Mossbauer effect

In 1957, Rudolph Mossbauer, during his graduate studies, discovered an outstanding effect [49] that has generated an entire field in physics, that is, a very high-resolution spectroscopy in the y-ray region of the spectrum named Mossbauer spectroscopy [50-56], The effect consists of the fact that a y photon emitted by an excited nucleus can be resonantly absorbed by another nucleus [50-56], This means that a recoilless emission and absorption has occurred. 

This methodology has been applied in many areas, such as the measurement of lifetimes of excited nuclear states and nuclear magnetic moments, the investigation of electric and magnetic fields in atoms and crystals, in the analysis of special relativity, the equivalence principle, and also in other applications [50-57], 

If the emitted photon hits on a system of the same nature as the one emitting, but in its ground state after that, exists a certain probability that it will be absorbed, then lifting up the system to an excited state, a phenomenon called resonant absorption of photons. 

FIGURE 1.41 Recoil of the emitter during an emission transition. 

For a system originally at rest, the conservation of momentum implies 

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J.J. Spijkerman, F.C. Ruegg and J.R. de Voe, Applications of Mossbauer Effect in Chemistry and Solid State Physics, International Atomic Energy Agency, Vienna, 1966, p. 254. 

Wertheim, G. K. Mossbauer Effect, Principles and Applications, Academic Press, New York, 1964... 

N. R. Smyrl and Gleb Mamontov The Mossbauer Effect in Supported... 

The Mossbauer effect, discovered by Rudolf L. Mossbauer in 1957, can in short be described as the recoil-free emission and resonant absorption of gamma radiation by nuclei. In the case of iron, the source consists of Co, which decays with a half-life of 270 days to an excited state of Fe (natural abundance in iron 2%). The latter, in turn, decays rapidly to the first excited state of this isotope. The final decay generates a 14.4 keV photon and a very narrow natural linewidth of the order of nano eV. 

The Mossbauer effect can only be detected in the solid state because the absorption and emission events must occur without energy losses due to recoil effects. The fraction of the absorption and emission events without exchange of recoil energy is called the recoilless fraction, f. It depends on temperature and on the energy of the lattice vibrations /is high for a rigid lattice, but low for surface atoms. 

Some of the transition metal macrocycles adsorbed on electrode surfaces are of special Interest because of their high catalytic activity for dloxygen reduction. The Interaction of the adsorbed macrocycles with the substrate and their orientation are of Importance In understanding the factors controlling their catalytic activity. In situ spectroscopic techniques which have been used to examine these electrocatalytlc layers Include visible reflectance spectroscopy surface enhanced and resonant Raman and Mossbauer effect spectroscopy. This paper Is focused principally on the cobalt and Iron phthalocyanlnes on silver and carbon electrode substrates. 

Mossbauer effect spectroscopy, MES, Is based on the ability of certain nuclei to undergo recoilless emission and absorption ofY rays (16). The energy and multiplicity of the ground and excited states of a given nucleus are modified by the chemical environment. It Is thus most often necessary to compensate for the differences In... 

The recollless fraction, that Is, the relative number of events In which no exchange of momentum occurs between the nucleus and Its environment. Is determined primarily by the quantum mechanical and physical structure of the surrounding media. It Is thus not possible to observe a Mossbauer effect of an active nucleus In a liquid, such as an Ion or a molecule In solution. This represents a serious limitation to the study of certain phenomena It allows, however, the Investigation of films or adsorbed molecules on solid surfaces without Interference from other species In solution. This factor In conjunction with the low attenuation of Y-rays by thin layers of liquids, metals or other materials makes Mossbauer spectroscopy particularly attractive for situ studies of a variety of electrochemical systems. These advantages, however, have not apparently been fully realized, as evidenced by the relatively small number of reports In the literature (17). 

Method employed to establish first-order character of transition cal = calorimetry DSC = differential scanning calorimetry X-ray = X-ray diffraction ME = Mossbauer effect... 

Up to the present time, the Mossbauer effect has been observed with nearly 100 nuclear transitions in about 80 nuclides distributed over 43 elements (cf. Fig. 1.1). Of course, as with many other spectroscopic methods, not all of these transitions are suitable for actual studies, for reasons which we shall discuss later. Nearly 20 elements have proved to be suitable for practical applications. It is the purpose of the present book to deal only with Mossbauer active transition elements (Fe, Ni, Zn, Tc, Ru, Hf, Ta, W, (Re), Os, Ir, Pt, Au, Hg). A great deal of space will be devoted to the spectroscopy of Fe, which is by far the most extensively used Mossbauer nuclide of all. We will not discuss the many thousands of reports on Fe... 

Number of isotopes in which - the Mossbauer effect has been observed... 

Fig. 1.1 Periodic table of the elements those in which the Mossbauer effect has been observed are marked appropriately. (Taken from the 1974 issue of [10])...

Stevens, J.G., Stevens, V.E. Mossbauer Effect Data Index 1965-1975. Adam Hilger, London... 

Stevens, J.G., Khasanov, A.M., Hall, N.F., Khasanova, I. Mossbauer Effect Reference and Data Journal. Mossbauer Effect Data Center, The University of North Carolina at Asheville, Asheville, NC (2009) (up to 2009)... 

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)...

In order to understand the Mossbauer effect and the importance of recoUless emission and absorption, one has to consider a few factors that are mainly related to the fact that the quantum energy of the y-radiation used for Mossbauer spectroscopy (Eo K, 10-100 keV) is much higher than the typical energies encountered, for instance, in optical spectroscopy (1-10 eV). Although the absolute widths of the... 

Given relative to a-Fe at 300 K taken from Mossbauer Effect Data Journal [8]... 

The true energy scale of the y-spectrum in units of keV usually cannot be derived directly from the pulse height spectrum because the overall amplification of the detection system is not known. Therefore, the y-lines eventually have to be identified by trial and error when a new system is set up by checking for the occurrence of the Mossbauer effect. 

So far, we have discussed only the detection of y-rays transmitted through the Mossbauer absorber. However, the Mossbauer effect can also be established by recording scattered radiation that is emitted by the absorber nuclei upon de-excitation after resonant y-absorption. The decay of the excited nuclear state proceeds for Fe predominantly by internal conversion and emission of a conversion electron from the K-shell ( 90%). This event is followed by the emission of an additional (mostly Ka) X-ray or an Auger electron when the vacancy in the K shell is filled again. Alternatively, the direct transition of the resonantly excited nucleus causes re-emission of a y-photon (14.4 keV). 

Taken from Mossbauer Effect Data Center (MEDC), Prof John Stevens, University of North Carolina, Asheville, NC, USA, September 2009 for a full list of the nuclear properties for all known Mossbauer isotopes see the MEDC web address http //orgs. unca.edu/medc/Resources.html, or the corresponding pdf file in the CD-ROM of this book An older report [46] states -720 mb the value reported by MEDC is -789 mb... 

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