Nuclear resonance energy

Following the heat-load monochromator, the X-ray bandwidth is narrowed to approximately 1 eV and centered on the nuclear resonance energy (14.4 kev for Fe). The high-resolution monochromator further reduces the X-ray bandwidth to about 1 meV and motorized scanning of this monochromator tunes the energy over a range (typically within 100 meV of the resonance) adequate to explore excitation or annihilation of vibrational quanta. The X-ray flux at the sample is about 10 photons/s ( 10 tW), which is very low compared to typical milliwatt beam powers in laser-based Raman experiments see Vibrational Spectroscopy). Additional X-ray optics may reduce the beam size. The cross section of the beam at the sample point is currently about 0.5 x 0.5 mm at station D of beam line 3ID at APS. 

One technique that is becoming increasingly important for the characterization of materials is that of solid-state nuclear magnetic resonance (NMR) spectroscopy, and the application of this methodology to topics of pharmaceutical interest has been amply demonstrated.The NMR spectra of polymorphs or solvatomorphs often contains non-equivalent resonance peaks for analogous nuclei since the intimate details of the molecular environments associated with differing crystal structures can lead to perturbations in nuclear resonance energies. 

All the above applications indicate that SRPAC is a promising new technique to study hyperfine interactions of Mossbauer isotopes, especially for the isotopes with small Lamb-Mossbauer factors, because of the unique feature of SRPAC— the intensity is independent of the Lamb-Mossbauer factors. However, future developments of this technique has to be done in the direction of increasing the efficiency and the detection area of the APD detector and optimizing the monochromators, especially for the high transition energy Mossbauer isotopes (such as Ni with 67.4 keV nuclear resonance energy) so that SRPAC can be regularly applied to protein studies. 

Half-life/ Decay Mode/ Particle Energy/ Spin Nuclear Resonance Energy (/MeV) Intensity (h/ln) Magnetic... 

With tlie development of femtosecond laser teclmology it has become possible to observe in resonance energy transfer some apparent manifestations of tire coupling between nuclear and electronic motions. For example in photosyntlietic preparations such as light-harvesting antennae and reaction centres [32, 46, 47 and 49] such observations are believed to result eitlier from oscillations between tire coupled excitonic levels of dimers (generally multimers), or tire nuclear motions of tire cliromophores. This is a subject tliat is still very much open to debate, and for extensive discussion we refer tire reader for example to [46, 47, 50, 51 and 55]. A simplified view of tire subject can nonetlieless be obtained from tire following semiclassical picture. 

Now the magnitude of the resonance integral, which determines the resonance energy and the resonance frequency, depends on the nature of the structures involved. In benzene it is large (about 36 kcal./mole) but it might have been much smaller. Let us consider what the benzene molecule would be like if the value of the resonance integral were very small, so that the resonance frequency were less than the frequency of nuclear... 

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). 

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... 

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. 

---