Mossbauer resonance fluorescence

Mosshauer effect The resonance fluorescence by y-radiation of an atomic nucleus, returning from an excited state to the ground state. The resonance energy is characteristic of the chemical environment of the nucleus and Mossbauer spectroscopy may be used to yield information about this chemical environment. Used particularly in the study of Fe. Sn and Sb compounds. 

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). 

The Mossbauer effect involves the resonance fluorescence of nuclear gamma radiation and can be observed during recoilless emission and absorption of radiation in solids. It can be exploited as a spectroscopic method by observing chemically dependent hyperfine interactions. The recent determination of the nuclear radius term in the isomer shift equation for shows that the isomer shift becomes more positive with increasing s electron density at the nucleus. Detailed studies of the temperature dependence of the recoil-free fraction in and labeled Sn/ show that the characteristic Mossbauer temperatures Om, are different for the two atoms. These results are typical of the kind of chemical information which can be obtained from Mossbauer spectra. 

Ti ossbauer spectroscopy is the term now used to describe a new ana-lytical technique which has developed using y-ray nuclear resonance fluorescence or the Mossbauer effect. For most of the time since Rudolf Mossbauer s discovery in 1958 it was the physicist who utilized this new tool. Starting approximately in 1962 some chemists realized the potential of this new technique. Since then they have applied Mossbauer spectroscopy to the study of chemical bonding, crystal structure, electron density, ionic states, and magnetic properties as well as other properties. It is now considered a complimentary tool to other accepted spectroscopic techniques such as NMR, NQR, and ESR. 

Mossbauer spectroscopy is the study of recoilless resonant fluorescence " Sn Mossbauer spectroscopy has been found to be a most usefifl method for studying the bonding and stereochemistry of tin compounds in the solid state. The two most important parameters are the isomer shift (5, mm s ) and the quadrupole sphtting (A q, nuns ), although the recoil-free fractions and temperature coefficients can also supply useful structural indications. 

Although more than 50 nuclei are known for which gamma-ray resonance fluorescence (the Mossbauer effect) has been observed,1 the present contribution will concern itself primarily with data pertinent to experiments using the 14.4 keV radiation of s7Fe and the 23.8 keV radiation of ll9Sn, since the vast majority of studies related to chemical structure problems have exploited these two nuclides. 

X 10 for up to 202 mms for Re. They are all very small when compared with the tremendous velocities ( 7 x 10 mm s ) used by Moon in 1950 to detect nuclear resonance fluorescence without recoilless emission, and show dramatically that the Mossbauer technique eUminates both recoil and thermal broadening. The Heisenberg relation means that an excited state with a shorter half-life has a greater uncertainty in the y-transition energy and hence a broader resonance line. 

Rudolph Mossbauer discovered the phenomenon of recoil-free nuclear resonance fluorescence in 1957-58 and the first indications of hyperfine interactions in a chemical compound were obtained by Kistner and Sunyar in 1960. From these beginnings the technique of Mbssbauer spectroscopy rapidly emerged and the astonishing versatility of this new technique soon led to its extensive application to a wide variety of chemical and solid-state problems. This book reviews the results obtained by MSssbauer spectroscopy during the past ten years in the belief that this will provide a firm basis for the continued development and application of the technique to new problems in the future. 

Figure 1. Schematic representation of nuclear resonance absorption of y-rays (MOssbauer effect) and nuclear resonance fluorescence...

After resonance absorption, the excited nucleus will decay by either emitting isotropically a y-quantum (as in the primary y-ray emission of Figure I) or a conversion electron e , preferentially from the K-shell. This phenomenon is termed nuclear resonance fluorescence and may be used in Mossbauer scattering experiments (surface investigations). 

The photons emitted by the de-excitation of nuclear levels that are populated in the course of radioactive decays can be resonantly scattered. Nuclear resonance fluorescence experiments can give information on the velocity distribution of recoil atoms and the chemical modifications following transmutations and on the slowing-down process of hot atoms. This technique can be applied in gaseous, liquid, and solid systems, giving an advantage over Mossbauer spectroscopy. Nuclear resonance fluorescence has been reviewed, with particular reference to the following systems ... 

The spectroscopic techniques that have been most frequently used to investigate biomolecular dynamics are those that are commonly available in laboratories, such as nuclear magnetic resonance (NMR), fluorescence, and Mossbauer spectroscopy. In a later chapter the use of NMR, a powerful probe of local motions in macromolecules, is described. Here we examine scattering of X-ray and neutron radiation. Neutrons and X-rays share the property of being found in expensive sources not commonly available in the laboratory. Neutrons are produced by a nuclear reactor or spallation source. X-ray experiments are routinely performed using intense synclirotron radiation, although in favorable cases laboratory sources may also be used. 

Noninvasive surface spectroscopies can be applied in the presence of liquid water most of them involve the input and detection of photons. The best known examples are nuclear magnetic resonance, electron spin resonance, Raman, Fourier transform infrared, UV-visible fluorescence, X-ray absorption, and Mossbauer spectroscopies, although Brown (28) enumerated many others that are available to detect adsorbed ions. These methods, some of which are listed in Table II along with citations of illustrative applications, can be used both noninvasively and in conjunction with in situ probes. 

A very suitable probe for calcium chemistry is europium (II). This cation has a good NMR nucleus, a good Mossbauer nucleus, useful spectra and fluorescence, and can be studied by EPR or by using its eflFect on proton (or other nuclear) resonances. We have shown. Table X, that europium(II) has a chemistry closely similar to that of calcium(II) 42). We are now studying the interaction between europium (II) and enzymes and proteins. We believe that europium (II) parallels calcium (II) to some degree in the triggering of muscle. 

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