Mossbauer theoretical

Another relatively recent technique, in its own way as strange as Mossbauer spectrometry, is positron annihilation spectrometry. Positrons are positive electrons (antimatter), spectacularly predicted by the theoretical physicist Dirac in the 1920s and discovered in cloud chambers some years later. Some currently available radioisotopes emit positrons, so these particles arc now routine tools. High-energy positrons are injected into a crystal and very quickly become thermalised by... 

Table 1. The 72-atom model examined by different theoretical methods. The energy differences (AE in kcal/mol) are calculated with respect to the lowest SCF energy. q(Fe) stands for Mulliken population charges on the Fe atoms q(S) and SS(b.i.) are the Mulliken population charges and the bond index for the bridging S atoms, respectively AEq is the calculated Mossbauer quadrupole splitting constant [mm/sec]. The PUHF spin states are those projected from the UHF wavefunction with 5 = 5,. 

During the past 20 years, considerable progress has been made toward understanding the electronic properties of iron-sulfur centers thanks to the fruitful interplay between various approaches such as synthetic analog chemistry, theoretical modeling, and of course spectroscopic studies. Modeling studies have been strongly stimulated by the permanent supply of complementary data provided especially by EPR, Mossbauer, ENDOR, MCD, and NMR experiments. However,... 

Two different approaches have been followed to calculate the lineshapes within a relaxation model. According to a phenomenological approach based on the modified Bloch equations [154, 155], the intensity distribution of the theoretical Mossbauer spectrum may be written as [156] ... 

In anisotropic crystals, the amplitudes of the atomic vibrations are essentially a function of the vibrational direction. As has been shown theoretically by Karyagin [72] and proved experimentally by Goldanskii et al. [48], this is accompanied by an anisotropic Lamb-Mossbauer factor/which in turn causes an asymmetry in quadra-pole split Mossbauer spectra, for example, in the case of 4 = 3/2, f = 1/2 nuclear transitions in polycrystalline absorbers. A detailed description of this phenomenon, called the Goldanskii-Karyagin effect, is given in [73]. The Lamb-Mossbauer factor is given by... 

Fig. 6.1 Mossbauer spectra of an amorphous frozen aqueous solution of 0.03 M Fe(N03)3, obtained at 4.5 K with various applied transverse magnetic fields. The bar diagrams indicate theoretical line positions of the spectral components. The lines are fits to the experimental data. (Reprinted with permission from [12] copyright 1977 by Elsevier)...

Fig. 6.2 Theoretical Fe Mossbauer relaxation spectra for longitudinal relaxation with the indicated relaxation times and with a hyperline field that can assume the values 55 T. The symmetry direction of the axially symmetric EFG is assumed parallel to the magnetic hyperfine field. (Reprinted with permission from [9] copyright 1966 by the American Physical Society)...

Fig. 6.5 Mossbauer spectra of NH4Fe(S04)2-12H20 at 4.2 K and with the indicated magnetic fields applied parallel to the y-ray direction. The lines indicate fits in accordance with a theoretical relaxation model [19, 29]. The bar diagrams indicate the theoretical line positions in the case of infinitely fast relaxation. (Adapted from [29] copyright 1973 by Springer-Verlag)...

The results are misleading simply because the basic set of functions used is inappropriate. The great value of the Mossbauer technique is that once SR/R is established, one then has a rather severe test to apply to theoretical calculations concerning the structure of compounds. If... 

Figure 7.12 Analysis of the zero-field Mossbauer spectrum at SOK of H2-reduced A. vinosum [NiFe] hydrogenase. (A) Dotted line experimental spectrum solid line theoretical spectrum for a [3Fe-4S] cluster. (B) Dotted spectrum experimental spectrum in A minus the theoretical spectrum or the reduced 3Fe cluster this was expected to represent the spectrum of the two reduced [4Fe-4S] clusters. Solid line theoretical spectrum for two reduced 4Fe clusters. The additional absorption in the experimental spectrum (arrows) comprised a doublet with a surprisingly small isomer shift (0.05-0.15 mm/s) and accounted for about 8 per cent of the total absorption, i.e. one Fe out of twelve (adapted from Surerus et al. 1994).The present interpretation is that this doublet represents the Fe atom in the Ni-Fe site.

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