Mossbauer effect experiments

Parent nuclides produced by the processes mentioned above can all be used for several half-lives. In contrast, one can also populate the Mossbauer excited state directly via Coulomb excitation (84). In this technique, a beam of high-energy ( 10 MeV) charged particles (e.g., O4+, Cl7 +) is directed onto the Mossbauer isotope and the electromagnetic field generated by these particles induces nuclear transitions, which can include transitions to the Mossbauer excited state. Subsequent decay to the nuclear ground state then provides the appropriate y radiation. The half-life of a source created in this manner is the half-life of the Mossbauer excited state (e.g., several nanoseconds), and thus Coulomb excitation is necessarily an in situ technique, i.e., the Mossbauer effect experiment must be performed at the location of the charged particle beam. 

The Mossbauer-effect experiment can also be applied to the study of surfaces in the variation known as conversion electron Mossbauer spectroscopy (CEMS). Here, what is monitored as a function of incident y-ray energy is not absorption, but the emission of electrons through a process of internal conversion (i.e., as a byproduct of the absorption of Mossbauer y rays). Since the conversion electrons can only escape from the surface layers of the solid, data are selectively acquired for the surface region, arising from the Mossbauer effect in the (most commonly iron) atoms of the surface layers. The monitoring of emitted electrons results in a mirror image of the usual absorption spectrum. Transmission and CEM spectra of vivianite [Ee3(P04)2-8H20] are illustrated in Fig. 2.49 (after Tricker et al., 1979]. 

In nuclear gamma resonance (Mossbauer effect) experiments, the nuclear hyperfine interactions are measured via their influence on the energy of the gamma ray emitted (or absorbed) when the nucleus undergoes a transition between an excited state and its ground state (or between two excited states). Since the nuclear hyperfine energies are small ( 10" -10 eV) compared with... 

The refined spectral parameters for the individual components observed in the room-temperature (300 K) Mossbauer effect experiments, including the spectral area (Arei), isomer shift (IS), quadrupole splitting (QS) as well as the hyperfine magnetic field (fihr), are listed in Table 4.29. The hyperfine parameters for the identified components (hematite, magnetite, goethite, lepidocrocite and feroxyhyte) are listed in [215]. 

In a conventional Fe Mossbauer experiment with a powder sample, one would observe a so-called quadrupole doublet with two resonance lines of equal intensities. The separation of the lines, as given by (4.36), represents the quadrupole splitting The parameter Afg is of immense importance for chemical applications of the Mossbauer effect. It provides information about bond properties and local symmetry of the iron site. Since the quadrupole interaction does not alter the mean energy of the nuclear ground and excited states, the isomer shift S can also be derived from the spectrum it is given by the shift of the center of the quadrupole spectrum from zero velocity. 

The nuclear decay of radioactive atoms embedded in a host is known to lead to various chemical and physical after effects such as redox processes, bond rupture, and the formation of metastable states [46], A very successful way of investigating such after effects in solid material exploits the Mossbauer effect and has been termed Mossbauer Emission Spectroscopy (MES) or Mossbauer source experiments [47, 48]. For instance, the electron capture (EC) decay of Co to Fe, denoted Co(EC) Fe, in cobalt- or iron-containing compormds has been widely explored. In such MES experiments, the compormd tmder study is usually labeled with Co and then used as the Mossbauer source versus a single-line absorber material such as K4[Fe(CN)6]. The recorded spectrum yields information on the chemical state of the nucleogenic Fe at ca. 10 s, which is approximately the lifetime of the 14.4 keV metastable nuclear state of Fe after nuclear decay. 

The conclusion of all this is the following. If we place our two atoms in a lattice and do the experiment under conditions where recoil energy of the photon emission and absorption are significantly smaller than the energy of the lattice vibrations, a fraction of the photons emitted by the source nucleus will be absorbed by the nucleus in the absorber. This is the Mossbauer effect, named after Rudolf L. Mossbauer, who discovered it in 1957 and received the Nobel Prize in 1961 [9]. 

Mossbauer spectroscopy of the 57Fe nucleus has been extensively used to investigate aspects of spin equilibria in the solid state and in frozen solutions. A rigid medium is of course required in order to achieve the Mossbauer effect. The dynamics of spin equilibria can be investigated by the Mossbauer experiment because the lifetime of the excited state of the 57Fe nucleus which is involved in the emission and absorption of the y radiation is 1 x 10 7 second. This is just of the order of the lifetimes of the spin states of iron complexes involved in spin equilibria. Furthermore, the Mossbauer spectra of high-spin and low-spin complexes are characterized by different isomer shifts and quad-rupole coupling constants. Consequently, the Mossbauer spectrum can be used to classify the dynamic properties of a spin-equilibrium iron complex. 

The role of iron clusters in Fischer-Tropsch catalysis has been the focus of considerable studies. Cagnoli et al. have recently studied the role of Fe clusters on silica and alumina supports for methanation.22 Chemisorption, catalysis and Mossbauer spectroscopy experiments were used to study the effect of dispersion and the role of various supports. Although several oxidation states of iron were observed, the focus of this research was on Fe clusters which were found to be on the order of 12 A crystal size. The authors proposed that metal support interactions were greater for silica than alumina supports and that selectivity differences between these catalysts were due to differences in surface properties of silica vs. alumina. Differences in selectivity for Fe/SiC>2 catalysts at different H2/CO ratios were attributed to differences in coadsorption of H2 and CO. Selectivity differences are difficult to explain in such systems even when only one metal is present. 

In a standard Mossbauer transmission experiment the absorber is placed between the source and the detector. In order to observe the effect. 

Although more than 50 nuclei are known for which gamma-ray resonance fluorescence (the Mossbauer effect) has been observed,1 the present contribution will concern itself primarily with data pertinent to experiments using the 14.4 keV radiation of s7Fe and the 23.8 keV radiation of ll9Sn, since the vast majority of studies related to chemical structure problems have exploited these two nuclides. 

In conducting Mossbauer effect spectroscopy experiments on platinum-iridium catalysts, one might incorporate the Mossbauer nuclides 195Pt and/or 193lr in the catalysts. However, experiments with these nuclides are more difficult because of short-lived sources and the requirement for measurements... 

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