Mössbauer spectroscopy is a technique based on an effect discovered by Rudolf Ludwig Mössbauer in 1958. The peculiarity is that the method consists in the return of resonant absorption and emission of gamma rays in solids.
Like magnetic resonance, Mössbauer spectroscopy examines tiny changes in the energy levels of an atomic nucleus in response to its environment. Generally, three types of interactions can be observed:
- isomer shift, formerly also called chemical shift;
- quadrupole splitting;
- ultrafine splitting
Due to the high energy and extremely narrow linewidth of gamma rays, Mössbauer spectroscopy is a very sensitive technique in terms of energy (and therefore frequency) resolution.
Basic Principle
Like a gun bounces when fired, maintaining momentum requires the core (e.g. in a gas) to recoil as it emits or absorbs gammaradiation. If an atom at rest emits a beam, its energy is less than the natural transition force. But in order for the core to absorb the gamma ray at rest, the energy would have to be slightly greater than the natural force, because in both cases the thrust is lost during the recoil. This means that nuclear resonance (the emission and absorption of the same gamma radiation by identical nuclei) is not observed with free atoms, because the energy shift is too large and the emission and absorption spectra do not have significant overlap.
Nucles in a solid crystal cannot bounce because they are bound by a crystal lattice. When an atom in a solid emits or absorbs gamma radiation, some energy may still be lost as a necessary recoil, but in this case it always occurs in discrete packets called phonons (quantized vibrations of the crystal lattice). Any integer number of phonons can be emitted, including zero, which is known as a "no recoil" event. In this case, conservation of momentum is performed by the crystal as a whole, so there is little to no energy loss.
Interesting discovery
Moessbauer found that a significant portion of the emission and absorption events will be without returns. This fact makes Mössbauer spectroscopy possible, since it means that gamma rays emitted by a single nucleus can be resonantly absorbed by a sample containing nuclei with the same isotope - and this absorption can be measured.
The recoil fraction of absorption is analyzed using nuclearresonant oscillatory method.
Where to conduct Mössbauer spectroscopy
In its most common form, a solid sample is exposed to gamma radiation and the detector measures the intensity of the entire beam that has passed through the standard. The atoms in the source emitting gamma rays must have the same isotope as in the sample that absorbs them.
If the radiating and absorbing nuclei were in the same chemical environment, the nuclear transition energies would be exactly equal, and resonant absorption would be observed with both materials at rest. The difference in the chemical environment, however, causes the nuclear energy levels to shift in several different ways.
Reach and pace
During the Mössbauer spectroscopy method, the source is accelerated over a range of velocities using a linear motor to obtain the Doppler effect and scan the gamma ray energy in a given interval. For example, a typical range for 57Fe could be ±11 mm/s (1 mm/s=48.075 neV).
It is easy to carry out Mössbauer spectroscopy there, where in the obtained spectra the intensity of gamma rays is presented as a function of the source rate. At velocities corresponding to the resonant energy levels of the sample, some of the gamma rays are absorbed, which leads to a drop in the measured intensity and a corresponding dip in the spectrum. The number and position of the peaks provide information about the chemical environment of the absorbing nuclei and can be used to characterize the sample. Therebythe use of Mössbauer spectroscopy made it possible to solve many problems of the structure of chemical compounds, it is also used in kinetics.
Choosing an appropriate source
The desired gamma ray base consists of a radioactive parent that decays to the desired isotope. For example, the source 57Fe consists of 57Co, which is fragmented by capturing an electron from an excited state from 57 Fe. It, in turn, decays into the main position of the radiating gamma ray of the corresponding energy. Radioactive cob alt is prepared on foil, often rhodium. Ideally, the isotope should have a convenient half-life. In addition, the energy of the gamma radiation must be relatively low, otherwise the system will have a low non-recoil fraction, resulting in a poor ratio and a long collection time. The periodic table below shows the elements that have an isotope suitable for MS. Of these, 57Fe is today the most common element studied using this technique, although SnO₂ (Mössbauer spectroscopy, cassiterite) is also often used.
Analysis of Mössbauer spectra
As described above, it has extremely fine energy resolution and can detect even slight changes in the nuclear environment of the corresponding atoms. As noted above, there are three types of nuclear interactions:
- isomer shift;
- quadrupole splitting;
- ultrafine splitting.
Isomeric shift
The isomer shift (δ) (also sometimes called chemical) is a relative measure describing the shift in the resonant energy of a nucleus due to the transfer of electrons within its s-orbitals. The entire spectrum is shifted in a positive or negative direction, depending on the charge density of the s-electron. This change is due to changes in the electrostatic response between orbiting electrons with a non-zero probability and the nucleus with a non-zero volume that they spin.
Example: when tin-119 is used in Mössbauer spectroscopy, then the detachment of a divalent metal in which the atom donates up to two electrons (the ion is designated Sn2+), and the connection of a four-valent (ion Sn4+), where the atom loses up to four electrons, have different isomeric shifts.
Only s-orbitals show a completely non-zero probability, because their three-dimensional spherical shape includes the volume occupied by the nucleus. However, p, d and other electrons can affect the density s through the screening effect.
Isomer shift can be expressed using the formula below, where K is the nuclear constant, the difference between Re2 and R g2 - effective nuclear charge radius difference between the excited state and the ground state, as well as the difference between [Ψs 2(0)], a and [Ψs2(0)]b difference of electron density on the nucleus (a=source, b=sample). Chemical shiftThe isomer described here does not change with temperature, but Mössbauer spectra are particularly sensitive due to a relativistic result known as the second-order Doppler effect. As a rule, the influence of this effect is small, and the IUPAC standard allows isomer shift to be reported without correcting it at all.
Explanation with an example
The physical meaning of the equation shown in the image above can be explained with examples.
While an increase in the density of s-electrons in the spectrum of 57 Fe gives a negative shift, since the change in the effective nuclear charge is negative (due to Re <Rg), an increase in the density of s-electrons in 119 Sn gives a positive shift due to a positive change in the total nuclear charge (due to R e> Rg).
Oxidized ferric ions (Fe3+) have smaller isomer shifts than ferrous ions (Fe2+) because the density of s-electrons in the core of ferric ions is higher due to the weaker shielding effect of d-electrons.
Isomer shift is useful for determining oxidation states, valence states, electron shielding, and the ability to withdraw electrons from electronegative groups.
Quadrupole splitting
Quadrupole splitting reflects the interaction between nuclear energy levels and the ambient electric field gradient. Nuclei in states with a non-spherical charge distribution, i.e., all those in which the angular quantum number is greater than 1/2, have a nuclear quadrupole moment. In this case, an asymmetric electric field (produced by an asymmetric electronic charge distribution or ligand arrangement) splits the nuclear energy levels.
In the case of an isotope with an excited state of I=3/2, such as 57 Fe or 119 Sn, the excited state is split into two substates: mI=± 1/2 and mI=± 3/2. Transitions from one state to an excited state appear as two specific peaks in the spectrum, sometimes referred to as a "doublet". Quadrupole splitting is measured as the distance between these two peaks and reflects the nature of the electric field in the nucleus.
Quadrupole splitting can be used to determine the oxidation state, state, symmetry and arrangement of ligands.
Magnetic ultrafine splitting
It is the result of the interaction between the nucleus and any surrounding magnetic field. A nucleus with spin I splits into 2 I + 1 subenergy levels in the presence of a magnetic field. For example, a nucleus with spin state I=3/2 will split into 4 non-degenerate substates with values mI +3/2, +1/2, - 1/2 and −3/2. Each partition is hyperfine, on the order of 10-7 eV. The selection rule for magnetic dipoles means that transitions between the excited state and the ground state can only occur where m changes to 0 or 1. This gives 6 possible transitions to go from3/2 to 1/2. In most cases, only 6 peaks can be observed in the spectrum produced by hyperfine splitting.
The degree of splitting is proportional to the intensity of any magnetic field on the nucleus. Therefore, the magnetic field can be easily determined from the distance between the outer peaks. In ferromagnetic materials, including many iron compounds, natural internal magnetic fields are quite strong and their effects dominate the spectra.
Combination of everything
Three main Mössbauer parameters:
- isomer shift;
- quadrupole splitting;
- ultrafine splitting.
All three items can often be used to identify a particular compound by comparing against standards. It is this work that is done in all laboratories of Mössbauer spectroscopy. A large database, including some of the published parameters, is maintained by the data center. In some cases, a compound may have more than one possible position for a Mössbauer active atom. For example, the crystal structure of magnetite (Fe3 O4) maintains two different locations for iron atoms. Its spectrum has 12 peaks, a sextet for each potential atomic site corresponding to two sets of parameters.
Isomeric shift
The Mössbauer spectroscopy method can be implemented even when all three effects are observed many times. In such cases, the isomeric shift is given by the average of all lines. quadrupole splitting when all fourexcited substates are equally biased (two substates are up and the other two are down) is determined by the offset of the two outer lines relative to the inner four. Usually, for precise values, for example, in the laboratory of Mössbauer spectroscopy in Voronezh, suitable software is used.
In addition, the relative intensities of the various peaks reflect the concentrations of compounds in the sample and can be used for semi-quantitative analysis. Because ferromagnetic phenomena are magnitude dependent, in some cases spectra can give insight into the size of crystallites and the grain structure of the material.
Mossbauer spectroscopy settings
This method is a specialized variant, where the emitting element is in the test sample, and the absorbing element is in the standard. Most often, this method is applied to the pair 57Co / 57Fe. A typical application is the characterization of cob alt sites in amorphous Co-Mo catalysts used in hydrodesulfurization. In this case, the sample is doped with 57Ko.
