Mossbauer absorption Intensities

The thermal properties of AU55 are treated in Sect. 3, using especially the results of MES measurements [24,25,42]. These are discussed in connection with the concept of bulk versus surface modes in small particles. An explanation of the temperature dependence of the MES [42] absorption intensities and the Cv results [25] on the basis of a model using the site coordination and the center-of-mass motion are briefly reviewed. The consequences of the Mossbauer results for surface Debye temperatures and for the melting temperature of small gold particles are also discussed. 

Mossbauer parameters of iron containing components in solid-state phthalocianinato-iron complexes (IS isomer shift, related to metallic a-iron, mm/s QS quadrupole splitting, mm/s, Rl relative contribution to the the spectrum, % A spectral absorption, intensity/base line, % A77/A30Q ratios of absorptions obtained from 77 K and 300 K spectra)... 

A Mossbauer spectrometer consists of a radioactive Co source on a transducer that continuously scans the desired velocity range, an absorber consisting of the catalyst and a detector to measure the intensity of the gamma radiation transmitted by the absorber as a function of the source velocity. This is the common mode of operation, called Mossbauer absorption spectroscopy, sometimes abbreviated as MAS. It is also possible to fix the Co containing source and move a single-line Fe absorber, in order to investigate Co-containing catalysts. This technique, called Mossbauer emission spectroscopy (MES), has successfully been applied to study Co-Mo hydrodesulphurization catalysts [42]. 

Fig. 35. Temperature dependence of relative intensity X of the 2nd and 5th Mossbauer absorption lines for 28 A Nd/39 A Fe. The corresponding angle G between the direction of Fe moments and the film normal is also indicated on the right side of the ordinate (after Mibu et al. 1989).

The total absorption intensity of the spectrum is a function of the concentration ofMossbauernuclei in the absorber and the cross-sections of the nuclear processes involved. This absorption intensity, together with the signal-to-noise ratio of the detection system and the total number of counts, determine the quality of the Mossbauer spectrum and the accuracy with which information can be obtained from it. It should be noted that Mossbauer spectroscopy is not generally an appropriate technique for measuring the total concentration of a certain nuclide within a system because it is usually relatively insensitive to small concentrations and because the absorption intensity depends on a number of other factors which may be difficult to quantify. However, relative concentrations of different chemical forms of the Mossbauer nuclide can frequently be obtained, such as the relative concentration of the element in different oxidation states. 

Since the Mossbauer effect is intimately related to any motion of the emitting or absorbing nucleus on either a microscopic or macroscopic scale, Mossbauer spectroscopy provides a potential means by which information on nuclear dynamics, and hence on the dynamics of a system in which the Mossbauer nucleus acts as a probe, can be obtained. Any motion of the Mossbauer nucleus can influence the Mossbauer spectrum in two ways. Firstly, because this motion may be related to the vibrational properties of the system it can influence the recoil-free fraction and hence the absorption intensity of the spectrum itself. Since the absolute absorption intensity is dependent on a large number of other factors, which may be diflicult to determine accurately, any change in recoil-free fraction is most usefully followed as a function of temperature in order to obtain information on the vibrational properties of the system. The second way in which the effects of any motion of the Mossbauer nucleus in the source or absorber are manifested is in the Mossbauer spectroscopic linewidths, as this motion can be thought of as an additional Doppler motion which may partially smear out the resonant absorption. Since the linewidths are also... 

The total absorption intensity depends on the concentration of the Mossbauer atoms in the absorber and their recoil-free fraction. The recoil-free fraction depends on the binding forces of the Mossbauer atom in the lattice (see Ap.l). The number and position of the absorption lines are determined by electric and magnetic electronic effects, the hyperfine interactions, which shift and split the nuclear levels. The allowed transitions between ground and excited substates are determined by the multipolarity of the nuclear transition. The = transition of... 

Any motion of the Mossbauer nucleus can influence the spectrum in two ways, by affecting the absorption intensity of the spectrum itself, and also the linewidth, eventually the lineshape, as a result of a kind of additional Doppler motion. 

The relative intensities of the Mossbauer absorption lines will depend on the angle between the direction of the observed magnetic field and the direction of propagation of the y-rays (see Ap.5). 

Fig. 82, The temperature dependence of the magnetie neutron diffraction peak at (3,1,0), the fraction of sextet intensity in Mossbauer absorption and thermoremanent magnetization, normalized by the value at 4.2 K. (Tanaka et al. 1984b.)...

Mossbauer spectroscopy has been used to characterize the iron clusters in fuscoredoxin isolated from D. desulfuricans (133). The authors explained why the iron nuclearity was incorrectly determined, and studied the protein in three different oxidation states fully oxidized, one-electron reduced, and two-electron reduced. The error made in determining the iron cluster nuclearity was caused by the assumption that in the as-purified fuscoredoxin, cluster 2 is in a pure S = state. This assumption was proven to be false and unnecessary. In fact, the observation of four resolved, equal intensity (8% of total Fe absorption) spectral components associated with the S = i species in the as-purified protein is consistent with cluster 2 being a tetranuclear Fe cluster. The 4x8 = 32% Fe absorption for the four components indicates that only 64% of clusters 2 are in the S = state (the total Fe absorption for cluster 2 is 50% of the total Fe absorption). The remaining clusters 2 are in a different oxidation state, the spectrum of which is unresolved from that of cluster 1. 

The thickness of a Mossbauer sample affects not only the strength of the Mossbauer signal but also the intensity of the radiation arriving at the detector because the y-rays are inherently attenuated by the sample because of nonresonant mass absorption caused by the photo effect and Compton scattering as mentioned earlier. The counting rate C in the detector decreases exponentially with the density of the absorber,... 

The intensity of a Mossbauer spectrum depends not only on the recoil-free fractions of the source and the absorber and on the number of absorbing nuclei, but also on the linewidth of the absorption lines and on whether or not saturation effects occur. The following approximate expression is valid for relatively thin absorbers [17] ... 

Thus making samples not too thick helps in getting sharper spectra and facilitates the quantitative interpretation. Finally, particularly in the Mossbauer spectra of small catalyst particles, one should be aware of the temperature dependence of the absorption area through the recoil-free fraction. If the spectrum contains contributions from surface and bulk phases, the intensity of the former will be greatly underestimated if the spectrum is measured at room temperature. The only way to obtain reliable concentrations of surface and bulk phases is to determine their spectral contributions as a function of temperature and make an extrapolation to zero Kelvin [13]. 

Recoilless Optical Absorption in Alkali Halides. Recently Fitchen et al (JO) have observed zero phonon transitions of color centers in the alkali halides using optical absorption techniques. They have measured the temperature dependence of the intensity of the zero phonon line, and from this have determined the characteristic temperatures for the process. In contrast to the Mossbauer results, they have found characteristic temperatures not too different from the alkali halide Debye temperatures. 

Atoms are not rigidly bound to the lattice, but rather vibrate around their equilibrium positions. If we were able to examine the crystal over a very brief observation time, we would see a slightly disordered lattice. Incident electrons see these deviations, and this is for example the reason that in low-energy electron diffraction (LEED) the spot intensities of diffracted beams depend on temperature. At high temperatures the atoms deviate more from their equilibrium position than at low temperatures, and a considerable number of atoms is not at the equilibrium position necessary for diffraction. Thus, spot intensities are low and the diffuse background high. Similar considerations apply in other scattering techniques, as well as in extended X-ray absorption fine structure (EXAFS) and in Mossbauer spectroscopy. 

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