Mdssbauer resonance

NaFeF4 was studied by Dance et al. who applied susceptibility, neutron diffraction and Mdssbauer resonance measurements130. J/k is equal to -11.5 K and the Neel temperature Tn = 111.5 K. The variation of the hyperfine field with temperature for 0.6 < T/Tn < 1 gives /9 = 0.25, indicating the presence of 2-D and 3-D correlations. 

Fig. 2.44. Mossbauer spectroscopy (a) nuclear transitions giving rise to the Mossbauer effect in Fe (b) principles involved in the Mossbauer spectrometer (c) Mdssbauer resonant absorption of iron in different crystal environments and the resulting spectral types. (After Vaughan and Craig, 1978 reproduced with the publisher s permission).

The absorption cross-section Uq should be large and the free-atom recoil energy should be small. Both of these factors have already been mentioned in connection with other quantities above. Appendix 1 lists 5 transitions for which CTo > 10 cm and few Mdssbauer resonances have been observed for transitions in which < 0 06 x 10 cm. Likewise only 5 transitions have a free-atom recoil > 6 x 10 eV and values are normally in the range (0-1-5) x 10 eV. 

Several experiments have been performed to observe a Mdssbauer resonance in the immediate time interval following an energetic nuclear reaction. Since the experimental conditions for these are severe compared to the Co source method, they are discussed here more as a curiosity than as a recommended means of attempting chemical studies. 

There are two iodine Mdssbauer resonances. The 27-72-keV transition of I was first observed in 1962 by Jha, Segnan, and Lang [67]. The 57-60-keV transition of I was reported in 1964 by Barros et al. [68]. Both have been used extensively in chemical investigations. The resonance has the better nuclear properties for MOssbauer work, with the unfortunate exception that the ground state is radioactive with a half-life of 1-7 x 10 y. Because of this, I absorbers must be specially prepared and handled, whereas natural iodine comprises the stable I in 100% abundance. 

A Mdssbauer resonance is known in at least one isotope of fourteen tran sition metals in addition to iron. However, none has been extensively used up to the present time. Several of them present extreme difficulties in measurement, but as this chapter will show, sufficient background information has been collected to assess the feasibility of chemical application. 

The high energy of the 145-43-keV transition in Pr is not conducive to a strong Mdssbauer resonance and indeed the first attempts to observe it... 

The second samarium Mdssbauer resonance, involving the 121-78-keV transition of Sm, was reported by two groups independently in 1967 [14, 15] and is difficult to observe because of the high energy of the y-ray causing the resonance to be weak even at 4-2 K. A convenient source is Eu in GdaOs, and the decay scheme is shown in Fig. 17.3. 

High-pressure Mdssbauer resonance studies with iron-57. H. G. Drickamer, R. W. Vaughan and A. R. Champion, Acc. Chem. Res., 1969, 2,40-47 (40). 

Fig. 6.19 Diffusional broadening of the Mdssbauer resonance in Fe self-diffusion [30]...

Typical required absorber thickness (in mg/cm of natural isotopic admixture of the resonant atoms) for selected Mdssbauer resonances in the lanthanides and actinides for different Debye temperatures 0 and measuring temperatures T. 

A useful feature of ns Sn Mdssbauer spectra is the quadrupole splittings, which can be informative on the environmental symmetry of tin in a compound. A non-zero value of field gradient is expected in any molecule which does not possess cubic symmetry, with an accompanying quadrupole splitting of the Mdssbauer resonance. 

However, in contrast, the resonance effect increased by cooling both the source and the absorber. Mdssbauer not only observed this striking experimental effect that was not consistent with the prediction, but also presented an explanation that is based on zero-phonon processes associated with emission and absorption of y-rays in solids. Such events occur with a certain probability/, the recoil-free fraction of the nuclear transition (Sect. 2.4). Thus, the factor/is a measure of the recoilless nuclear absorption of y-radiation - the Mdssbauer effect. 

In an actual Mdssbauer transmission experiment, the radioactive source is periodically moved with controlled velocities, +u toward and —d away from the absorber (cf. Fig. 2.6). The motion modulates the energy of the y-photons arriving at the absorber because of the Doppler effect Ey = Eq + d/c). Alternatively, the sample may be moved with the source remaining fixed. The transmitted y-rays are detected with a y-counter and recorded as a function of the Doppler velocity, which yields the Mdssbauer spectrum, r(u). The amount of resonant nuclear y-absorption is determined by the overlap of the shifted emission line and the absorption line, such that greater overlap yields less transmission maximum resonance occurs at complete overlap of emission and absorption lines. 

In a Mdssbauer transmission experiment, the absorber containing the stable Mdssbauer isotope is placed between the source and the detector (cf. Fig. 2.6). For the absorber, we assume the same mean energy q between nuclear excited and ground states as for the source, but with an additional intrinsic shift A due to chemical influence. The absorption Une, or resonant absorption cross-section cr( ), has the same Lorentzian shape as the emission line and if we assume also the same half width , cr( ) can be expressed as ([1] in Chap. 1)... 

Mdssbauer sources for the 6.2 keV resonance of Ta have mostly been produced by diffusing, under ultrahigh vacuum conditions, neutron-activated into... 

Table 7.12 Isomer shift 5 (with respect to the source of Au in Pt metal at 4.2 K), line width F, and percentage resonance effect / from Pt (99 keV) Mdssbauer measurements on platinum compounds at 4.2 K by Riiegg and coworkers [329, 332, 333]...

The electron density centered at M is the only central contributor at the nuclear position M, as in this case the nucleus coincides with the field point P, which is excluded from the integrals. For transition metal atoms, the central contributions are the largest contributors to the properties at the nuclear position, which can be compared directly with results from other experimental methods. The electric field gradient at the nucleus, for instance, can be measured very accurately for certain nuclei with nuclear quadrupole resonance and/or Mdssbauer spectroscopic methods, while the electrostatic potential at the nucleus is related to the inner-shell ionization energies of atoms, which are accessible by photoelectron and X-ray spectroscopic methods. 

Mdssbauer spectroscopy, 47 118-119 NMR spectroscopy, 47 134-137 resonance Raman spectroscopy, 47 119-121... 

The substitution of a different metal into an enzyme provides a very useful method for studying the immediate environment of the metal site. In addition to the use of Co2 for spectral studies, appropriate substitution allows the use of physical methods such as electron paramagnetic resonance (Co . Cu2 ). the Mdssbauer effect tFe2 ). proton magnetic resonance relaxation techniques (Mir ), or X-ray crystallography (with a heavy metal atom to aid in the structure solution). ... 

If the above three conditions are satisfied, the Mdssbauer effect is observed, and the cross section for resonant absorption resonant absorption per unit flux of impinging y photons with energy E) is given by... 

The preceding discussions form the basis for deciding under what conditions the Mdssbauer effect will be observed. However, observation of resonance between the nuclear levels of a source and absorber is just the first step in the application of this effect to chemical and catalytic phenomena. The perturbation of the nuclear levels by the chemical environment must now be considered, since it is in the latter that information pertinent to catalysis is found. 

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