Recoilless nuclear resonance absorption

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...

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. 

The recoilless nuclear resonance absorption of y-radiation (Mossbauer effect) has been verified for more than 40 elements, but only some 15 of them are suitable for practical applications [33, 34]. The limiting factors are the lifetime and the energy of the nuclear excited state involved in the Mossbauer transition. The lifetime determines the spectral line width, which should not exceed the hyperfine interaction energies to be observed. The transition energy of the y-quanta determines the recoil energy and thus the resonance effect [34]. 57Fe is by far the most suited and thus the most widely studied Mossbauer-active nuclide, and 57Fe Mossbauer spectroscopy has become a standard technique for the characterisation of SCO compounds of iron. 

Recoilless nuclear resonance absorption of y-radiation (used by MoBbauer in his Nobel lecture 1961)... 

Rudolf Mossbauer, while working on his Ph.D. thesis, carried out experiments of this kind with lr. But surprisingly, on lowering the temperature he found an increase in the absorption effect rather than a decrease. Mossbauer was able to explain this unexpected phenomenon, and was awarded the Nobel Prize in 1961 for the observation and correct interpretation of the recoilless nuclear resonance absorption (fluorescence) which is now known as the Mossbauer Effect and which provided the basis for a powerful technique in solid state research. 

Figure 4. Schematic illustration of the experimental arrangement for recoilless nuclear resonance absorption, and relative transmission of recoilless y-quanta as a function of Doppler velocity...

For free atoms, or atoms in molecules, these recoil effects are typically about five to six orders of magnitude larger than the natural line width, and so there is no possibility of a resonance phenomenon (but having said that, under very special conditions resonances for gases have in fact been achieved). Even in the liquid state, atom or molecule movements are generally too large. However, when both emitter and absorber atoms are bound in solid samples recoilless nuclear resonance absorption becomes observable. But even here the atomic movements due to lattice and molecular vibrations lead to broadening of emission and absorption lines. We must therefore expect temperature-dependent effects in Mossbauer spectroscopy. 

The examples of chemical applications of Mossbauer spectroscopy discussed in this Tutorial Lecture have mostly been selected from the authors own research work and can, of course, only provide the reader with ideas about the kind of problems that can be solved with this nuclear resonance technique. Since the discovery of recoilless nuclear resonance absorption ( Mossbauer effect ) by the german physicist Rudolf Mossbauer more than fifty years ago, Mossbauer spectroscopy has developed to a poweifril tool in solid state research, making use of more than twenty Mossbauer-active nuclides from the list of more than fourty isotopes for which the Mossbauer effect has been observed [17]. Mossbauer spectroscopy has mostly been employed in conjunction with other physical techniques in order to gain more conclusive information in certain studies, but also in cases where certain problems could not be solved by other techniques. 

Shortly after the discovery of recoilless nuclear resonance emission/absorption on Ir by Rudolf Mossbauer (Mossbauer 1958a) the Mossbauer effect was shown with Fe (Shiffer and Marshall 1959 Pound and Rebka 1959 Hanna et al. 1960 de Pasquali et al. 1960) and... 

However, in contrast, the resonance effect increased by cooling both the source and the absorber. Mdssbauer not only observed this striking experimental effect that was not consistent with the prediction, but also presented an explanation that is based on zero-phonon processes associated with emission and absorption of y-rays in solids. Such events occur with a certain probability/, the recoil-free fraction of the nuclear transition (Sect. 2.4). Thus, the factor/is a measure of the recoilless nuclear absorption of y-radiation - the Mdssbauer effect. 

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. 

Mossbauer effect is the recoilless (also called recoil-free) nuclear resonance emission/ absorption of y rays (see O Pig. 25.1). In the case of a nuclear transition, the de-excited nucleus is normally recoiled by the momentum of the y photon emitted, which makes its resonance absorption impossible by another ground-state nucleus of the same type. In solids, however, recoilless photons can be emitted (and reabsorbed by another ground-state nucleus) with some probability. 

In Chap. 1 is written by Saburo Nasu, the reader will find a general introduction to Mossbauer Spectroscopy. What is the Mossbauer effect and What is the characteristic feature of Mossbauer spectroscopy These questions are answered briefly in this chapter. Mossbauer spectroscopy is based on recoilless emission and resonant absorption of gamma radiation by atomic nuclei. Since the electric and magnetic hyperfine interactions of Mossbauer probe atom in solids can be described from the Mossbauer spectra, the essence of experiments, the hyperfine interactions and the spectral line shape are discussed. A few typical examples are also given for laboratory experiments and new nuclear resonance techniques with synchrotron radiation. 

A recent development in physical techniques which may be of aid in evaluating the relative merits of theory is the Mossbauer effect. This effect is based upon recoilless y-ray emission (absorption) resulting from a nuclear transition in a particular atom with the resonance condition of zero-phonon processes. Since such nuclear transitions can be obtained with... 

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