Mössbauer Effect

Mössbauer effect is the recoil-free emission and resonant absorption of gamma ( γ) rays from the nuclei of certain radioactive isotopes such as ⁵⁷Fe. These γ-rays, as emitted, do not lose energy from the recoil of the nuclei because the recoil momentum due to their emission or absorption is taken up by the whole bulky sample rather than by the emitting or absorbing nucleus itself. Under such conditions, the nuclei are bound rigidly in the lattice of a crystal, and the energy of recoil is negligible. Spectra based on Mössbauer effect have found numerous applications in studying the valence of an element in a chemical compound, the state of the irons and their electronic structure, as well as structural and magnetic properties of magnetic bulk and thin-film materials.

Mössbauer effect was first reported by Rudolph Mössbauer in 1958. Three years later, he won the Nobel Prize with his discovery. Since then, it is believed that nuclear γ-ray emission and absorption process can take place in recoil-free fashion. In reality, of course we have both recoil and recoil-free events. Mössbauer also utilized the Doppler (velocity) shift to modulate the γ-ray energy so that Mössbauer effect could be developed into a spectroscope for material characterization. The emission of γ-rays with natural or nearly natural line width allows for observing in the γ-ray spectra the interaction between the nucleus and its atom in solids and viscous liquids.

A radioactive parent or Co, Ta, serves as the primary source of the excited nuclear state of the iron isotope ⁵⁷Fe*. ⁵⁷Fe* decays rapidly and emits a γ-ray that strikes another ⁵⁷Fe (or of the absorber or of the sample to be studied) in its ground state. If the energy of the γ-ray matches the ⁵⁷Fe-⁵⁷Fe* energy separation, it will be absorbed via a resonant absorption process and ⁵⁷Fe is excited. Since we need to match the γ-ray frequency to the absorber energy levels, the energy of emitted γ-ray is modulated via the Doppler effect by oscillating the radioactive source. Therefore, absorption is observed as a function of relative velocities. The γ-radiation of interest is detected by a counter, and the data are processed by a multichannel analyzer in which each channel corresponds to a different velocity and a mechanism for synchronization is also installed. Certain elements and compounds have to be examined at cryogenic temperatures such as - 452°F (-269°C) to see the Mössbauer effect. The final results, or Mössbauer spectra, are usually plotted as curves of transmission (T) in % versus relative velocity and the transmitted intensity is a minimum at the velocity of exact resonance. These spectra consist of sharp lines due to recoil-free emission or absorption and of a featureless background due to recoil events and irrelevant radiation.

The transitions involved in the Mössbauer effect and the basic principle of Mössbauer spectroscopy, occur where the area is enclosed by a dash box and kept at cryogenic temperature.

In Mössbauer spectra, the resonance is shifted relative to the source frequency, generally called "isomer shift." Such a chemical shift is due to a difference in the s-electron density or the shielding of s-electrons between two cases. Therefore, a measure in the isomer shift provides us with the information on s-orbitals involved in the chemical bonding. In addition to the isomer shift, there are the second-order Doppler shift caused by temperature effects and the gravitational red shift caused by differences in the potential energy between the source and the absorber. Magnetic effects due to a net electron spin or an unpaired electron density, on the other hand, often make the spectra split into several lines. For a quadrupolar nucleus under certain conditions, or a change in the symmetry of the electron distribution, the energy of the nucleus exhibits a dependence on the nuclear orientation. This will then lead to the electric quadrupole splitting. When we look at Mössbauer spectra, the effect due to magnetic field and quadrupole splitting can be easily identified.

Based on the aforementioned mechanisms, Mössbauer spectroscopy has been applied successfully in all fields of natural science. For instance, Mössbauer effect study provides local information on both magnetism via the hyperfine interaction and structure via the electric quadrupole interaction in ferromagnetic films. It evaluates electromagnetic moments of excited states. The differences in chemical bonding due to the change in valence state lead to variations of the occupation of the conduction band and the magnetic exchange coupling also manifests in the Mössbauer spectra. When a small amount of impurities is introduced in order to improve the magnetic properties and chemical stability of the material, Mössbauer spectroscopy can help find at which crystallographic sites of the matrix material the impurity atoms are located. Since iron occurs in a variety of biologically important compounds or heme proteins, Mössbauer spectroscopy also has found use in molecular biology and clinical diagnosis.