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Mössbauer spectroscopy (German: Mößbauer) is a spectroscopic technique based on the resonant emission and absorption of gamma rays in solids. This resonant emission and absorption was first observed by Rudolf Mössbauer in 1957 and is called the Mössbauer effect in his honor. Mössbauer spectroscopy is similar to NMR spectroscopy in that it probes nuclear transitions and is thus sensitive to similar electron-nucleus interactions as cause the NMR chemical shift. Furthermore, due to the high energy and extremely narrow line widths of gamma rays, it is one of the most sensitive techniques in terms of energy resolution having the capability of detecting changes of just a few parts per 10¹¹.

[edit] Typical Method

In its most common form, Mössbauer Absorption Spectroscopy, a solid sample is exposed to a beam of gamma radiation, and a detector measures the intensity of the beam transmitted through the sample. The atoms in the source emitting the gamma rays must be of the same isotope as the atoms in the sample absorbing them. In accordance with the Mössbauer effect, a significant fraction (given by the Lamb-Mössbauer factor) of the emitted gamma rays will not lose energy to recoil and thus will have approximately the right energy to be absorbed by the target atoms, the only differences being attributable to the chemical environment of the target, which is what we wish to observe. The gamma-ray energy of the source is varied through the Doppler effect by accelerating it through a range of velocities with a linear motor. A typical range of velocities for ⁵⁷Fe may be +/-11 mm/s (1 mm/s = 48.075 neV).

In the resulting spectra, gamma-ray intensity is plotted as a function of the source velocity. At velocities corresponding to the resonant energy levels of the sample, some of the gamma-rays are absorbed, resulting in a drop in the measured intensity and a corresponding dip in the spectrum. The number, positions, and intensities of the dips (also called peaks) provide information about the chemical environment of the absorbing nuclei and can be used to characterize the sample.

A major limitation of Mössbauer spectroscopy is finding a suitable gamma-ray source. Usually, this consists of a radioactive parent that decays to the desired isotope. For example, the source for ⁵⁷Fe consists of ⁵⁷Co, which undergoes beta-decay to an excited state of ⁵⁷Fe and subsequently decays to the ground state emitting the desired gamma-ray. Ideally the parent will have a sufficiently long half-life to be usable, but will also have a sufficient decay rate to supply the needed intensity of radiation. Also, the gamma-ray energy should be relatively low, otherwise the system will have a low recoil-free fraction (see Mössbauer effect) resulting in a poor signal-to-noise ratio and requiring long collection times. The periodic table below indicates those elements having an isotope suitable for Mössbauer spectroscopy. Of these, ⁵⁷Fe is by far the most common element studied using the technique, although ¹²⁹I, ¹¹⁹Sn, and ¹²¹Sb are also frequently studied.

[edit] Analysis of Mössbauer spectra

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. Typically, there are three types of nuclear interactions that are observed. The first of these is called the Isomer Shift (or chemical shift). It reflects the chemical bonding of the atoms and is related to the electron density at the nucleus. The Isomer Shift is observed in the spectra as a shift (either to the left or the right) of all the peaks corresponding to a particular atomic environment. The second interaction is called Quadrupole Splitting and reflects the interaction between the nuclear quadrupole and the surrounding electric field gradient (EFG). As its name implies, this interaction "splits" the otherwise degenerate nuclear transitions from a single peak into two peaks. The Quadrupole Splitting is measured as the separation between these two peaks and reflects the character of the electric field at the nucleus. The third interaction is called Hyperfine Splitting (also Zeeman Splitting) and is a result of the interaction between the nucleus and any surrounding magnetic field. Typically, it splits the single peak into six non-degenerate peaks (as shown in the image above). The hyperfine splitting is usually measured as the distance between the outermost of these six peaks. Hyperfine splitting is especially important in the Mössbauer spectroscopy of Iron-containing compounds, which are frequently ferromagnetic or anti-ferromagnetic, resulting in strong internal magnetic fields. In cases where both Quadrupole Splitting and Hyperfine Splitting occur the spectrum will still consist of six peaks, although the peak positions will shift depending on the relative amounts of each type of splitting.

The three Mössbauer parameters (Isomer Shift, Quadrupole Splitting, and Hyperfine Splitting) can often be used to identify a particular compound. A large database including most of the published Mössbauer parameters available in the literature is maintained by The Mössbauer Effect Data Center at the University of North Carolina-Asheville. Note that in some cases, a compound may have more than one type of site which the relevant atoms occupy. In such cases, because each site has a unique environment it will have its own set of peaks. For example, Hematite (Fe₂O₃) contains two unique sites for the Iron atoms and the corresponding spectrum has twelve peaks, six corresponding to each type of site. Thus, Hematite also has two sets of Mössbauer parameters, one for each site.

In addition to identification, the relative intensities of the various peaks reflect the relative concentrations of compounds in the sample and can be used for semi-quantitative analysis. Also, since ferromagnetic phenomena are size-dependent, in some cases spectra can provide insight into the crystallite size and grain structure of a material.

[edit] Applications of Mössbauer spectroscopy

Mössbauer spectroscopy has been applied in a wide variety of scientific endeavors. It has been especially useful in the field of geology for identifying the composition of iron-containing minerals. In particular it has been used to study a number of meteors and moon rocks and has even been used by NASA on Mars^[1].

Another application of Mössbauer spectroscopy is the study of the phase transformations that occur in Iron Catalysts during Fisher-Tropsch synthesis. While these catalysts initially consist of Hematite (Fe₂O₃), during reaction they are transformed into a mixture of Magnetite (Fe₃O₄) and several Iron Carbides. The formation of carbides appears to improve catalytic activity, however it can also lead to the mechanical break-up and attrition of the catalyst particles, causing difficulties in the separation of the catalyst and the desired reaction products^[2].

Due to the very high energy resolution, Mössbauer spectroscopy has been used to observe the second-order Transverse Doppler effect predicted by the theory of Relativity.^[3].

[edit] Mössbauer spectrometers

A Mössbauer spectrometer is a device that performs Mössbauer spectroscopy, or a device that uses the Mössbauer effect to determine the chemical environment of Mössbauer nuclei present in the sample. Dr. Göstar Klingelhöfer at the Johannes Gutenberg University in Mainz, Germany, developed a miniature Mössbauer Spectrometer, named (MB) MIMOS II, that was used by the two rovers in NASA's Mars Exploration Rover mission. ^[4]

[edit] Notes on iron Mössbauer spectroscopy

The Mössbauer parameters—chemical isomer shift and quadrupole splitting—are generally evaluated with respect to a reference material. For example, in iron compounds, the Mössbauer parameters were evaluated using iron foil (thickness less than 40 micrometer). The centroid of the six lines spectrum from metallic iron foil is -0.1 mm/s (for Co/Rh source). All shifts in other iron compounds are computed relative to this -0.10 mm/s (at room temperature), i.e., in this case isomer shifts are relative to Co/Rh source. In other words, the centre point of the Mössbauer spectrum is zero. The shift values may also be reported relative to 0.0 mm/s, here shifts are relative to Fe foil. Calculation of outerline distance from six line iron spectrum:

$V=\frac{cH_\text{int}\mu_N}{E_\gamma}(3g_n^e+g_n)$

where c is the velocity of light in m/s, H_int is the internal magnetic field of the metallic iron in T (= 33 T), μ_N is the nuclear magneton in eV/T (= 3.1524512326×10⁻⁸ eV/T), E_γ is the excitation energy in keV (= 14.412497 keV), g_n (= 0.09062/I_1/2) is the ground state (I_1/2 = 1/2) nuclear splitting factor and g_n^* (0.1549/I_3/2) is the excited state (I_3/2 = 3/2) splitting factor for ⁵⁷Fe nucleus. By substituting the above values one would get 10.6257738983924 mm/s. PS: The unit is in mm/s, since we have taken c in m/s, E in keV and nuclear magneton in eV/T and hence the final unit for V will be in mm/s. But it is not very uncommon that V is taken as 10.6 or 10.66 or 10.2 mm/s. One reason for this may be the different quality of iron foil used in different laboratories. In any case that changing this V only affects the quadrupole splitting and not the isomer shift. Since IBAME, the authority for Mössbauer society does not mention any accurate value and so people often take a value between 10.6 to 10.669 mm/s.

[edit] References

^ Klingelhöfer G. (2004). "Mössbauer in situ studies of the surface of Mars". Hyperfine Interactions 158: 117–124. doi:10.1007/s10751-005-9019-1.
^ Amitava Sarkar, Gary Jacobs, Yaying Ji, Hussein H. Hamdeh and Burtron H. Davis (2007). "Fischer–Tropsch Synthesis: Characterization Rb Promoted Iron Catalyst". Catalysis Letters 121: 1–11. doi:10.1007/s10562-007-9288-1.
^ Chen, Yi-Long; Yang, De-Ping (July 2007). "Recoilless Fraction and Second-Order Doppler Effect". Mössbauer Effect in Lattice Dynamics (1 ed.). Wiley InterScience. doi:10.1002/9783527611423.ch5. ISBN 9783527611423.
^ Klingelhöfer G., Bernhardt B., Foh J., Bonnes U., Rodionov D., De Souza P. A., Schroder C., Gellert R., Kane S., Gutlich P., Kankeleit E. (2002). "The miniaturized Mossbauer spectrometer MIMOS II for extraterrestrial and outdoor terrestrial applications: A status report". Hyperfine Interactions 144: 371–379. doi:10.1023/A:1025444209059.

[edit] External links

[0] Klingelhöfer G. (2004). "Mössbauer in situ studies of the surface of Mars". Hyperfine Interactions 158: 117–124. doi:10.1007/s10751-005-9019-1.

[1] Amitava Sarkar, Gary Jacobs, Yaying Ji, Hussein H. Hamdeh and Burtron H. Davis (2007). "Fischer–Tropsch Synthesis: Characterization Rb Promoted Iron Catalyst". Catalysis Letters 121: 1–11. doi:10.1007/s10562-007-9288-1.

[2] Chen, Yi-Long; Yang, De-Ping (July 2007). "Recoilless Fraction and Second-Order Doppler Effect". Mössbauer Effect in Lattice Dynamics (1 ed.). Wiley InterScience. doi:10.1002/9783527611423.ch5. ISBN 9783527611423.

[3] Klingelhöfer G., Bernhardt B., Foh J., Bonnes U., Rodionov D., De Souza P. A., Schroder C., Gellert R., Kane S., Gutlich P., Kankeleit E. (2002). "The miniaturized Mossbauer spectrometer MIMOS II for extraterrestrial and outdoor terrestrial applications: A status report". Hyperfine Interactions 144: 371–379. doi:10.1023/A:1025444209059.

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