Mössbauer spectroscopy
This effect, discovered by Rudolf Mössbauer (sometimes written "Moessbauer", German: "Mößbauer") in 1958, consists of the nearly recoil-free emission and absorption of nuclear gamma rays in solids.
The consequent nuclear spectroscopy method is exquisitely sensitive to small changes in the chemical environment of certain nuclei.
[1] Mössbauer found that a significant fraction of emission and absorption events will be recoil-free, which is quantified using the Lamb–Mössbauer factor.
[2] 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.
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.
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.
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.
Generally, the impact of this effect is small, and the IUPAC standard allows the Isomer Shift to be reported without correcting for it.
[5] Quadrupole splitting reflects the interaction between the nuclear energy levels and the surrounding electric field gradient (EFG).
Nuclei in states with non-spherical charge distributions, i.e. all those with a spin quantum number (I) greater than 1/2, may have a nuclear quadrupole moment.
The splitting can be measured, for instance, with a sample foil placed between an oscillating source and a photon detector (see Fig.
In ferromagnetic materials, including many iron compounds, the natural internal magnetic fields are quite strong and their effects dominate the spectra.
Its spectrum has 12 peaks, a sextet for each potential atomic site, corresponding to two sets of Mössbauer parameters.
Also, since ferromagnetic phenomena are size-dependent, in some cases, spectra can provide insight into the crystallite size and grain structure of a material.
[10] Among the technique's drawbacks are the limited number of gamma-ray sources and the requirement that samples be solid to eliminate the nucleus's recoil.
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.
During calcination, all the Sb ions in an antimony-containing tin dioxide catalyst transform into the +5 oxidation state.
[14] This technique has also been used to observe the second-order transverse Doppler effect predicted by the theory of relativity, because of very high energy resolution.
[15] Mössbauer spectroscopy has been widely applied to bioinorganic chemistry, especially for the study of iron-containing proteins and enzymes.
Examples of prominent iron-containing biomolecules are iron-sulfur proteins, ferritin, and hemes including the cytochromes.
It is formed by three main parts; a source that moves back and forth to generate a Doppler effect, a collimator that filters out non-parallel gamma rays and a detector.
To calculate the outer line distance from the six-line iron spectrum: where c is the speed of light, Bint is the internal magnetic field of the metallic iron (33 T), μN is the nuclear magneton (3.1524512605×10−8 eV/T), Eγ is the excitation energy (14.412497(3) keV[21]), gn is the ground state nuclear splitting factor (0.090604/(I), where Isospin I = 1⁄2) and gen is the excited state splitting factor of 57Fe (-0.15532/(I), where I = 3⁄2).
