Mössba uera effe kt, resonant absorption of g-quanta, observed when the source and absorber of g-radiation are solids, and the energy of g-quanta is low (~ 150 keV). Sometimes the Mössbauer effect is called resonant absorption without recoil, or nuclear gamma resonance (NGR).

In 1958, R. Mössbauer discovered that for nuclei that are part of solids, at low energies of g-transitions, the emission and absorption of g-quanta can occur without loss of energy due to recoil. In the emission and absorption spectra, unshifted lines with an energy exactly equal to the energy of the g transition are observed, and the widths of these lines are equal to (or very close to) the natural width G. In this case, the emission and absorption lines overlap, which makes it possible to observe the resonant absorption of gamma rays.

This phenomenon, called the Mössbauer effect, is due to the collective nature of the movement in solid body. Due to the strong interaction in solids, the recoil energy is not transferred to a separate nucleus, but is converted into the energy of vibrations of the crystal lattice, in other words, recoil leads to the birth of phonons. But if the recoil energy (calculated per nucleus) is less than the average phonon energy characteristic of a given , then recoil will not lead to the birth of a phonon every time. In such “phononless” cases, the recoil does not change. The kinetic energy that is acquired as a whole, perceiving the recoil impulse of the g-quantum, is negligible. The transfer of momentum in this case will not be accompanied by the transfer of energy, and therefore the position of the emission and absorption lines will exactly correspond to the energy E of the transition.

The probability of such a process reaches several tens of percent if the energy of the g transition is sufficiently low; In practice, the Mössbauer effect is observed only at D E » 150 keV (with increasing E, the probability of phonon production during recoil increases). The probability of the Mössbauer effect also strongly depends on . Often, to observe the Mössbauer effect, it is necessary to cool the gamma quanta source and absorber to liquid or liquid, however for g-transitions of very low energies (for example, E = 14.4 keV for the g-transition of the 57 Fe nucleus or 23.8 kev for g- transition of the 119 Sn nucleus) the Mössbauer effect can be observed up to temperatures exceeding 1000 °C. All other things being equal, the probability of the Mössbauer effect is greater, the stronger the interaction in a solid, i.e., the greater the phonon energy. Therefore, the greater the probability of the Mössbauer effect, the higher the .

The essential property of resonant absorption without recoil, which turned the Mössbauer effect from a laboratory experiment into important method research, is the extremely small line width. The ratio of the line width to the energy of the g-quantum with the Mössbauer effect is, for example, for 57 Fe nuclei the value "3´ 10 -13", and for 67 Zn nuclei "5.2´ 10 -16". Such line widths have not been achieved even in gas, which is the source of the narrowest lines in the infrared and visible range of electromagnetic waves. With the help of the Mössbauer effect, it turned out to be possible to observe processes in which the energy of the g-quantum differs by an extremely small amount (»G or even small fractions of G) from the transition energy of the absorber nuclei. Such energy changes lead to a displacement of the emission and absorption lines relative to each other, which entails a change in the magnitude of the resonant absorption, which can be measured.

The capabilities of methods based on the use of the Mössbauer effect are well illustrated by an experiment in which it was possible to measure in laboratory conditions the change in the frequency of a quantum predicted by relativity theory electromagnetic radiation into the Earth's gravitational field. In this experiment (R. Pound and G. Rebki, USA, 1959), the source of g-radiation was located at a height of 22.5 m above the absorber. The corresponding change in the gravitational potential should have led to a relative change in the energy of the g-quantum by 2.5´ 10 -15. The shift of the emission and absorption lines turned out to be in accordance with the theory.

Under the influence of internal electric and magnetic fields acting on nuclei in solids (see), as well as under the influence of external factors (external magnetic fields), shifts and splitting of nuclear energy levels can occur, and, consequently, changes in the transition energy. Since the magnitude of these changes is related to the microscopic structure of solids, studying the displacement of emission and absorption lines makes it possible to obtain information about the structure of solids. These shifts can be measured using Mössbauer spectrometers ( rice. 3). If g -quanta are emitted by a source moving with a speed v relative to the absorber, then as a result of the Doppler effect, the energy of g -quanta incident on the absorber changes by the amount Ev/c (for nuclei usually used in observing the Mössbauer effect, the change in energy E by the amount G corresponds to speed values ​​v from 0.2 to 10 mm/sec). By measuring the dependence of the magnitude of resonant absorption on v (spectrum of Mössbauer resonant absorption), one finds the velocity value at which the emission and absorption lines are in exact resonance, i.e., when absorption is maximum. The value of v determines the shift D E between the emission and absorption lines for a stationary source and absorber.

On rice. 4, and shows an absorption spectrum consisting of one line: the emission and absorption lines are not shifted relative to each other, that is, they are in exact resonance at v = 0. The shape of the observed line can be described with sufficient accuracy by the Lorentz curve (or Breit - Wigner formula) with a width at half height of 2G. Such a spectrum is observed only in the case when the source and absorber are chemically identical and when the nuclei in these are not affected by either magnetic or inhomogeneous electric fields. In most cases, several lines (hyperfine structure) are observed in the spectra, caused by interaction with extranuclear electric and magnetic fields. The characteristics of the hyperfine structure depend both on the properties of nuclei in the ground and excited states, and on the structural features of solids, which include emitting and absorbing nuclei.

The most important types of interactions with extranuclear fields are electric monopole, electric quadrupole and magnetic dipole interactions. Electric monopole interaction is the interaction of a nucleus with an electrostatic field created in the region of the nucleus by those surrounding it; it leads to the appearance of a line shift d in the absorption spectrum ( rice. 4, b), if the source and absorber are not chemically identical or if the distribution of electric charge in the nucleus is unequal in the ground and excited states (see). This so-called the isomeric or chemical shift is proportional in the region of the nucleus, and its magnitude is an important characteristic in solids (see). By the magnitude of this shift one can judge the ionic and covalent character, the contents of the composition, etc. The study of chemical shifts also allows one to obtain information about the charge distribution in .

An important characteristic of the Mössbauer effect for solid state physics is also its probability. Measuring the probability of the Mössbauer effect and its dependence on the atoms of the isotopes of 41 elements; the lightest among them is 40 K, the heaviest is 243 At.

Lit.: Mossbauer effect. Sat. Art., ed. Yu. Kagana, M., 1962; Mössbauer R., The RK effect and its significance for precise measurements, in the collection: Science and Humanity, M., 1962; Frauenfelder G., The Mossbauer Effect, trans. from English, M., 1964; Wertheim G., The Mossbauer Effect, trans. from English, M., 1966; V.S., Resonance of gamma rays in, M., 1969; Chemical Applications, trans. from English, ed. V. I. Goldansky [and others], M., 1970; Mossbauer effect. Sat. translations of articles, ed. N. A. Burgov and V. V. Sklyarevsky, trans. from English, German, M., 1969.

N. N. Delyagin.

Rice. 3. Simplified diagram of a Mössbauer spectrometer; The source of g-quanta, using a mechanical or electrodynamic device, is set into reciprocating motion at a speed v relative to the absorber. Using a g-radiation detector, the dependence on the speed v of the intensity of the flux of g-quanta passing through the absorber is measured.

Rice. 4. Spectra of Mössbauer resonant absorption of g-quanta: I - intensity of the flux of g-quanta passing through the absorber, v - speed of movement of the source of g-quanta; a - single emission and absorption lines, not shifted relative to each other at v = 0; b - isomeric or chemical shift of the line. The shift d is proportional in the core region and varies depending on the features in the solid; c - quadrupole doublet observed for 57 Fe, 119 Sn, 125 Te, etc. The magnitude of splitting D is proportional to the electric field gradient in the core region: d - magnetic hyperfine structure observed in absorption spectra for magnetically ordered materials. The distance between the components of the structure is proportional to the strength of the magnetic field acting on the nuclei in the solid.

Rice. 1. Schematic representation of the processes of emission and resonant absorption of g-quanta; The emitting and absorbing nuclei are the same, therefore the energies of their excited states E" and E"" are equal.

Rice. 2. Displacement of emission and absorption lines relative to the energy of the E g transition; G - line widths.

MÖSSBAUER EFFECT
and its application in chemistry

A new phenomenon discovered in 1958 by the German physicist Rudolf Ludwig Mössbauer - the resonant absorption of gamma quanta by the atomic nuclei of solids without changing the internal energy of the body (or without losing part of the quantum energy due to the recoil of the nucleus in a solid) - was called the Mössbauer effect and led to the creation a completely new direction of research in science. The main areas of application of this effect were solid state physics and chemistry.

Background of the issue

The ideological foundations of gamma resonance spectroscopy began to take shape a long time ago, and its development, of course, was influenced by the fundamental concepts of optical spectroscopy, especially advances in the field of so-called resonance fluorescence.
Since the 1850s It was known that some gases, liquids and solids (for example, fluoride compounds) absorb electromagnetic radiation (usually visible light) and immediately emit it again (a phenomenon called fluorescence). In a special case known as resonant fluorescence, the absorbed and emitted radiation have the same energy, wavelength and frequency.
The first assumptions about the existence of resonant scattering in atoms appeared in the works of the English physicist J. W. Rayleigh, and the first experiments in this direction were carried out by the famous American experimental physicist R. W. Wood in 1902–1904. He used mechanical analogies to explain resonant scattering.
The phenomenon of resonant fluorescence was well explained by the theory of N. Bohr (quantum model of the atom) that replaced the old ideas. An atom transitioning from an excited state IN to the ground state A, emits a photon of a strictly defined frequency. When such a photon passes through a gas consisting of the same atoms as the emitter, it can be absorbed, causing one of the target atoms to transition to the state IN. After a short period of time, this excited target atom in turn decays, emitting a photon of the same frequency. Thus, the primary and secondary radiation have the same frequency, but the processes of absorption and subsequent emission of a photon are independent, and there is no specific phase relationship between the incident and emitted waves.
Many aspects of the phenomenon of resonant radiation were correctly described on the basis of Bohr's theory and quantum mechanics, which then began to develop. A complete description of the processes of emission, absorption and resonant fluorescence was carried out somewhat later, in the late 1920s and early 1930s. German physicists W.F. Weiskopf and J.P. Wigner.
The idea that the energy levels of nuclei are similar to the electronic levels of atoms and transitions between them, according to Bohr’s postulate, are accompanied by radiation or absorption, was first voiced in the works of the English physicist C.D. Ellis in the early 1920s. At the end of the 1920s. The search for the corresponding nuclear resonance fluorescence was undertaken by the Swiss photochemist Werner Kuhn, who had been working in Germany since 1927. He showed that the phenomena of atomic and nuclear resonance fluorescence seem extremely similar, but there are significant differences between them that make experiments on nuclei much more complex.
As a result, only in 1950 were scientists finally able to carry out a successful experiment on gold-198 nuclei for the first time and understand the obstacles that existed along this path. This problem was finally solved only by Mössbauer.

Mössbauer's discovery

What exactly the problem was and how it was solved by Mössbauer will be more obvious if we look at the structure of the kernel.
Among the many theoretical constructions, the stereotype of the Bohr atomic model attracts attention - the “shell” model of the atomic nucleus M. Goeppert-Mayer and H. Jensen, laureates Nobel Prize in physics for 1963. According to this model, nucleons in the nucleus are located at certain energy levels, mainly in pairs with antiparallel spins (Pauli principle), and transitions between levels are accompanied by the emission or absorption of gamma quanta. Unlike the electronic levels of the states of atoms or molecules, the excited states of nuclei do not live long (on the order of the characteristic “nuclear time” ~ 10–23 s), and, therefore, the uncertainty in the energy of the levels should be very large in accordance with the Heisenberg uncertainty principle.
All this would be of significance only for nuclear physics, but not for structural organic chemistry, and, probably, not for chemistry in general, if not for one important circumstance. Namely: there are also long-lived excited nuclei, the excess energy of which does not manifest itself nearly as quickly as during ordinary transitions of nucleons from one state to another. Such kernels are called isomers, they have the same charge and mass numbers, but different energy and different lifetime. Nuclear isomerism was discovered by O. Gan (1921) while studying the beta decay of thorium-234 and I. V. Kurchatov with his colleagues L. V. Mysovsky and L. I. Rusinov when observing artificial radioactivity of bromine nuclei (1935–1936). The theory of nuclear isomerism was developed by K.F. von Weizsäcker in 1936
It is the lifetime of metastable states of nuclei (isomers) that plays a key role in the formation of spectral lines of gamma spectroscopy. According to the same Heisenberg uncertainty principle, the uncertainty in the energy of the levels, and therefore the natural width of the spectral line, should be extremely small. In particular, a simple calculation using the example of the iron-57 isotope shows a negligible value, on the order of 5–10–9 eV. It was this unprecedented narrowness of the spectral lines that caused the failure of all work before Mössbauer.
Scientist in his famous work entitled “Resonant absorption of gamma quanta in solids without recoil” wrote about this: “Gamma quanta emitted during the transition of a nucleus from an excited state to the ground state are usually not suitable for transferring the same nucleus from the ground state to excited by the reverse process of resonant absorption. This is a consequence of the recoil energy loss that the -quantum experiences during the process of emission or absorption due to the fact that it transfers the recoil momentum to the emitting or absorbing atom. These energy losses due to recoil are so great that the emission and absorption lines are significantly shifted relative to each other.” As a result, resonant absorption (or fluorescence), as he noted, is usually not observed in X-rays. In order to make resonant absorption of gamma rays observable, it is obvious that conditions must be artificially created so that the emission and absorption lines overlap.
Thus, already in 1951, P.B. Moon from the University of Birmingham (England) proposed to compensate for the recoil of nuclei during radiation by mechanically moving the source as it moves towards the receiver nuclei. In this case, the kinetic energy of the source motion is added to the energy of the gamma quantum, and, therefore, it is possible to select a speed at which the resonance condition is completely restored. But a few years later, Mössbauer unexpectedly found a simpler way to solve this problem, in which recoil loss was prevented from the very beginning. The scientist achieved the fluorescence of gamma rays using atoms of the radioactive isotope of the metal iridium-191 as their source.
Iridium is a crystalline solid, so the emitting and absorbing atoms occupy a fixed position in the crystal lattice. After cooling the crystals with liquid nitrogen, Mössbauer was surprised to find that the fluorescence increased markedly. Studying this phenomenon, he found that individual nuclei emitting or absorbing gamma rays transmit the interaction impulse directly to the entire crystal. Since the crystal is much more massive compared to the nucleus, due to the strong interaction of atoms in solids, the recoil energy is not transferred to an individual nucleus, but is converted into vibrational energy of the crystal lattice, as a result, no frequency shift is observed in the emitted and absorbed photons. In this case, the emission and absorption lines overlap, which makes it possible to observe the resonant absorption of gamma rays.
This phenomenon, which Mössbauer called “elastic nuclear resonant absorption of gamma radiation,” is now called the Mössbauer effect. Like any effect that occurs in a solid, it depends on the crystal structure of the substance, temperature and even the presence of the smallest impurities. The scientist also showed that suppressing nuclear recoil using the phenomenon he discovered makes it possible to generate gamma rays, the wavelength of which is constant to within one billionth ( = 10–9 cm). In Fig. Figure 1 shows a diagram of its experimental setup.
In fact Full description The Mössbauer effect requires the use of knowledge from various branches of quantum mechanics, so in this article we focused only on the most general provisions his approach.

In subsequent experiments (following iridium, other objects were studied: 187 Re, 177 Hf, 166 Er, 57 Fe and 67 Zn, in which resonant absorption without recoil was also observed), Mössbauer finally confirmed the correctness of the explanation of the effect of resonant gamma fluorescence without recoil observed by him and at the same time provided the basis for the experimental methodology for all subsequent studies of this phenomenon.
By studying the displacements of emission and absorption lines, one can obtain extremely useful information about the structure of solids. Shifts can be measured using Mössbauer spectrometers (Fig. 2).

The source of gamma quanta, using a mechanical or electrodynamic device, is set into reciprocating motion at a speed relative to the absorber. Using a gamma radiation detector, the speed dependence of the intensity of the flow of gamma quanta passing through the absorber is measured.
All experiments on observing Mössbauer spectra come down to observing the dependence of absorption (less often, scattering) of gamma rays in the sample under study on the speed of movement of this sample relative to the source. Without going into details of the design of various experimental setups, it should be noted that classic scheme A Mössbauer spectrometer includes the following main elements: a radiation source, an absorber, a system for moving the source relative to the absorber, and a detector.

General Applications method

After the publication of Mössbauer's first paper, it took about a year before other laboratories began to repeat and expand his experiments. The first verification experiments were carried out in the USA (Los Alamos Scientific Laboratory and Argonne National Laboratory). Moreover, interestingly, research at the Los Alamos Laboratory began with a bet between two physicists, one of whom did not believe in Mössbauer’s discovery, and the other repeated his experiment and thus won the bet (they observed a gamma line at 67 Zn). A significant increase in publications on this topic was observed after the discovery of the Mössbauer effect in 57 Fe, carried out independently also at Harvard University, Argonne National Laboratory, etc. The ease with which the effect can be observed in 57 Fe, its enormous magnitude and its presence up to temperatures exceeding 1000 °C, made this area of ​​research accessible even to laboratories with very modest equipment.
Physicists soon discovered that using the Mössbauer effect, it was possible to determine the lifetimes of excited states of nuclei and the sizes of the nuclei themselves, the exact values ​​of magnetic and electric fields near emitter-nuclei, and the phonon spectra of solids. For chemists, the most important two parameters turned out to be the chemical shift of the resonance signal and the so-called quadrupole splitting.
As a result, in solid state physics, research using the Mössbauer effect of the magnetic structure and magnetic properties of elements and compounds, especially alloys, has received the greatest development. Particularly noticeable progress in this direction has been achieved in work on rare earth elements. The second most important area of ​​research was the study of crystal lattice dynamics.
Things were completely different in chemistry. As it turned out, using gamma resonance spectroscopy signals, it is possible to draw certain conclusions about the electric field at the center of the atom and solve typical chemistry problems related to the nature of the chemical bond. Mössbauer spectroscopy made it possible to solve many questions about the structure of chemical compounds; it has found its application in chemical kinetics and radiation chemistry. This method has proven to be indispensable in determining the structures of biological macromolecules with particularly large molecular weights.
It should be added to this that gamma resonance spectroscopy has proven to have incredibly high sensitivity (5–6 orders of magnitude higher than nuclear magnetic resonance), therefore, one can understand the excitement of chemists in the early 1960s and 1970s. Passions, however, subsided a little when the chemists got used to the situation and found out the limitations in using the method. In particular, V.I. Goldansky, in his book devoted to the applications of the Mössbauer effect in chemistry, wrote: “The main objects of application of the Mössbauer effect in chemistry, apparently, are organoelement compounds and complex compounds. In the field of organoelement compounds, the comparison of general element-carbon bonds, which differs greatly between transition metals and main group metals.” But 30 years have passed since then, and gamma resonance spectroscopy has confirmed its promising use for a wide variety of purposes and objects of chemistry.

Chemical applications of the method

The position of the resonant signal depends on the electronic environment in which the nucleus emitting the quantum is located. Obtaining a new type of physical information about the electronic environment of nuclei has undoubtedly always been of significant interest for chemistry.
Resolving issues of the nature of chemical bonds and the structure of chemical compounds. Since the main parameters of Mössbauer spectra - such as chemical shifts and quadrupole splittings - are largely determined by the structure of the valence electron shells of atoms, the first natural possibility for the chemical application of this effect was to study the nature of the bonds of these atoms. In this case, the simplest approach to the problem was to distinguish between two types of bonds - ionic and covalent - and evaluate the contribution of each of them. But it should be noted that this is the simplest approach, since we should not forget that the very distinction between chemical bonds into ionic and covalent is a rather gross simplification, since it does not take into account educational opportunities, for example, donor-acceptor bonds, bonds involving multicenter orbits (in polymers) and others discovered in recent decades.
A parameter such as a chemical shift can be correlated with the degree of oxidation of elemental atoms in the molecules of the substances under study. Correlation diagrams of isomeric (chemical) shifts of 57 Fe for iron compounds are especially well developed. As is known, iron is an integral part of many biosystems, in particular hemoproteins and systems of non-protein nature (for example, contained in microorganisms). In the chemistry of life processes, a significant role is played by the redox reactions of porphyrin iron complexes, in which iron is also found in various valence states. Biological function of these compounds can be revealed only when there is detailed information about the structure of the active center and the electronic states of iron at different stages of biochemical processes.
As mentioned above, important applications of the Mössbauer effect in chemistry are organoelement and complex compounds. In the field of organoelement compounds, a comparison of the general nature of elemental-carbon bonds, which are very different for transition metals and metals of the main groups (for example, the work of A.N. Nesmeyanov), was of significant interest.
Thus, using the Mössbauer effect, comparisons were made of acetylenide complexes of a number of transition metals. Particularly successful studies have been carried out for metal cyclopentadienylides M(C 5 H 4) 2, in particular ferrocene-like “sandwich” structures.
An important application of this effect is the elucidation of the structure of iron dodecacarbonyl. The results of preliminary X-ray diffraction studies showed that iron atoms are localized at the corners of the triangle in these molecules. That is why it took so long to reconcile these results with the Mössbauer spectra of iron dodecacarbonyl, since the latter excluded any symmetrical triangular structure. Repeated experiments simultaneously using the methods of X-ray diffraction analysis and Mössbauer spectroscopy showed that the choice can definitely be made only on linear structures.
We especially note the use of the Mössbauer effect in determining the structures of biomolecules. Currently, the structure of proteins is determined almost exclusively by X-ray diffraction on single crystals of these proteins (see about this: Direct methods in X-ray crystallography. Chemistry, 2003, No. 4).
However, this method has limitations due to molecular weight systems being studied. For example, the molecular weight of 150,000 g/mol, which gamma immunoglobulin has, is the upper limit for determining the structure by the method of successive isomorphic substitutions. For proteins with a higher molecular weight (for example, catalase, hemocyanin, tobacco mosaic virus, etc.), it is necessary to use other methods. It is here that the method of resonant scattering of gamma radiation without recoil on 57 Fe nuclei has successfully proven itself. This method uses interference between gamma radiation scattered on the electron shells of all atoms in the crystal and on some 57Fe nuclei embedded in the crystal at specific positions in the unit cell (Mössbauer scattering).
Chemical kinetics and radiation chemistry. Along with questions of the structure of chemical compounds, the Mössbauer effect is actively used in chemical kinetics and radiation chemistry. In addition to the possibility of directly obtaining kinetic curves entirely in one experiment (based on the frequency of samples at some fixed characteristic speed of movement), observations of unstable intermediate products are especially interesting here. When carrying out reactions in the liquid phase, it becomes necessary to stop the process by freezing the mixture for each observation of the Mössbauer spectrum. In the case of topochemical processes (especially for radiation-topochemical processes), continuous observation of changes in the Mössbauer spectrum during the reaction is possible.
Undoubtedly, other quite promising applications of the Mössbauer spectroscopy method should also be mentioned. First of all, this effect has become a useful tool for solving a number of problems in the physical chemistry of polymers, in particular the problem of polymer stabilization. It is also used as an analyzer in the tagged atom method. In particular, experiments were carried out to study the metabolism of iron included in the red blood cells of mammals and in the mitochondria of bacteria.

Afterword

Of course, the method of Mössbauer spectroscopy is not as widely used in chemical research as, for example, the well-known methods of NMR, infrared and mass spectroscopy. This is due both to the low availability and complexity of equipment, and to the limited range of objects and tasks to be solved. After all, the effect itself is observed on the nuclei of not all elements and isotopes9. However, its use is very relevant in combination with other research methods, especially radio spectroscopy.
In recent years, studies of Mössbauer spectra at high pressures have developed. Although the latter have a relatively weak effect on the electron shells of atoms, nevertheless, the parameters of the Mössbauer spectra measured depending on pressure carry new information on the interaction of the nucleus with the electronic environment. Compared to other methods, Mössbauer spectroscopy in high-pressure studies is even more sensitive to energy changes.

LITERATURE

R.L. Rckstossfreie Kernresonanzabsorption von Gammastrahlung. Nobelvortrag 11 December 1961. Le Prix Nobel en 1961. Stockholm: Impremerie Royale P.A.Norstedt & Sner, 1962,
S. 136–155;
Goldansky V.I.. Mossbauer effect. M.: Publishing House of the USSR Academy of Sciences, 1963;
Mössbauer R.L. Resonant nuclear absorption of -quanta in solids without recoil. Uspekhi Fizicheskikh Nauk, 1960, v. 72, no. 4, p. 658–671.

MÖSSBAUER Rudolf Ludwig(b. 31.I.1929) was born in Munich (Germany) in the family of photographic technician Ludwig Mössbauer and his wife Erna, née Ernst. Having received his initial secondary education in one of the Munich suburban schools (Pasing district), he then entered the gymnasium, which he graduated from in 1948.
Then Mössbauer worked for an optical company for one year and then, having submitted documents to the physics department of the Higher Technical School in Munich (now the Technical University), in 1949 he was enrolled as a student. In 1952 he received a bachelor's degree, in 1955 he completed a master's degree, and in 1958, after defending his dissertation, he received a doctorate of philosophy.
During runtime thesis in 1953–1954 the young man worked as a mathematics teacher at the Mathematical Institute in Alma Mater. After graduation, from 1955 to 1957 he was an assistant at the Institute of Physics of Medical Research named after. M. Planck in Heidelberg, and in 1959 he became an assistant Technical University in Munich.
The doctoral dissertation, in which the effect bearing his name was discovered, was carried out by the scientist under the guidance of the famous Munich physicist H. Mayer-Leibniz.
At first, the results obtained by Mössbauer were not supported by most scientists and were questioned. However, a year later, having recognized the potential importance of this effect, some of its opponents fully confirmed their validity with their experimental studies. Soon the importance of the discovery was recognized by all physicists, the “Mossbauer effect” became a sensation, and dozens of scientists from various laboratories around the world began to work in this area.
In 1961, Mössbauer received the Nobel Prize in Physics “for his study of the resonant absorption of gamma radiation and the discovery in this connection of the effect that bears his name.”
Mössbauer was supposed to become a professor at the Technical University in Munich, but, disillusioned with the bureaucratic and authoritarian principles of the organizational structures of German universities, he, taking a sabbatical in Heidelberg in 1960, went to the USA to the California Institute of Technology on a scientific grant. The next year he received the title of professor there.
In 1964, the scientist returned to his homeland and headed the physics department of the Technical University in Munich, transforming it according to the type organizational structures American universities. Some scholars jokingly called this change in the structure of German academic education the “second Mössbauer effect.” He worked at the university until 1971.
In 1972–1977 Mössbauer headed the Max Laue-Paul Langevin Institute in Grenoble (France). In 1977 he returned to Alma Mater, where he continued to work as a professor of physics and at the same time the scientific director of an institute specially created to develop problems in the field of Mössbauer spectroscopy and Mössbauerography. In the 1980s–1990s. headed the Mössbauer–Parak–Hoppe project to study the diffraction of Mössbauer gamma quanta on biological objects (Mössbauerography of proteins).
In 1957, the scientist married Elisabeth Pritz, a designer. The couple have one son and two daughters.
Mössbauer is a member of the American, European and German Physical Societies, the Indian Academy of Sciences and the American Academy of Arts and Sciences. The scientist was awarded honorary doctorates from the Universities of Oxford, Leicester and Grenoble.
In addition to the Nobel Prize, Mössbauer received an award for scientific achievements American Research Corporation (1960), E. Gresson Medal of the Franklin Institute (1961). He is also a recipient of the Roentgen Prize of the University of Giessen (1961).

Gamma radiation is short-wave electromagnetic radiation with a wavelength less than or equal to 10–8 cm; has pronounced corpuscular properties, i.e. it behaves like a stream of particles - gamma quanta or photons.
One of the ways to describe quantum mechanical phenomena; indicates how quickly certain parameters that characterize the state of the system change over time (in relation to this case, for example, spectral linewidth).
It should be noted that the young scientist had difficulty obtaining this isotope of iridium for experiments from his English colleagues. It was a difficult post-war time in Germany; Many substances were missing, as well as instruments necessary for research.
The results obtained contradicted the then accepted ideas about resonant nuclear fluorescence, although they did not raise doubts about their correctness. All that was missing was a theoretical interpretation of the effect. Then, on the advice of your scientific supervisor Mössbauer became familiar with W. Lamb's article (1939) on the theory of the interaction of slow neutrons with crystals. As it turned out, his theory could be successfully applied to the phenomenon observed by Mössbauer. The paradox was that the researchers working with neutrons were very familiar with this work of Lamb, but it did not occur to them to apply its results to the study of gamma fluorescence; at the same those time, who were engaged in resonant scattering and absorption of gamma quanta, did not turn to the achievements of the neighboring field of nuclear physics. By applying Lamb's calculations to gamma rays, Mössbauer was able to explain his results.
Phonon is a quantum of vibrational motion of crystal atoms.
The change in the energy of the nuclear transition, i.e., the energy of the gamma quantum absorbed by the sample compared to that emitted, associated with the difference in the electronic environment of the nuclei in the sample and the source, is called an isomeric, or chemical, shift and is measured as the value of the speed of the source at which a maximum is observed absorption of gamma rays.
The interaction of the nuclear quadrupole moment (which is understood as a quantity characterizing the deviation of the distribution of electric charge in an atomic nucleus from a spherically symmetrical one) with an inhomogeneous electric field leads to the splitting of nuclear levels, as a result of which not one, but several lines are observed in the absorption spectra. The study of quadrupole splitting allows one to obtain information about the electronic configurations of atoms and ions.
Solid-phase reactions occurring locally in the same place where the solid phase of the product is formed.

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Mössbauer effect ( JGR- nuclear gamma resonance) - the emission or absorption of gamma quanta by atomic nuclei in a solid, which is not accompanied by the emission or absorption of phonons. Opened in 1958. Rudolf Mössbauer in Germany. It is worth saying that to observe the effect, low-lying, long-lived nuclear levels with an energy of no more than 200 keV and lifetimes are used. -natural level width. For the iron nucleus the energy is gamma quanta.

The effect is observed for 73 isotopes of 41 elements. It is worth saying that to observe resonant absorption and obtain spectra, the states of Mössbauer atoms in the emitter and absorber must be identical. Tuning into resonance occurs by moving the source or absorber at a speed V. Energy change due to the Doppler effect. For core level width and operating speeds .

In the adsorption version JGR The source of radiation is nuclei, which, when capturing their own electron from the K-shell, turn into iron nuclei in an excited state with an energy of 136.4 KeV. This state forms a metastable state with an energy of 14.4 keV, which is used in Mössbauer spectroscopy of iron. Mössbauer spectra make it possible to determine the sizes of nanoclusters in the region of 1-10 nm with a known anisotropy constant of the substance. Figure shows superparamagnetic Mössbauer spectra of iron oxide nanoclusters at different measurement temperatures. Nanoclusters were obtained by a solid-state chemical reaction of the decomposition of iron oxalate at a decomposition temperature of .

Mössbauer spectroscopy– a set of methods for studying microscopic objects of nuclei and ions. chemical and biological complexes in solids.

The most important applications are shifts and hyperfine splitting of Mössbauer lines associated with the interaction of electric and magnetic moments of the nucleus with intracrystalline fields causing splitting of nuclear levels.

Chemical (isomeric) shift The Mössbauer line is observed when the source and absorber are not chemically identical.

The shift of the emission and absorption line, for example, when the ion charge changes, is 32 mm/s with a measurement accuracy of 0.1 mm/s. This makes it possible to establish a correlation between the magnitudes and electronegativity of nearby ions.

Fig. Chemical isomer shift of the Mössbauer line for two neptunium ions.

Quadrupole splitting of nuclear levels, leading to splitting of the lines of the Mössbauer spectrum, arises due to the interaction of the electric quadrupole moment of the nucleus with the gradient of the electric field of the crystal (with non-cubic symmetry of the environment). The distance between the split lines is for a nucleus with spin 3/2.

Where - z-component of the electric field gradient tensor (EFG) on the nucleus. - asymmetry parameter of the GEP tensor.

Due to the polarization of the own electron shell of the ion containing the resonant nucleus, the gradient of the gap can change several times, and even change sign. .

Sternheimer factor– the anti-shielding factor depends on the chemical state of the resonant ion.

Measuring quadrupole splitting spectra provides information about the structure and electronic properties of the solid matrix. For example, in the absorption spectrum of the nuclei of a high-temperature superconductor (superconducting transition temperature 72 TO) there are 3 quadrupole doublets corresponding to the ions Fe replacing ions Cu in structural positions with different oxygen environments. Chemical shifts for three positions Fe are identical and close to the shear in metallic iron, ᴛ.ᴇ. density s-electrons are approximately the same at all lattice sites. This means that the valence electrons for a given superconductor are delocalized throughout the crystal.

Magnetic ultrafine splitting nuclear levels and Mössbauer lines are caused by the interaction of the magnetic moment of the nucleus and the magnetic field at the location of the nucleus. The energy of magnetic hyperfine interaction is proportional to the product of the nuclear magnetic moment and the local magnetic moment, which is usually called hyperfine magnetic field. This interaction splits the nuclear state into 2I+1 Zeeman sublevels the distance between which is equal to ( I-spin of the nucleus). The number of hyperfine structure components in the Mössbauer spectrum is equal to the number of transitions between the Zeeman sublevels of the excited and ground states of the nucleus, allowed by the selection rule for the magnetic quantum number. For a magnetic dipole transition between states ( ) 6 components of the magnetic hyperfine structure are observed in the Mössbauer spectrum.

Ultrafine structure of lines in the Mössbauer spectrum in paramagnesics

The spectrum of impurity iron ions in aluminum nitrate is presented, consisting of the spectra of three Kramers doublets into which the ground state of the iron ion Fe 3+ is split

Conclusion. Mössbauer spectroscopy allows one to determine in one experiment the probabilities of the Mössbauer effect, the magnitude of the temperature shift, and the chemical shift. Quadrupole and magnetic splittings, line shapes of individual components. This is combined with the ability to influence Mössbauer spectra with temperature, pressure, magnetic and electric fields, ultrasound and radio frequency radiation. The ability to study objects ranging in size from a single carbonaceous plant to a massive sample makes Mössbauer spectroscopy a unique method for analyzing the physical and chemical properties of solids.

Mössbauer effect (nuclear gamma resonance) - concept and types. Classification and features of the category "Mossbauer effect (nuclear gamma resonance)" 2017, 2018.

The energy of nuclei is quantized. When a nucleus transitions from an excited state to a ground state, a -quantum with energy is emitted. The most probable value of this energy for an infinitely heavy free nucleus is equal to the difference between the energies of its ground and excited states: . The reverse process corresponds to the absorption of a g-quantum with an energy close to .

When a collection of identical nuclei is excited to the same level, the energy of the emitted quanta will be characterized by some spread around the average value.

Fig. 1.13 Diagram illustrating quantum transitions with emission and absorption of electromagnetic quanta (a) and the appearance of emission and absorption lines in the optical (b) and nuclear (c) cases.

The absorption line contour is described by the same relationship as the emission line contour (Fig. 1.13). It is clear that the effect of resonant absorption of electromagnetic radiation in the optical range, when optical quanta emitted during the transition of electrons of excited atoms to lower electronic levels are resonantly absorbed by a substance containing atoms of the same type. The phenomenon of static resonant absorption is well observed, for example, in sodium vapor.

Unfortunately, the phenomenon of resonant nuclear absorption on free nuclei is not observed. The reason is that the model of heavy nuclei (atoms), when energy losses due to recoil are small in relation to, is valid for optical resonance and is completely inapplicable for nuclear resonance. Gamma rays emitted in nuclear transitions have a significantly higher energy - tens and hundreds of keV (compared to several tens of eV for quanta in the visible region). With comparable lifetime values ​​and, accordingly, close values ​​of the natural width of electronic and nuclear levels in the nuclear case, the recoil energy plays a much more significant role in emission and absorption:

where is the recoil momentum of the nucleus equal in magnitude to the momentum of the emitted quantum, m is the mass of the nucleus (atom).

Therefore, in the optical case, resonance on free nuclei is not observed (see Fig. 1.13 b and c).

Rudolf Mössbauer, studying the absorption of -quanta emitted by the Ir isotope, discovered in an Ir crystal, contrary to predictions classical theory, increased scattering of -quanta at low temperatures (T≈77K). He showed that the observed effect is associated with the resonant absorption of -quanta by the nuclei of Ir atoms and gave an explanation of its nature.

In experiments on the Mössbauer effect, it is not the emission (or absorption) lines themselves that are measured, but the resonant absorption curves (Mössbauer spectra). The unique applications of the nuclear gamma resonance method in chemistry and solid state physics are due to the fact that the width of the individual resonance lines that make up the Mössbauer spectrum is less than the energies of the magnetic and electrical interactions of the nucleus with the electrons surrounding it. Mössbauer effect – effective method studies of a wide range of phenomena influencing these interactions.

The simplest scheme observation of the Mössbauer effect in transmission geometry includes a source, an absorber (a thin sample of the material under study) and a g-ray detector (Fig. 1.14).

Rice. 1.14 Scheme of the Mössbauer experiment: 1 – electrodynamic vibrator, setting different meanings source speed; 2 – Mössbauer source; 3 – absorber containing nuclei of the Mössbauer isotope; 4 – detector of g-quanta passing through the absorber (usually a proportional counter or photomultiplier).

The source of rays must have certain properties: have long period half-life of a nucleus, in the event of its decay, a nucleus of a resonant isotope is born in an excited state. The energy of the Mössbauer transition should be relatively low (so that the recoil energy does not exceed the energy required to displace the atom and the crystal lattice site), the emission line should be narrow (this ensures high resolution) and the probability of background-free radiation should be high. The source of g-quanta is most often obtained by introducing a Mössbauer isotope into a metal matrix through diffusion annealing. The matrix material must be dia- or paramagnetic (magnetic splitting of nuclear levels is excluded).

Thin samples in the form of foil or powders are used as absorbers. When determining the required sample thickness, the probability of the Mössbauer effect must be taken into account (for pure iron, the optimal thickness is ~20 µm). The optimum thickness is the result of a compromise between the need to work with a thin absorber and to have a high absorption effect. To register photons passing through a sample, scintillation and proportional counters are most widely used.

Obtaining a resonant absorption spectrum (or Mössbauer spectrum) involves changing the resonance conditions, for which it is necessary to modulate the energy of the -quanta. The currently used modulation method is based on the Doppler effect (most often, the movement of the g-ray source relative to the absorber is specified).

The energy of the g-quantum due to the Doppler effect changes by the amount

where is the absolute value of the speed of movement of the source relative to the absorber; с – speed of light in vacuum; – the angle between the direction of movement of the source and the direction of emission of g-quanta.

Since in the experiment the angle takes only two values ​​=0 and , then ∆E = ( positive sign corresponds to approaching, and negative - to the removal of the source from the absorber).

In the absence of resonance, for example, when there is no nucleus of a resonant isotope in the absorber or when the Doppler velocity is very high (corresponding to the destruction of the resonance due to too large a change in the energy of the -quantum), the maximum part of the radiation emitted in the direction of the absorber hits the one located behind it. detector. The signal from the detector is amplified, and pulses from individual photons are recorded by the analyzer. Usually the number of photons is recorded for equal periods of time at different . In the case of resonance, g-quanta are absorbed and re-emitted by the absorber in arbitrary directions (Fig. 1.14). The fraction of radiation entering the detector decreases.

The Mössbauer experiment examines the dependence of the intensity of radiation transmitted through an absorber (the number of pulses registered by the detector) on the relative velocity of the source. The absorption effect is determined by the ratio

where is the number of g-quanta recorded by the detector in a certain time at the Doppler velocity value (in the experiment a discrete set of velocities is used); – the same for , when there is no resonant absorption. The dependences and determine the shape of the resonant absorption curve of iron alloys and compounds and lie within ±10 mm/s.

The probability of the Mössbauer effect is determined by the phonon spectrum of the crystals. In the region of low temperatures () the probability reaches values ​​close to unity, and in the region of high temperatures () it is very small. All other things being equal, the probability of backgroundless absorption and emission is greater in crystals with a high Debye temperature (which determines the rigidity of the interatomic bond).

The probability of the effect is determined by the spectrum of elastic vibrations of atoms in the crystal lattice. The Mössbauer line is intense if the amplitude of atomic vibrations is small compared to the wavelength of the z-quanta, i.e. at low temperatures. In this case, the emission and absorption spectrum consists of a narrow resonance line (background-free processes) and a wide component due to changes in the vibrational states of the lattice during the emission and absorption of z-quanta (the width of the latter is six orders of magnitude greater than the width of the resonance line).

The anisotropy of the interatomic bond in the lattice determines the anisotropy of the amplitude of atomic vibrations and, consequently, the different probability of backgroundless absorption in different crystallographic directions. For single crystals, not only average but also angular dependences can be measured in this way.

In the thin absorber approximation, the probability of backgroundless transitions is proportional to the area under the resonant absorption curve.

Nuclear gamma resonance can be used to study the vibrational properties of the lattice of a solid or impurity atoms in this lattice. The most convenient experimental parameter in this case is the spectral area S, since it is an integral characteristic and does not depend on the shape of the emission spectrum of resonant quanta and self-absorption in the source. This area is preserved when the spectrum is split into several components as a result of hyperfine interactions.

The simplest resonant absorption spectrum of a thin absorber is a single line of Lorentzian shape. The intensity of radiation transmitted through the absorber is minimal at the absorption maximum.

As an example in Fig. Figure 1.15 shows the Mössbauer spectra of pure iron.

Rice. 1.15 Mössbauer spectra of pure iron.

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Nuclear gamma resonance (NGR) - the emission or absorption of gamma rays by a solid body without the production of phonons in it - is not one of the magnetic resonances.

Nuclear gamma resonance (Mössbauer effect) allows one to obtain valuable information about the structure of the electron shells of atoms containing Mössbauer nuclei. A significant disadvantage of the method is the limited number of elements practically available for research. IN this work An attempt was made to overcome this limitation, using the results of Mössbauer measurements on the Sn119 and Sb121 nuclei of tin and antimony atoms included in the compounds, as well as on the Fe57 nuclei of impurity iron atoms as a criterion for the applicability of various approaches in the theoretical calculation of the effective charges of atoms in compounds of the type under consideration.

Nuclear gamma resonance spectroscopy (Mössbauer spectroscopy) detects weak disturbances energy levels iron nuclei by surrounding electrons. This effect is the phenomenon of emission or absorption of soft v-radiation without nuclear recoil. The nuclear transition of interest to us with an energy of 14 36 keV occurs between the states / 3 / 2 and / 1 / 2 of the Mössbauer isotope 57Fe, where / is the nuclear spin quantum number. For a protein with a molecular weight of 50,000 that binds 1 iron atom per molecule, and in the absence of isotopic enrichment, this corresponds to a sample weight of 2 5 g. The multinuclear proteins considered here contain much more iron and are quite suitable for study by nuclear gamma resonance spectroscopy. Four possible types of interaction between the 57Fe nucleus and its electronic environment are widely studied: isomer shift, quadrupole splitting, nuclear magnetic hyperfine interactions, nuclear Zeeman interactions.

The essence of nuclear gamma resonance, or the so-called Mössbauer effect, is that quanta emitted during the transition of an excited nucleus to the ground state can be absorbed in equilibrium by unexcited nuclei with the transition of the latter to an excited state. A similar phenomenon is well known in conventional optics; the only important thing is that with a relatively large impulse of the y-quanta one would expect a strong recoil like that of the emitting one; and at the absorbing core and thereby the impossibility of resonant absorption due to the Doppler effect. Mössbauer showed that, at least in a significant proportion of cases, the recoil is absorbed by the crystal (or heavy molecule) as a rigid whole, and the recoil phenomenon can naturally be neglected.

The phenomenon of nuclear gamma resonance on atomic nuclei consists of a sharp increase in the probability of absorption or scattering of gamma quanta with an energy corresponding to the excitation of nuclear transitions.

A nuclear gamma resonance study showed that the iron particles studied were not oxidized.

Using X-ray diffraction analysis and nuclear gamma resonance, it was established that this change in the crystal structure is not associated with a change in the carbon concentration in the solid solution, but is caused by reversible transitions of interstitial atoms (carbon) from octahedral interstices to radiation defects. Such transitions do not require diffusion of carbon over significant distances - it occurs within the unit cell. An increased concentration of point defects created by irradiation in the crystal lattice of martensite stimulates transitions of interstitial atoms from one position to another, which is energetically more favorable at given temperatures.

We have carried out observations of nuclear gamma resonance in samples of various massive multicomponent tin-containing glasses and glass fibers of the same chemical composition. The glass compositions are given in the table.

We conducted a study of nuclear gamma resonance in complex compounds of iron with 4-butyroyl - and 4-benzoyl - 1 2 3-tri-azole anions. The spectra were obtained on a mechanical NGR spectrometer using a Co57 source in chromium.

Processing of experimental data on nuclear gamma resonance is possible only if the NGR spectrometer has been calibrated by velocities and the positions of the absorption lines of any substances selected as a standard have been determined. Typically, substances that can be quite easily manufactured and reproduced under identical conditions are used as a standard. They must be stable, must have a sufficiently large probability of absorption - y-quanta without loss of energy due to recoil, their Mössbauer spectra must be a narrow line, characterized by a small temperature shift.

Although quadrupole splitting complicates the appearance of nuclear gamma resonance (NGR) spectra (Fig. 111 6), it helps to draw a number of important conclusions about the structure and symmetry of the compounds under study. This compound (which served as a scavenger) was synthesized using the 1291 isotope, a long-lived fission reaction product. Complex view spectrum is due both to quadrupole splitting and to the fact that iodine is located in two different positions in this compound.

We undertook a systematic study by the nuclear gamma resonance (NGR) method of tin compounds with elements of the fifth and sixth groups, as well as chalcogenide semiconductor glasses in the arsenic - selenium - tin system in order to obtain information about the chemical bond and internal crystal fields in these compounds.

The study of narrow lines is carried out using the method of nuclear gamma resonance, which is commonly called Mössbauer spectroscopy. In Fig. Figure 8.14 shows a typical experimental setup.

The method of Mössbauer spectroscopy, sometimes called nuclear gamma resonance spectroscopy (NGR), is based on the study of the absorption of y-radiation from a source nucleus by a nucleus of the same isotope located in the sample under study. Resonance conditions are met only when the recoil effect of nuclei during the emission and absorption of y-quanta is also eliminated, and the Doppler effect is also compensated in some way. The method was developed precisely from the moment when this was understood, and even earlier a simple and almost the only possible way to eliminate recoil losses was found experimentally.