The Mossbauer spectrum

The isomer shift provides considerable valuable information concerning oxidation states and bond properties of certain atoms as well as the interaction with surrounding ligands thereof since it is a measurement of the difference of both the radius of the nucleus and the electronic charge density near the nucleus between the source and absorber. 

Experimentally, isomer shift is presented in the dimensions of velocity rather than in energy dimensions. Useful diagrams of its ranges as a function of oxidation states of atoms is introduced to help analyze the Mossbauer spectrum 

Electric quadruple splitting is related to the electric quadruple moment, the inherited property of a certain type of atom, and the EFG, a measurement of the electric field surrounding an atom. This parameter concerns the electronic structure of the nucleus and the chemical environment and is hence quite useful for determining the fingerprint for valence and co-ordination while correlated with isomer shift, i.e. local distortions, local defects, spin reorientations, and crystal field asymmetry.  

Magnetic dipole splitting provides information eoneeming spin interaction processes. Information can be derived with referenee to the electron configuration, magnetic relaxation, magnetic moment, spin value, magnetic transition 

This section will introduce the application of Mossbauer spectroscopy for the study of elemental speciation in environmental and biological samples. The 

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Figure 4.54 The effect of an electric field gradient (EFG) creating asymmetry in the electron distribution round a gold nucleus, leading to a quadrupole splitting in the Mossbauer spectrum. (Reproduced with permission from Gold Bull., 1982,15, 53, published by World Gold Council.)...

Figure 4.55 The Mossbauer spectrum of Cs2(AuI2)(AuI4) showing separate resonance from (i) the gold and (ii) the gold(III) sites. (Reproduced with permission from J. Chem. Soc., Dalton...

The Mossbauer spectrum of the rearranged compound corresponds with that of a Sn(IV) compound, and the most probable structure appears to be that of a chlorine-bridged dimer. 

Some early studies [126] on the S-methyldithiocarbazate of 2-formylpyridine, 6, showed it to form low spin iron(III) species, [Fe(6-H)2]C104 and [Fe(6-H)2] [FeCm. The Mossbauer spectrum of the latter showed the presence of two different iron centers, and the compound was a 1 1 electrolyte in nitromethane. 

Most Mossbauer spectra are split because of the hyperfine interaction of the absorber (or source) nuclei with their electron shell and chemical environment which lifts the degeneracy of the nuclear states. If the hyperfine interaction is static with respect to the nuclear lifetime, the Mossbauer spectrum is a superposition of separate lines (i), according to the number of possible transitions. Each line has its own effective thickness t i), which is a fraction of the total thickness, determined by the relative intensity W of the lines, such that t i) = Wit. 

The number of beatings in the output signal U t) can be recorded as a function of the channel number simultaneously to the Mossbauer spectrum (usually in a second part of the MCA memory). Accordingly, the number of beats stored in each channel per velocity sweep divided by the dwell time of the MCA channels yields the specific average velocity for each detection channel. 

As discussed in Sects. 3.1.1-3.1.3, successful acquisition of Mossbauer spectra depends on accurate knowledge of the relative velocity of the source and sample. External vibrations that impart differential velocity components to the source and sample would degrade the quality of the Mossbauer spectrum. This degradation... 

Radioactive Ru is dilutely doped in Fe304. The value of the magnetic hyperfine field (at the Ru nucleus) obtained by TDPAC at 10 K (—16 T) is essentially in agreement with that derived from the Mossbauer spectrum at 5 K (14.5 T). The negative sign of means that the Ru ions... 

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... 

Fig. 8.16 Fe Mossbauer spectra of [Fe2 (PMAT)2](BF4)4-DMF at selected temperatures. At 298 K, the only quadrupole doublet is characteristic of iron(II) in the HS state. SCO from HS to LS occurs at one Fe(II) site of the dinuclear complex at ca. 225 K. The second Fe(II) site remains in the HS state, but feels the spin state conversion of the neighboring atom by local distortions communicated through the rigid bridging ligand, giving rise to a new quadrupole doublet (dark gray), i.e., HS in [HS-LS], in the Mossbauer spectrum. The intensity ratio of the resonance signals of HS in [HS-LS] to that of LS (black) in [HS-LS] is close to 1 1 at all temperatures (from [32])...

Lowering temperature to 200 K, the thermally induced ST at the central Fe(ll) site sets in with the appearance of two new doublets in the Mossbauer spectrum (1) the poorly resolved doublet termed LS refers to central Fe(ll) sites now being in the LS state, and (2) the doublet termed arises from the outer Fe(ll) sites of... 

The Mossbauer spectrum of the dianion was a surprise because the isomer shift (5 = 0.48 mm s ) was substantially more negative than that of the monoanion, and resembles more that of Fe(II)(TPP) than that of [Fe(I)TPP] . Reed and Scheldt therefore suggested a resonance hybrid of iron(I) and iron(II) for the electronic structure of the dianion molecule instead of the formulation [Fe(0)(TPP)] [290, 291]. 

Cosine smearing. Because instrument volume and experiment time must both be minimized for a planetary Mossbauer spectrometer, it is desirable in backscatter geometry to illuminate as much of the sample as possible with source radiation. However, this requirement at some point compromises the quality of the Mossbauer spectrum because of an effect known as cosine smearing [327, 348, 349] (see also Sects. 3.1.8 and 3.3). The effect on the Mossbauer spectrum is to increase the linewidth of Mossbauer peaks (which lowers the resolution) and shift their centers outward (affects the values of Mossbauer parameters). Therefore, the diameter of the source y-ray beam incident on the sample, which is determined by a... 

Fig. 8.31 Left) In the Mossbauer spectrum taken in the Columbia Hills at a rock called Clovis the mineral goethite (GT) (a-FeOOH) could be identified. GT is a clear mineralogical evidence for aqueous processes on Mars. Right) The rock Clovis is made out of rather soft material as indicated by the electric drill-current when drilling the - - I cm deep hole seen in the picture. Drill fines are of brownish color. The pattern to the right of the drill hole was made by bmshing the dust off the surface by using the RAT...

Fig. 8.38 (Left) The Mossbauer spectrum of the rock called Heat Shield rock, clearly shows with high intensity the mineral Kamacite, an Fe-Ni alloy with about 6-7% Ni (Right) The iron-nickel meteorite Meridiani Planum (originally called Heat Shield Rock ) at Opportunity landing site, close to the crater Endurance. The meteorite is about 30 cm across (Courtesy NASA, JPL, Cornell University)...

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