Isomer shift reference scale

For a comparison of experimental Mossbauer isomer shifts, the values have to be referenced to a common standard. According to (4.23), the results of a measurement depend on the type of source material, for example, Co diffused into rhodium, palladium, platinum, or other metals. For Fe Mossbauer spectroscopy, the spectrometer is usually calibrated by using the known absorption spectrum of metallic iron (a-phase). Therefore, Fe isomer shifts are commonly reported relative to the centroid of the magnetically split spectrum of a-iron (Sect. 3.1.3). Conversion factors for sodium nitroprusside dihydrate, Na2[Fe(CN)5N0]-2H20, or sodium ferrocyanide, Na4[Fe(CN)]6, which have also been used as reference materials, are found in Table 3.1. Reference materials for other isotopes are given in Table 1.3 of [18] in Chap. 1. 

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Fig. 7.68 Ir Mossbauer spectra of Ba2CaIr06 at 4.2 and 65 K (notice the different velocity scale) and of Ir metal (the isomer shift reference) at 4.2 K (from [310])...

Fig. 17. Some representative Mossbauer spectra of (Fe(2-CH30-phen)3](ClC>4)2 H2O at various temperatures the zero point of the velocity scale refers to the isomer shift of Na2[Fe(CN)sNO] 2 H2O relative to the 57Co/Cu source at 298 K. The doublets A, B and C are assigned to the 1 Aj, 5Aj and 5E states, serially105)...

By combining data for a range of almost completely ionic iodides, it has been possible to calibrate the isomer shift scales in terms of the electron configurations. The closed-shell configuration of the iodide ion is again taken as the reference point, and the following relationships have been derived1,5 ... 

To analyze the recorded spectra, the spectrometer needs to be calibrated. The three main calibration parameters are the velocity scale, the center point of the spectrum and the nonlinearity of the velocity/time profile of the oscillation compared to a standard reference. The calibration is performed using a spectrum recorded from an a-iron foil at room temperature using the well defined line positions of the sextet from a-iron, which occur at 5.312mms , 3.076mms , and 0.840mms The center of this a-iron spectrum at room temperature is taken as the reference point (0.0 nun s ) for isomer shift values of sample spectra. The typical Mossbauer spectrum of the 14.4 keV transition of Fe in natural iron (Fig. 4.10) represents a simple example of pure nuclear Zeeman effect. Because of the cubic symmetry of the iron lattice, there is no quadrupole shift of the nuclear energy levels. The relative intensities of the six magnetic dipole transitions are... 

A peak position of a component in the Doppler velocity scale provides us information both on the different lattice sites of Fe atoms and their charge states. This shift is called center shift , depending on the isomer shift and the second-order Doppler (SOD) shift. As a reference, the broken line in Fig. 6.28 shows the... 

The equivalence of the oxidation state in both protein active sites and tetranuclear synthetic analogues is also clearly established by comparing the isomer shifts Fe observed in the corresponding Mossbauer spectra. Isomer shifts may be indeed considered to be a measure of the electron density in vicinity of the iron atom in a given species relative to that in a reference compound. For iron compounds with equivalent coordination spheres, the isomer shifts correlate with iron formal oxidation states. Isomers shifts observed for both proteins and model compounds are consistent with the hypothesis that synthetic species [Fe4S4(SR)4] are isoelectronic with the active sites in the 4Fe and 8-Fe-Ferredoxins and HP protein. Mossbauer spectra confirm moreover that all Fe-atoms in Fe4S4 units in synthetic and natural species are strictly equivalent in the time-scale of this technique (ca. 10 s) so they are, as mentioned above, electronically delocalized systems. 

It is important to realize that isomer shifts (in mm/s) cannot be compared for different resonances. Not only does ot vary widely (and can have a positive or negative sign) but the transition energy q so enters [e.g. in eq. (19)]. For any given isotope the isomer shifts are commonly referred to a certain compound representing the zero point of the isomer shift scale. For example, in Np the intermetallic NpAl2 serves as reference. For other Mossbauer resonances see Cohen and Kalvius (1970). 

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