Mössbauer spectroscopy is an effect-based method discovered by Rudolph Ludwig Mössbauer in 1958. The peculiarity lies in the fact 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 the atomic nucleus in response to the environment. As a rule, three types of interactions can be observed:
- isomeric shift, also called previously chemical;
- quadrupole splitting;
- ultrafine splitting
Due to its high energy and extremely narrow gamma-ray line width, Mössbauer spectroscopy is a very sensitive method in terms of energy (and therefore frequency) resolution.
The basic principle
Like a shotgun bounces when fired, to maintain momentum it is required that the core (for example, in a gas) recoil during radiation or absorption of gamma radiation. If an atom at rest emits a ray, its energy is less than the natural transition force. But in order for the core to absorb the gamma ray at rest, the energy must be a little more than the natural force, because in both cases the pressure is lost during the recoil. This means that nuclear resonance (emission and absorption of the same gamma radiation by the same nuclei) is not observed with free atoms, since the energy shift is too large and the emission and absorption spectra do not have a significant overlap.
Nuclei in a solid crystal cannot bounce off because they are connected by a crystal lattice. When an atom in a solid emits or absorbs gamma radiation, some energy can still be lost as a required return, but in this case it always occurs in discrete packets called phonons (quantized crystal lattice vibrations). Any integer number of phonons can be emitted, including zero, which is known as the “no return” event. In this case, the conservation of momentum is performed by the crystal as a whole, so the energy is practically not lost.
Interesting discovery
Mossbauer found that a significant portion of emission and absorption events would be without recoil. This fact makes Mössbauer spectroscopy possible, since this 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 absorption recoil fraction is analyzed using the nuclear resonance vibrational 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 passing through the standard. Atoms in the source emitting gamma rays must have the same isotope as in the sample that absorbs them.
If the emitting and absorbing nuclei were in the same chemical media, the energies of the nuclear transitions were exactly equal, and resonant absorption would be observed with both materials at rest. The difference in the chemical environment, however, causes a shift in the levels of nuclear energy in several different ways.
Reach and pace
During the method of Mössbauer spectroscopy, the source is accelerated in the speed range using a linear motor to obtain the Doppler effect and scan the energy of gamma radiation in a given interval. For example, a typical range for 57 Fe may be ± 11 mm / s (1 mm / s = 48.075 neV).
It is easy to conduct Mössbauer spectroscopy there, where in the obtained spectra the intensity of gamma rays is presented as a function of the source tempo. At speeds corresponding to the resonant energy levels of the sample, part 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 on the chemical environment of the absorbing nuclei and can be used to characterize the sample. Thanks to this, the use of Mössbauer spectroscopy made it possible to solve many problems of the structure of chemical compounds; it is also used in kinetics.
Choosing the Right Source
The desired gamma radiation base consists of a radioactive parent that decays to the desired isotope. For example, a 57 Fe source consists of 57 Co, which is fragmented by trapping an electron from an excited state from 57 Fe. It, in turn, splits into the main position of the emitting gamma ray of the corresponding energy. Radioactive cobalt is prepared on foil, often from rhodium. Ideally, the isotope should have a convenient half-life. In addition, the energy of gamma radiation should be relatively low, otherwise the system will have a low proportion without recoil, which will lead to a poor ratio and a long collection time. The periodic table below shows elements having an isotope suitable for MS. Of these, 57 Fe is today the most common element studied using this technique, although SnO₂ (Mössbauer spectroscopy, cassiterite) is also often used.
Mössbauer Spectrum Analysis
As described above, it has an extremely fine energy resolution and can detect even minor changes in the nuclear environment of the corresponding atoms. As noted above, there are three types of nuclear interactions:
- isomeric shift;
- quadrupole splitting;
- ultrafine splitting.
Isomer shift
The shift of the isomer (δ) (also sometimes called chemical) is a relative measure describing the shift in the resonance energy of a nucleus due to the transition 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 non-zero probability and a nucleus with a non-zero volume that they rotate.
Example: when tin-119 is used in Mössbauer spectroscopy, detachments of a divalent metal, in which an atom gives up two electrons (an ion is Sn 2+ ), and four valence compounds (Sn 4+ ion), where an atom loses up to four electrons, have various isomeric shifts.
Only on s-orbitals demonstrate a completely nonzero probability, because their three-dimensional spherical shape includes the volume occupied by the nucleus. However, p, d, and other electrons can influence the density s through the screening effect.
The isomer shift can be expressed using the formula below, where K is the nuclear constant, the difference between R e 2 and R g 2 is the effective difference in the radius of the nuclear charge between the excited state and the ground position, as well as the difference between [Ψ s 2 (0)], a and [Ψ s 2 (0)] b is the difference in electron density at the core (a = source, b = sample). The shift of the chemical isomer described here does not change with temperature, however, the Mössbauer spectra are especially sensitive due to the relativistic result, known as the second-order Doppler effect. As a rule, the effect of this effect is small, and the IUPAC standard allows reporting an isomeric shift without correcting it at all.
Example explanation
The physical meaning of the equation shown in the image above can be explained using 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 charge of the nucleus is negative (due to R e <R g ), 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 > R g ).
Oxidized ferric ions (Fe 3+ ) have smaller isomer shifts than ferrous ions (Fe 2+ ), since the density of s electrons in the core of ferric ions is higher due to the weaker screening effect of d electrons.
Isomer shift is useful for determining the degree of oxidation, valence states, electron screening, and the ability to pull electrons of electronegative groups.
Quadrupole splitting
Quadrupole splitting reflects the interaction between the levels of nuclear energy and the gradient of the surrounding electric field. Nuclei in states with a non-spherical charge distribution, i.e., all those with an angular quantum number greater than 1/2, have a nuclear quadrupole moment. In this case, an asymmetric electric field (created by an asymmetric electronic charge distribution or arrangement of ligands) splits the levels of nuclear energy.
In the case of an isotope with an excited state I = 3/2, such as 57 Fe or 119 Sn, the excited state is divided into two substates: m I = ± 1/2 and m I = ± 3/2. Transitions from one state to an excited state manifest themselves in the form of two specific peaks in the spectrum, which are sometimes called the “doublet”. Quadrupole splitting is measured as the distance between these two peaks and reflects the nature of the electric field in the nucleus.
Quadrupole cleavage can be used to determine the degree of oxidation, state, symmetry and arrangement of ligands.
Magnetic ultrafine splitting
It is the result of the interaction between the core and any surrounding magnetic field. A nucleus with spin I in the presence of a magnetic field splits into 2 I + 1 subenergy levels. For example, a nucleus with a spin state of I = 3/2 will split into 4 non-degenerate substates with values of m I +3/2, +1/2, - 1/2 and −3/2. Each partition is hyperfine, of the order of 10 -7 eV. The rule for selecting 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 for the transition from 3/2 to 1/2. In most cases, only 6 peaks can be observed in the spectrum created by hyperfine splitting.
The degree of splitting is proportional to the strength of any magnetic field on the core. Therefore, the magnetic field can be easily determined by the distance between the external peaks. In ferromagnetic materials, including many iron compounds, natural internal magnetic fields are quite strong, and their effects dominate in the spectra.
Combination of everything
Three main Mössbauer parameters:
- isomeric shift;
- quadrupole splitting;
- ultrafine splitting.
All three points can often be used to identify a particular compound by comparison for standards. It is this work that is being 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, the compound may have more than one possible position for the Mössbauer active atom. For example, the crystal structure of magnetite (Fe 3 O 4 ) supports two different sites for iron atoms. Its spectrum has 12 peaks, a sextet for each potential atomic site, corresponding to two sets of parameters.
Isomeric displacement
The Mössbauer spectroscopy method can also be implemented when all three effects are observed many times. In such cases, isomeric displacement is given by the average of all lines. Quadrupole splitting, when all four excited substates are shifted equally (two substates are raised and the other two are lowered) is determined by the displacement of two outer lines relative to the inner four. Usually, suitable software is used for accurate values, for example, at the Mössbauer spectroscopy laboratory in Voronezh.
In addition, the relative intensities of the various peaks reflect the concentration of compounds in the sample and can be used for semi-quantitative analysis. Since ferromagnetic phenomena depend on the magnitude, in some cases the spectra can give an idea of the crystallite size and grain structure of the material.
Mossbauer Spectroscopy Settings
This method is a specialized option, where the radiating element is in the test sample, and the absorbing element is in the standard. Most often, this method is applied to a pair of 57 Co / 57 Fe. A typical application is characterization of cobalt sites in amorphous Co-Mo catalysts used in hydrodesulfurization. In this case, the sample is doped with 57 Ko.