Nuclear resonances

This method, introduced originally in an analysis of nuclear resonance reactions, has been extensively developed [H, 16 and F7] over the past 20 years as a powerful ab initio calculational tool. It partitions configuration space into two regions by a sphere of radius r = a, where r is the scattered electron coordinate. 

P. Diehl in T. Axenrod and G. Webb, eds. Nuclear Resonance Spectroscopy of Nuclei Other Than Protons, John Wiley Sons, Inc., New York, 1974, pp. 275-285. 

Chemical shifts in nuclear resonances of caesium ions in solution, Proc. Royal Soc. London. 313 45 (1969). 

Winkler H, Chumakov AI, Trautwein AX (2004) Nuclear Resonant Forward and Nuclear Inelastic Scattering Using Synchrotron Radiation for Spin Crossover Systems. 235 105-136... 

The spin-echo experiment therefore leads to the refocusing not only of the individual nuclear resonances but also of the field inhomogeneity components lying in front or behind those resonances, a maximum negative amplitude being observed at time 2t after the initial 90° pulse. The frequency of rotation of each signal in the rotating frame will depend on its chemical shift and after the vector has been flipped by the 180° pulse, it... 

Britt and Yen , using the pulsed nuclear resonance technique, have obtained exchange data comparable to those obtained by the isotopic method. An observed... 

Unfortunately, many other nuclei of importance in chemistry, such as and 0, have nuclear spin values of 0 and hence do not give nuclear resonance signals in a magnetic field. 

Fig. 2.1 Nuclear resonance absorption of y-rays (Mossbauer effect) for nuclei with Z protons and N neutrons. The top left part shows the population of the excited state of the emitter by the radioactive decay of a mother isotope (Z, N ) via a- or P-emission, or K-capture (depending on the isotope). The right part shows the de-excitation of the absorber by re-emission of a y-photon or by radiationless emission of a conversion electron (thin arrows labeled y and e , respectively)...

Resonant y-ray absorption is directly connected with nuclear resonance fluorescence. This is the re-emission of a (second) y-ray from the excited state of the absorber nucleus after resonance absorption. The transition back to the ground state occurs with the same mean lifetime t by the emission of a y-ray in an arbitrary direction, or by energy transfer from the nucleus to the K-shell via internal conversion and the ejection of conversion electrons (see footnote 1). Nuclear resonance fluorescence was the basis for the experiments that finally led to R. L. Mossbauer s discovery of nuclear y-resonance in ir ([1-3] in Chap. 1) and is the basis of Mossbauer experiments with synchrotron radiation which can be used instead of y-radiation from classical sources (see Chap. 9). 

Hence, nuclear resonance absorption of y-photons (the Mbssbauer effect) is not possible between free atoms (at rest) because of the energy loss by recoil. The deficiency in y-energy is two times the recoil energy, 2Er, which in the case of Fe is about 10 times larger than the natural line width F of the nuclear levels involved (Fig. 2.4). 

Fig. 2.6 Schematic illustration of a Mossbauer transmission experiment in five steps. The Absorption bars indicate the strength of recoilless nuclear resonant absorption as determined by the overlap of emission and absorption lines when the emission line is shifted by Doppler modulation (velocities Uj,. .., 1)5). The transmission spectrum T v) is usually normalized to the transmission T oo) observed for v oo by dividing T(v)IT(oo). Experimental details are found in Chap. 3...

Leupold et al. were the first to report on coherent nuclear resonant scattering of synchrotron radiation from the 67.41 keV level of Ni. The time evolution of the forward scattering was recorded by employing the so-called nuclear lighthouse... 

Potzel et al. [Ill] have established recoil-free nuclear resonance in another ruthenium nuclide, ° Ru. This isotope, however, is much less profitable than Ru for ruthenium chemistry because of the very small resonance effect as a consequence of the high transition energy (127.2 keV) and the much broader line width (about 30 times broader than the Ru line). The relevant nuclear properties of both ruthenium isotopes are listed in Table 7.1 (end of the book). The decay... 

The first report on hafnium Mossbauer measurements by Wiedemann et al., in 1963 [162], deals with the 88.36 keV transition in Observations of the Hf and Hf nuclear resonances were described in 1966, and that of the Hf resonance in 1968 by Gerdau and coworkers [163, 164]. In the majority of hafnium... 

Nuclear resonance absorption for the 136 keV transition has been established by Steiner et al. [174]. The authors used a metal source and an absorber of metallic tantalum to determine the mean lifetime of the 136 keV level from the experimental line width ( 52.5 mm s for zero effective absorber thickness) and found a value of 55 ps. This has been the only report so far on the use of the 136 keV excited state of Ta for Mossbauer experiments. 

Ta foil First successful observation of nuclear resonance scattering of synchrotron radiation (NRS) with Ta... 

There are two iridium isotopes, ir and Ir, suitable for Mossbauer spectroscopy. Each of them possesses two nuclear transitions with which nuclear resonance absorption has been observed. Figure 7.58 (from [266]) shows the (simplified) nuclear decay schemes for both iridium Mossbauer isotopes the Mossbauer transitions are marked therein with bold arrows. The relevant nuclear data known to date for the four Mossbauer transitions are collected in Table 7.1 at the end of the book. 

It is a matter of historical interest that Mossbauer spectroscopy has its deepest root in the 129.4 keV transition line of lr, for which R.L. Mossbauer established recoilless nuclear resonance absorption for the first time while he was working on his thesis under Prof. Maier-Leibnitz at Heidelberg [267]. But this nuclear transition is, by far, not the easiest one among the four iridium Mossbauer transitions to use for solid-state applications the 129 keV excited state is rather short-lived (fi/2 = 90 ps) and consequently the line width is very broad. The 73 keV transition line of lr with the lowest transition energy and the narrowest natural line width (0.60 mm s ) fulfills best the practical requirements and therefore is, of all four iridium transitions, most often (in about 90% of all reports published on Ir Mossbauer spectroscopy) used in studying electronic stractures, bond properties, and magnetism. 

Mossbauer et al. [294] studied Ir-Fe and Ir-Ni alloy systems over the whole composition range by means of Ir (73 keV) and Fe nuclear resonance... 

Nearly aU Mossbauer measurements on iridium compounds and alloys have been carried out with the Ir (73 keV) and not with the lr (129 keV) transition, though R.L. Mossbauer originally discovered the nuclear resonance phenomenon with the latter nuchde. An overview of noteworthy pubhcations, which were published after 1978, when the first edition of this book appeared, is collected in Table 7.10. 

Nuclear Resonance Scattering Using Synchrotron Radiation (Mossbauer Spectroscopy in the Time Domain)... 

NIS of synchrotron radiation yields details of the dynamics of Mossbauer nuclei, while conventional MS yields only limited information in this respect (comprised in the Lamb-Mossbauer factor /). NIS shows some similarity with Resonance Raman- and IR-spectroscopy. The major difference is that, instead of an electronic resonance (Raman and IR), a nuclear resonance is employed (NIS). NIS is site-selective, i.e., only those molecular vibrations that contribute to the overall... 

Nuclear Resonance Scattering Using Synchrotron Radiation... 

Some of the basic features of nuclear resonance scattering using synchrotron radiation (NFS and NIS) and of conventional MS are compared in Table 9.1. 

Fig. 9.1 Schematic representation of the experimental arrangement for nuclear resonant scattering, both for NIS and NFS...

Equation (9.1) documents that quadmpole splittings A q exhibit quantum-beat spectra with period H/IuAEq superimposed over the time dependence of the nuclear decay exp(—f/t) with mean decay time t = 141 ns for Fe. In Fig. 9.2, quadmpole splittings A q = 0 and 2 mm s in the energy domain (conventional MS) are compared with those in the time domain (MS using synchrotron radiation) [7]. The QBs in the time domain spectmm for A q = 2 mm s are the result of the interference between the radiation scattered by different nuclear resonances. Consequently, their frequencies correspond to the energetic differences between these resonances. 

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