Mossbauer Relaxation Spectra

For relaxation times t 1 ns, the spectra can be described as three Lorentzian lines with different line width, and for relaxation times around 10 ° s, the spectra appear as asymmetric doublets with line widths that decrease with decreasing relaxation time. In the theoretical spectra in Fig. 6.2, the EFG was assumed uniaxial 

In samples with negligible quadmpole interaction, isotropic relaxation with relaxation times around s results in spectra consisting of a single, 

Relaxation phenomena can also be studied by nuclear forward scattering of synchrotron radiation [16, 30]. This is discussed in Chap. 9. 

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Fig. 6.2 Theoretical Fe Mossbauer relaxation spectra for longitudinal relaxation with the indicated relaxation times and with a hyperline field that can assume the values 55 T. The symmetry direction of the axially symmetric EFG is assumed parallel to the magnetic hyperfine field. (Reprinted with permission from [9] copyright 1966 by the American Physical Society)...

Two different approaches have been followed to calculate the lineshapes within a relaxation model. According to a phenomenological approach based on the modified Bloch equations [154, 155], the intensity distribution of the theoretical Mossbauer spectrum may be written as [156] ... 

Dr. Spijkerman I would be wilhng to have hemoglobin 7-coordinated because we worked on the iron EDTA salts, which show a very asymmetric quadrupole splitting, and they are 7-coordinated. The spectra of hemoglobin and the iron EDTA salts are quite similar. I think that spin-spin relaxation occurs, which is more pronounced with 7-coordination than one would expect for 6 or 4. If this is the case, there might be some indication of it, but the Mossbauer spectrum certainly doesn t indicate it from the chemical shift or quadrupole splitting. 

To extract the desired relaxation time information from the Mbssbauer spectrum, there are essentially two approaches. The first requires a theoretical calculation of the Mossbauer spectra for various relaxation times spanning the range from tu/tl 1 to th/tl 1 (e.g., 56, 159, 160). As shown earlier [Eq. (42)], for a given particle of size d0, the relaxation time can be estimated by... 

These spectra, taken at variable temperatures and a small polarizing applied magnetic field, show a temperature-dependent transition for spinach ferredoxin. As the temperature is lowered, the effects of an internal magnetic field on the Mossbauer spectra become more distinct until they result at around 30 °K, in a spectrum which is characteristic of the low temperature data of the plant-type ferredoxins (Fig. 11). We attribute this transition in the spectra to spin-lattice relaxation effects. This conclusion is preferred over a spin-spin mechanism as the transition was identical for both the lyophilized and 10 mM aqueous solution samples. Thus, the variable temperature data for reduced spinach ferredoxin indicate that the electron-spin relaxation time is around 10-7 seconds at 50 °K. The temperature at which this transition in the Mossbauer spectra is half-complete is estimated to be the following spinach ferredoxin, 50 K parsley ferredoxin, 60 °K adrenodoxin, putidaredoxin, Clostridium. and Axotobacter iron-sulfur proteins, 100 °K. 

In Mossbauer spectroscopy, we encounter two types of expectation values for the electronic spin4 6 that we illustrate briefly for an iron site with S = 1/2 and g 2, taking the applied field along z. If the spin relaxation rate (spin flips between the Ms= + 1/2 and Ms= —1/2 sublevels) is slow compared to the nuclear precession frequency (which is typically 10—30 MHz Larmor precession around Bint or quadrupole precession), the nucleus senses the Fe atom in either the Ms= + 1/2 or Ms =1/2 state during the absorption process. In this case, we have (Sz) = + 1/2 for spin up and (Sz) = —1/2 for spin down. Each electronic level produces a Mossbauer spectrum, and these two spectra are weighted by the probability (given by the... 

Whether the relaxation is fast or slow at 4.2 K can be checked experimentally, using the following arguments. Figures 2.6b shows a 4.2 K Mossbauer spectrum recorded for B = 8.0 T. The solid line outlines the contribution of Z. The remaining absorption pattern, well understood, originates from the [Feni(H20)6]3 + contaminant. The sharpness of the absorption lines of Z shows that the intermediate has nearly axial symmetry thus, we can set EID = 0 in Equation 2.2 and Ax = Ay in Equation 2.3. We have recorded, and published, spectra in variable applied fields and at different temperatures.11 Since the spectra are a bit noisy, it will aid the reader if we... 

The 19F NMR resonance has been measured from 115 to 245 K at different field strengths of solid OsF6 and spin-lattice relaxation times determined.785 The electron affinity of OsF6 has been compared with those of other metal hexafluorides.762 786 787 The Mossbauer spectrum has been recorded.184... 

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