Nuclear forward scattering

12 SYNCHROTRON RADIATION-BASED NUCLEAR RESONANT SCATTERING 

TABLE 12.1 Hyperfine Interaction Parameters Obtained from Zero-Field Conventional Fe Mossbauer Parameters Fe NFS Spectra Compared with the  

The numbers in parentheses give the error in the last significant digit. 

The NFS spectra of oxidized Rc FdVI reveaied a diamagnetic [2Fe-2S] center with an averaged A q of +0.64 mms and t] = 0.S, which is similar with the Mossbauer parameters reported on several other oxidized [2Fe-2S] ferridoxins [44-48], indicating the [2Fe-2S] center in the oxidized Rc FdVI has very similar structural and electronic environments with the [2Fe-2S] center in other types of oxidized [2Fe-2S] ferridoxins. 

The best fit of the zero-field spectrum was obtained by introducing four different iron sites. The obtained parameters are listed in Table 12.1. Since no reference sample was used, the absolute isomer shift information cannot be obtained. However, the relative isomer shift of each iron site can still be obtained by setting one of the four sites as a reference. The best fit gave four quadrupole splittings as 1.48, 1.29, I. I, and 0.63 mm s . The relative isomer shift of the last three sites relative to the first site were —0.007, 0.012, and 0.042 mm s , indicating they have very similar isomer shifts. These values are consistent with the zero-field 80 K Mossbauer measurement on the same sample (the inset of Fig. 12.8a), where the four quadrupole splittings were 1.40, 1.13,0.84, and 0.54 mm s , the corresponding isomer shifts were 0.42, 

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

Fig. 7.17 Time evolution of the nuclear forward scattering for metallic Ni foil. All measurements except for the upper curve were performed with external magnetic field B = 4 T. The solid lines show the fit. The arrows emphasize stretching of the dynamical beat structure by the applied magnetic field. The data at times below 14.6 ns had to be rescaled (from [34])...

Ta foil Nuclear forward scattering of synchrotron radiation (NFS) at Ta resonance in Ta foil without and with applied magnetic field, point out advantages over conventional Ta Mossbauer spectroscopy... 

Ta metal Stroboscopic detection of nuclear forward scattered synchrotron radiation is used to detect the high-resolution 6.2 keV Mossbauer resonance of Ta in Ta metal... 

Fig. 9.24 Theoretical calculations of nuclear forward scattering for the relaxation rates as indicated for a system with electron spin S = 1/2, hyperfine parameters A y jg fi = 50 T, and AF.q = 2 mm s in an external field of 75 mT applied perpendicular to k and O . The transition probabilities co in ((9.8a) and (9.8b)) are expressed in units of mm s , with 1 mm corresponding to 7.3 10 s. (Taken Ifom [30])...

Nuclear inelastic scattering (NIS), nuclear forward scattering (NFS), nuclear lighthouse effect (NLE), synchrotron radiation-based perturbed angular correlation (SRPAC)... 

About twenty years ago we reported on the di-isothiocyanato iron(II) complex of the tetradentate ligand tpa (tris(2-pyridylmethyl)amine) [7] (6). It was shown that this complex exhibits the spin crossover phenomenon with a critical temperature Tm of about 170 K. Several different solvated phases of the same system have since been characterized by Chansou et al. [8]. The unsolvated phase which can be isolated from an aqueous solution has been investigated by nuclear forward scattering (NFS), nuclear inelastic scattering (NIS) [9], extended x-ray absorption fine structure (EXAFS) spectroscopy, conventional Mossbauer spectroscopy, and by measurements of the magnetic susceptibility (SQUID) [10-13]. The various measurements consistently show that the transition is complete and abrupt and it exhibits a hysteresis loop between 102 and 110 K. 

Other improvements in spatial resolution in the future should allow more accurate investigation of conventional powdered samples such as catalysts. This will likely be made possible as a result of developments in synchrotron investigations of nuclear forward scattering (presented in the next section) (215). The development of a Mossbauer electron microscope that would focus conversion electrons using conventional electron optics has also been mentioned 216). 

Fig. 15. Basic equipment for measuring a nuclear inelastic scattering spectrum. Detector 1 measures the intensity of the incoherent nuclear forward scattering, which proceeds both elastically and inelas-tically detector 2 measures only the intensity of the coherent nuclear forward scattering, which proceeds elastically. Figure according to Ruffer and Chumakov (224).

CEMS = conversion electron Mossbauer spectroscopy DFT = density functional theory EFG = electric field gradient EPR = electron paramagnetic resonance ESEEM = electron spin echo envelope modulation spectroscopy GTO = Gaussian-type orbitals hTH = human tyrosine hydroxylase MIMOS = miniaturized mossbauer spectrometer NFS = nuclear forward scattering NMR = nuclear magnetic resonance RFQ = rapid freeze quench SAM = S -adenosyl-L-methionine SCC = self-consistent charge STOs = slater-type orbitals TMP = tetramesitylporphyrin XAS = X-ray absorption spectroscopy. 

This chapter has presented a review of the parameters involved in Mossbauer spectroscopy within the context of phase transformations. Although the Mossbauer effect can be considered a mature technique, now more than forty years old, technical developments continue to expand the experimental possibilities. Spatial resolution has improved within the last decade. The development of the Mossbauer milliprobe, for example, has enabled spatial resolution to be increased by more than two orders of magnitude (McCammon et al. 1991, McCammon 1994). Further improvement of spatial resolution may be anticipated with advances in nuclear forward scattering. Other possibilities on the horizon include development of a Mossbauer electron microscope which would focus conversion electrons using conventional electron optics (Rancourt and Klingelhofer 1994). 

In a nuclear resonant scattering experiment all resonant levels of the Mossbauer nuclei in the sample are simultaneously excited by a short pulse of synchrotron radiation, creating the nuclear exciton. The time dependence of the delayed intensity emitted upon de-excitation of the nuclear exciton in forward direction is the time spectrum of nuclear forward scattering (NFS). 

Calculated Mossbauer transmission spectra (a), nuclear forward scattering spectra in energy (b) and in time (c) domain for the case of a quadrupole doublet in a 0.2 jtm thick stainless steel foil 100% enriched in Fe. (d), (e), and (f) are the corresponding spectra for a 3.0 ji,m thick stainless steel foil 100% enriched in Fe. (Reproduced from Ref. 46 with permission of Kluwer Academic Publishers.)... 

In general, the time dependence of the nuclear forward scattering comprises both quantum and dynamical beats, as depicted in Fig. l.lOf for the case of electric quadrupole interaction. If the resonances are well separated, the time development of the amplitude of nuclear forward scattering from a sample of thickness d can be expressed as [22]... 

Time spectra of nuclear forward scattering from a-Fe foil with various thicknesses. The polycrystalline foils are in magnetic state where all six transitions are allowed. (Reproduced from Ref. 22 with permission of Springer.)... 

Time evolution of nuclear forward scattering from iron foils (95% enriched in Fe) of different thicknesses in a vertical magnetic field. Only the two Am = 0 transitions are excited. The solid lines are computations based on (1.15), while the dashed lines indicate the exponential decay of the envelope as calculated using (1.19). (Reproduced from Ref. 47 with permission of Kluwer Academic Publishers.)... 

Figure 1.24 shows time spectra of nuclear forward scattering from a single crystal of FeAl measured at the indicated temperatures [85,86,90]. With increasing temperature one clearly observes an accelerated decay of the nuclear exciton. The maximal values of the diffusion coefficient D determined by this method are limited by the accessibility of early times after the excitation within which the fast decay can be observed. Assuming start time for the time spectra at 20 ns sets an upper limit for D < 10 m s. ... 

Y.V. Shvyd ko, U. van Burck, Hybrid forms of beat phenomena in nuclear forward scattering of synchrotron radiation, Hyperfine Interact. 1999, /23(1-8), 51 1-527. 

Y.V. Shvyd ko, MOTIF evaluation of time spectra for nuclear forward scattering, Hyperfine Interact 2000, /25(l-4), 173-188. 

Fig. 1.10 Coherent excitation and coherent de-excitation of the nucleus for the nuclear forward scattering and the nuclear Bragg scattering. Coherent de-excitation of the resonant scattering creates the quantum beat with a frequency of Q...

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