Hyperfine interaction, Mossbauer spectroscopy

Similar to bulk materials, through the recoilless fi-action and static hyperfine interactions, Mossbauer spectroscopy can be sensitive to a wide range of phenomena relevant to fine-particle systems. The main differences with respect to bulk studies come from the specific properties of the atoms near the surface. 

The third prominent interaction in iron Mossbauer spectroscopy is the magnetic hyperfine interaction of the Fe nucleus with a local magnetic field. As explained in detail in Chap. 4, it can be probed by performing the Mossbauer experiment in the presence of an applied external magnetic field. 

The first comprehensive review article on Zn Mossbauer spectroscopy, covering extensively the theory of hyperfine interactions, describing the spectrometer and cryogenic systems and reviewing the Zn Mossbauer effect studies of the early stage appeared in 1983 [64]. 

We have learned from the preceding chapters that the chemical and physical state of a Mossbauer atom in any kind of solid material can be characterized by way of the hyperfine interactions which manifest themselves in the Mossbauer spectrum by the isomer shift and, where relevant, electric quadrupole and/or magnetic dipole splitting of the resonance lines. On the basis of all the parameters obtainable from a Mossbauer spectrum, it is, in most cases, possible to identify unambiguously one or more chemical species of a given Mossbauer atom occurring in the same material. This - usually called phase analysis by Mossbauer spectroscopy - is nondestructive and widely used in various kinds of physicochemical smdies, for example, the studies of... 

The recoilless nuclear resonance absorption of y-radiation (Mossbauer effect) has been verified for more than 40 elements, but only some 15 of them are suitable for practical applications [33, 34]. The limiting factors are the lifetime and the energy of the nuclear excited state involved in the Mossbauer transition. The lifetime determines the spectral line width, which should not exceed the hyperfine interaction energies to be observed. The transition energy of the y-quanta determines the recoil energy and thus the resonance effect [34]. 57Fe is by far the most suited and thus the most widely studied Mossbauer-active nuclide, and 57Fe Mossbauer spectroscopy has become a standard technique for the characterisation of SCO compounds of iron. 

Rodionov, D. et al. 2006. Automated Mossbauer spectroscopy in the field and monitoring of fougerite. Hyperfine Interactions, 167, 869-873. 

In this chapter, we will first describe what the Mossbauer effect is, then explain why it can only be observed in the solid state and in a limited number of elements. Next we discuss the so-called hyperfine interactions between the nucleus and its environment, which make the technique so informative. After a few remarks on spectral interpretation we go systematically through a number of examples which show what type of information Mossbauer spectroscopy yields about catalysts. 

J. van de Loosdrecht, P. J. van Berge, M. W. J. Craje and A. M. van der Kraan, The application of Mossbauer emission spectroscopy to industrial cobalt based Fischer-Tropsch catalysts, Hyperfine Interact., 2002, 139/140, 3-18. 

As will be explained in Chapter 7, spectroscopic methods are a powerful way to probe the active sites of the hydrogenases. Often spectroscopic methods are greatly enhanced by judicious enrichment of the active sites with a stable isotope. For example, Mossbauer spectroscopy detects only the isotope Fe, which is present at only 2.2 per cent abundance in natural iron. Hydrogen atoms, which cannot be seen by X-ray diffraction for example, can be studied by EPR and ENDOR spectroscopy, which exploit the hyperfine interactions between the unpaired electron spin and nuclear spins. More detailed information has been derived from hyperfine interactions with nuclei such as Ni and Se, in the active sites. In FTTR spec-... 

Figure 14 shows three Fe case studies of the time behavior of the photons reemitted in the forward direction and a comparison with the typical spectra obtained in Mossbauer spectroscopy. Figure 14a corresponds to the case for which there is no hyperfine interaction. The nuclear levels are not split, and only one transition between ground and excited state is possible. In that case, the Mossbauer spectrum shows a single-absorption line and contains only y-quanta of equal energy. In the presence of an electric field gradient (Fig. 14b), the splitting of the excited state is... 

Mossbauer spectroscopy senses the hyperfine interactions, which are present at the nucleus of the Mossbauer isotope. The electrical monopole interaction causes the isomer shift and the electric quadrupole interaction leads to the quadrupole splitting, which in the case of Fe causes a two-line Mossbauer pattern. The magnetic dipole interaction leads to a magnetically split six-line pattern (Figure 4). In the following text, these interactions and their deduction from Mossbauer spectra will be discussed. 

Figure 1 depicts a typical sequence of events started by absorption of an incident photon with an energy near the nuclear excited state energy Eq. The Fe nucleus has an excited state lifetime of 141ns, and excited nuclei have two decay channels. About 10% of them reemit a 14.4kev photon. For recoilless absorption, where no vibrational levels are excited, time-resolved measurements of 14.4kev photons scattered in the forward direction reveal information on hyperfine interactions comparable to conventional Mossbauer spectroscopy (see Mossbauer Spectroscopy). The remaining nuclei expel electrons from the atomic K shell, followed by... 

Lalonde AE, Rancourt DG, Ping JY (1998) Accuracy of ferric/ferrous determinations in micas A comparison of Mossbauer spectroscopy and the Pratt and Wilson wet-chemical methods. Hyperfine Interactions 117 175-204... 

Pfannes H-D (1997) Simple theory of superparamagnetism and spin-tuimeling in Mossbauer spectroscopy. Hyperfine Interactions 110 127-134... 

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