Resonance absorption, Mdssbauer

In a Mdssbauer transmission experiment, the absorber containing the stable Mdssbauer isotope is placed between the source and the detector (cf. Fig. 2.6). For the absorber, we assume the same mean energy q between nuclear excited and ground states as for the source, but with an additional intrinsic shift A due to chemical influence. The absorption Une, or resonant absorption cross-section cr( ), has the same Lorentzian shape as the emission line and if we assume also the same half width , cr( ) can be expressed as ([1] in Chap. 1)... 

If the above three conditions are satisfied, the Mdssbauer effect is observed, and the cross section for resonant absorption resonant absorption per unit flux of impinging y photons with energy E) is given by... 

Fig. 2.44. Mossbauer spectroscopy (a) nuclear transitions giving rise to the Mossbauer effect in Fe (b) principles involved in the Mossbauer spectrometer (c) Mdssbauer resonant absorption of iron in different crystal environments and the resulting spectral types. (After Vaughan and Craig, 1978 reproduced with the publisher s permission).

The Mossbauer effect is the emission and resonant absorption of nuclear y-rays studied under conditions such that the nuclei have negligible recoil velocities when y-rays are emitted or absorbed. This is only achieved by working with solid samples in which the nuclei are held rigidly in a crystal lattice. The energy, and thus the frequency of the y-radiation involved, corresponds to the transition between the ground state and the short-lived excited state of the nuclide concerned. Table 2.4 lists properties of several nuclei which can be observed using Mdssbauer spectroscopy. 

There are few direct spectroscopic probes for iron for instance, in most complexes charge transfer bands obscure the d-d transitions and magnetic measurements cannot provide information on low-spin iron(II) complexes. Therefore when the Mdssbauer technique became generally available in the late 1960s, it was widely used for the study of iron complexes. The effect arises from the resonant absorption of y-rays by the Fe nucleus, which has a natural abundance of 2%. When the y-ray source and sample nuclei are in different environments, the shift in the energy of resonant absorption is expressed as a velocity relative to some arbitrary zero such as stainless steel or Na2[Fe(CN)5N0] 2H20. Whilst the value of the shift is temperature dependent, it is also affected by the electronic environment. 

Spectroscopic methods for hydration of ions were reviewed for structural aspects and dynamic aspects of ionic hydration by Ohtaki and Radnai (150). They discussed X-ray diffraction, neutron diffraction, electron diffraction, small-angle X-ray (SAXS) and neutron-scattering (SANS), quasi-elastic neutron-scattering (QENS) methods, extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES) spectroscopies, nuclear magnetic resonance (NMR), Mdssbauer, infrared (IR), Raman, and Raleigh-Brillouin spectroscopies. The clay interlayer molecular modeling where clay surface is interfaced with aqueous solution also includes ions that are also solvated by interlayer water. 

However, in contrast, the resonance effect increased by cooling both the source and the absorber. Mdssbauer not only observed this striking experimental effect that was not consistent with the prediction, but also presented an explanation that is based on zero-phonon processes associated with emission and absorption of y-rays in solids. Such events occur with a certain probability/, the recoil-free fraction of the nuclear transition (Sect. 2.4). Thus, the factor/is a measure of the recoilless nuclear absorption of y-radiation - the Mdssbauer effect. 

In an actual Mdssbauer transmission experiment, the radioactive source is periodically moved with controlled velocities, +u toward and —d away from the absorber (cf. Fig. 2.6). The motion modulates the energy of the y-photons arriving at the absorber because of the Doppler effect Ey = Eq + d/c). Alternatively, the sample may be moved with the source remaining fixed. The transmitted y-rays are detected with a y-counter and recorded as a function of the Doppler velocity, which yields the Mdssbauer spectrum, r(u). The amount of resonant nuclear y-absorption is determined by the overlap of the shifted emission line and the absorption line, such that greater overlap yields less transmission maximum resonance occurs at complete overlap of emission and absorption lines. 

The conclusion that the cobalt and iron complexes 2.182 and 2.183 are formally TT-radical species is supported by a wealth of spectroscopic evidence. For instance, the H NMR spectrum of the cobalt complex 2.182 indicated the presence of a paramagnetic system with resonances that are consistent with the proposed cobalt(III) formulation (as opposed to a low-spin, paramagnetic cobalt(IV) corrole). Further, the UV-vis absorption spectrum recorded for complex 2.182 was found to be remarkably similar to those of porphyrin 7r-radicals. In the case of the iron complex 2.183, Mdssbauer spectroscopy was used to confirm the assignment of the complex as having a formally tetravalent metal and a vr-radical carbon skeleton. Here, measurements at 120 K revealed that the formal removal of one electron from the neutral species 2.177 had very little effect on the Mdssbauer spectrum. This was interpreted as an indication that oxidation had occurred at the corrole ligand, and not at the metal center. Had metal oxidation occurred, more dramatic differences in the Mdssbauer spectrum would have been observed. 

The absorption cross-section Uq should be large and the free-atom recoil energy should be small. Both of these factors have already been mentioned in connection with other quantities above. Appendix 1 lists 5 transitions for which CTo > 10 cm and few Mdssbauer resonances have been observed for transitions in which < 0 06 x 10 cm. Likewise only 5 transitions have a free-atom recoil > 6 x 10 eV and values are normally in the range (0-1-5) x 10 eV. 

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