst transform into the +5 oxidation state. Following the catalytic reaction, almost all Sb ions revert from the +5 to the +3 oxidation state. A significant change in the chemical environment surrounding the antimony nucleus occurs during the oxidation state change which can easily be monitored as an isomer shift in the Mössbauer spectrum.
This technique has also been used to observe the second-order
transverse Doppler effect predicted by the
theory of relativity
The theory of relativity usually encompasses two interrelated theories by Albert Einstein: special relativity and general relativity, proposed and published in 1905 and 1915, respectively. Special relativity applies to all physical phenomena in ...
, because of very high energy resolution.
Bioinorganic chemistry
Mossbauer spectroscopy has been widely applied to bioinorganic chemistry, especially for the study of iron-containing proteins and enzymes. Often the technique is used to determine the oxidation state of iron. Examples of prominent iron-containing biomolecules are
iron-sulfur protein
Iron–sulfur proteins (or iron–sulphur proteins in British spelling) are proteins characterized by the presence of iron–sulfur clusters containing sulfide-linked di-, tri-, and tetrairon centers in variable oxidation states. Iron–sulfur cl ...
s,
ferritin
Ferritin is a universal intracellular protein that stores iron and releases it in a controlled fashion. The protein is produced by almost all living organisms, including archaea, bacteria, algae, higher plants, and animals. It is the primary ' ...
, and
heme
Heme, or haem (pronounced / hi:m/ ), is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver.
In biochemical terms, heme is a coordination complex "consis ...
s including the
cytochrome
Cytochromes are redox-active proteins containing a heme, with a central Fe atom at its core, as a cofactor. They are involved in electron transport chain and redox catalysis. They are classified according to the type of heme and its mode of bi ...
s. These studies are often supplemented by analysis of related model complexes. An area of particular interest is the characterization of intermediates involved in
oxygen activation by iron proteins.
Vibrational spectra of
57Fe-enriched biomolecules can be acquired using
nuclear resonance vibrational spectroscopy
Nuclear resonance vibrational spectroscopy is a synchrotron-based technique that probes vibrational energy levels. The technique, often called NRVS, is specific for samples that contain nuclei that respond to Mössbauer spectroscopy, most commonl ...
(NRVS), in which the sample is scanned through a range of synchrotron-generated X-rays, centered at the Mössbauer absorbance frequency. Stokes and anti-Stokes peaks in the spectrum correspond to low frequency vibrations, many below 600 cm
−1 with some below 100 cm
−1.
Mössbauer spectrometers
![Mossbauer Spectrometer](https://upload.wikimedia.org/wikipedia/commons/2/2f/Mossbauer_Spectrometer.jpg)
A Mössbauer spectrometer is a device that performs Mössbauer spectroscopy, or a device that uses the Mössbauer effect to determine the chemical environment of Mössbauer nuclei present in the sample. It is formed by three main parts; a source that moves back and forth to generate a
Doppler effect, a
collimator
A collimator is a device which narrows a beam of particles or waves. To narrow can mean either to cause the directions of motion to become more aligned in a specific direction (i.e., make collimated light or parallel rays), or to cause the spati ...
that filters out non-parallel
gamma rays
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...
and a detector.
A miniature Mössbauer Spectrometer, named (MB)
MIMOS II MIMOS II is the miniaturised Mössbauer spectrometer, developed by Dr. Göstar Klingelhöfer at the Johannes Gutenberg University in Mainz, Germany, that is used on the Mars Exploration Rovers '' Spirit'' and '' Opportunity'' for close-up investiga ...
, was used by the two rovers in
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
's
Mars Exploration Rover
NASA's Mars Exploration Rover (MER) mission was a robotic space mission involving two Mars rovers, ''Spirit (rover), Spirit'' and ''Opportunity (rover), Opportunity'', exploring the planet Mars. It began in 2003 with the launch of the two rover ...
missions.
57Fe Mössbauer spectroscopy
The chemical isomer shift and quadrupole splitting are generally evaluated with respect to a reference material. For example, in iron compounds, the Mössbauer parameters were evaluated using iron foil (of a thickness less than 40 micrometers). The centroid of the six-line spectrum from metallic iron foil is −0.1 mm/s (for a
Co/
Rh source). All shifts in other iron compounds are computed relative to this −0.10 mm/s (at room temperature), i.e., in this case isomer shifts are relative to the Co/Rh source. In other words, the centre point of the Mössbauer spectrum is zero. The shift values may also be reported relative to 0.0 mm/s; here, shifts are relative to the iron foil.
To calculate the outer line distance from the six-line iron spectrum:
:
where ''c'' is the speed of light, ''B''
int is the internal magnetic field of the metallic iron (), ''μ''
N is the
nuclear magneton
The nuclear magneton (symbol ''μ'') is a physical constant of magnetic moment, defined in SI units by:
:\mu_\text =
and in Gaussian CGS units by:
:\mu_\text =
where:
:''e'' is the elementary charge,
:''ħ'' is the reduced Planck constant ...
(), ''E''
γ is the excitation energy (14.412497(3) keV
[Mössbauer Effect Data Center]
20.08.2013), ''g''
n is the ground state nuclear splitting factor (/(''I''), where
Isospin
In nuclear physics and particle physics, isospin (''I'') is a quantum number related to the up- and down quark content of the particle. More specifically, isospin symmetry is a subset of the flavour symmetry seen more broadly in the interactions ...
''I'' = ) and ''g'' is the excited state splitting factor of
57Fe (-0.15532/(''I''), where ''I'' = ).
By substituting the above values one would get ''V'' = .
Other values are sometimes used to reflect different qualities of iron foils. In all cases any change in ''V'' only affects the isomer shift and not the quadrupole splitting. As the
IBAME, the authority for Mössbauer spectroscopy, does not specify a particular value, anything between 10.60 mm/s to 10.67 mm/s can be used. For this reason it is highly recommended to provide the isomer shift values relative to the source used, not to the iron foil, mentioning the details of the source (centre of gravity of the folded spectrum).
See also
*
Alpha-particle spectroscopy
Alpha spectrometry (also known as alpha(-particle) spectroscopy) is the quantitative study of the energy of alpha particles emitted by a radioactive nuclide that is an alpha emitter.
As emitted alpha particles are mono-energetic (i.e. not emitt ...
*
Gamma probe
*
Gamma ray spectrometer
A gamma-ray spectrometer (GRS) is an instrument for measuring the distribution (or spectrum—see figure) of the intensity of gamma radiation versus the energy of each photon.
The study and analysis of gamma-ray spectra for scientific and techni ...
*
Isomeric shift
The isomeric shift (also called isomer shift) is the shift on atomic spectral lines and gamma spectral lines, which occurs as a consequence of replacement of one nuclear isomer by another. It is usually called isomeric shift on atomic spectral line ...
*
Liquid scintillation counting Liquid scintillation counting is the measurement of radioactive activity of a sample material which uses the technique of mixing the active material with a liquid scintillator (e.g. zinc sulfide), and counting the resultant photon emissions. The pu ...
*
Mass spectrometry
Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...
*
Mössbauer effect
The Mössbauer effect, or recoilless nuclear resonance fluorescence, is a physical phenomenon discovered by Rudolf Mössbauer in 1958. It involves the resonant and recoil-free emission and absorption of gamma radiation by atomic nuclei bound in ...
*
Perturbed angular correlation
The perturbed γ-γ angular correlation, PAC for short or PAC-Spectroscopy, is a method of nuclear solid-state physics with which magnetic and electric fields in crystal structures can be measured. In doing so, electrical field gradients and the L ...
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Pandemonium effect
The pandemonium effect is a problem that may appear when high resolution detectors (usually germanium detectors) are used in beta decay studies. It can affect the correct determination of the feeding to the different levels of the daughter nuc ...
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Total absorption spectroscopy
Total absorption spectroscopy is a measurement technique that allows the measurement of the gamma radiation emitted in the different nuclear gamma transitions that may take place in the daughter nucleus after its unstable parent has decayed by mean ...
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Scintillation counter
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X-ray spectroscopy
X-ray spectroscopy is a general term for several spectroscopic techniques for characterization of materials by using x-ray radiation.
Characteristic X-ray spectroscopy
When an electron from the inner shell of an atom is excited by the energy o ...
References
External links
Mössbauer Effect Data Center page, including periodic table of Mössbauer isotopesIntroduction to Mössbauer Spectroscopy — RSC siteMössbauer Spectroscopy: A Powerful Tool in Scientific Research"Mossbauer Spectroscopy – A Rewarding Probe of Morphological Structure of Semiconducting Glasses ", P. Boolchand in Physical Properties of Amorphous Materials (Institute for Amorphous Studies Series), Springer US, Eds.: David Adler, Brian B. Schwartz, Martin C. Steele
The program MossA provides a straightforward approach to the fitting of 57Fe conventional and synchrotron energy-domain Mössbauer spectraMossA is written in the MATLAB programming language. The source code can be obtained from its github repositoryMössbauer Spectroscopy – Principles and Applications – Prof. Dr. Philipp Gütlich Emeritus Professor Mainz University – Institut für Anorganische Chemie und Analytische Chemie Johannes Gutenberg-Universität Mainz
{{DEFAULTSORT:Mossbauer spectroscopy
Scientific techniques