Mossbauer chemical influence

Fig. 2.8 (a) Fractional absorption of a Mossbauer absorption line as function of the effective absorber thickness t. (b) The depth of the spectrum is determined by fs. The width for thin absorbers, t 1, is twice the natural line width F of the separate emission and absorption lines (see (2.30)). AE is the shift of the absorption line relative to the emission line due to chemical influence... 

The study of chemical speciation requires that the irradiation energy be in the range of chemical bonds. There are some methods in which chemical structure can influence the properties of the nuclei. The most important method is nuclear magnetic resonance spectroscopy (NMR). Another example is Mossbauer spectroscopy, in which the oxidation state and the chemical environment affect the... 

The interactions between the nucleus and the atomic electrons depend strongly on the electronic, chemical and magnetic state of the atom. Information from these hyperfine interactions is provided by the hyperfine parameters, which can be determined experimentally from the line positions in a Mossbauer spectrum. The following section gives only a brief description of the parameters themselves, since this information is widely available in the references listed in Table 1. The section focuses on the structural and electronic information available from the parameters, and on the influence of... 

The chemical or isomer shift (8) is a linear function of electron density (due to electrons occupying s orbitals) at the nucleus. This, in turn, is influenced by many factors (oxidation state, spin state, -character i n the c bonds, dn back-bonding, ionicity), and correlations are not simple. For low-spin complexes 8 is rather independent of oxidation state from —2 to +2. Even Fe(CN) and Fe(CN) have almost identical 8 values. For high-spin compounds, however, 8 varies markedly with formal oxidation state and indeed provides an excellent means of establishing it. A few examples of how Mossbauer spectra can be employed in studying the chemistry of iron will now be given others will be mentioned as appropriate below. 

The name chemical isomer shift is related to the fact that the electron density at the place of the nucleus is strongly influenced by the valence state and the chemical bonding of the MOssbauer atom. 

The study of the functionalization of nanoparticles by molecules requires a good knowledge of both the structure and the chemical nature of the surface state but the chemical procedure may influence on the density of grafting, the chemical and structural evolution of the surface and the chemical bonding at the interface defined by the molecule and the nanoparticulate substrate. It is important to emphasize that the characterization of the chemical bonding is crucial in order to make the functionalized nanoparticles useful and suitable for some biomedical applications as drug delivery. Some relevant information can be obtained by comparing Mossbauer spectra recorded in the same conditions on nanoparticles before and after functionalization, combined to further spectroscopic techniques. 

The observed lineshift is not entirely caused by the chemical isomer shift. There is another generally smaller contribution termed the second-order Doppler shift. The emitting or absorbing nucleus is not stationary - it is vibrating at its position in the lattice. The period of its vibration is much shorter than the Mbssbauer lifetime, so the average displacement and velocity are effectively zero, but the mean-squared values of the velocity, (v ), are finite, v) is zero, so only the second-order term in (v ) can influence the Mossbauer resonance. This gives rise to a shift in the Mossbauer line of... 

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