Mossbauer nuclei

Scattered radiation. In a transmission experiment, the Mossbauer sample emits a substantial amount of scattered radiation, originating from XRF and Compton scattering, but also y-radiation emitted by the Mossbauer nuclei upon de-excitation of the excited state after resonant absorption. Since scattering occurs in 4ti solid angle, the y-detector should not be positioned too close to the absorber so as not to collect too much of this unwanted scattered radiation. The corresponding pulses may not only uimecessarily overload the detector and increase the counting dead time, but they may also affect the y-discrimination in the SCA and increase the nonresonant background noise. 

Mossbauer nuclei per square centimetre, the Debye-Waller factor/a of the absorber material, and the resonance cross-section Gq of the Mossbauer isotope. For a multiline spectrum, the result must be split into separate values for each line, which are obtained by weighting t with the relative transition probability of each line. 

Interestingly, the curve shape of SNR( ) does not depend explicitly on the concentration of Mossbauer nuclei in the sample. The function has a maximum at f = 2 (see Fig. 3.13), as one can easily verify from the root of the first derivative. 

Provided the electron densities absorber material are different because of the different chemical... 

Table 4.1 Quadrupole moments for the ground state (g) and the excited state (e) of some Mossbauer nuclei quoted in millibam (1 mb = 10 m )...

On the other hand, internal magnetic fields at the iron nucleus arising from the magnetic moments of unpaired valence electrons can be much stronger than any applied field and their effect can easily exceed the quadrupole interaction. For instance, Mossbauer nuclei in magnetic materials such as metals or oxides may experience fields of 30-50 T even without applied field. Similarly, the typical... 

Because of the different properties of the nuclear ground and excited states, the hyperfine coupling constants (A-values) for Mossbauer nuclei are often quoted in units of the internal field, where /I represents an energy and is the... 

The use of synchrotron radiation overcomes some of the limitations of the conventional technique. The high brilliance of up to 10 ° photons s mm mrad /0.1% bandwidth of energy, and the extremely collimated synchrotron beam lead to a large flux of photons through a very small cross section (0.1-1 mm ). This allows measurements with samples of small volume if isotopi-cally enriched (with the relevant Mossbauer isotope, e.g., Fe). Measurements that were described earlier [4] and that require a polarized Mossbauer source now become experimentally more feasible by making use of the polarization of the synchrotron radiation. Additionally, the energy can be tuned over a wide range. This facilitates measurements with those Mossbauer nuclei for which conventional sources are available but with life times that are too short for most experimental purposes, e.g., 99 min for Co —> Ni and 78 h for Ga —> Zn. 

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... 

Table 5.1 Mossbauer nuclei, sources, half life times and energies.

So by measuring the second-order Doppler shift of the Mossbauer nuclei in a material it is possible to determine their average velocity and thus their average vibrational kinetic energy, /2, where the mass of the Mossbauer nucleus. The... 

It is shown that the semi-empirical model of Miedema and v.d. Woude [66], which was developed for predicting the I.S. changes of Mossbauer nuclei in alloys and intermetallic compounds, gives a remarkably good prediction of the observed I.S. values for the surface sites [67]. 

The aim of this chapter is to provide a brief background to Mossbauer spectroscopy within the context of phase transformations. The relevant parameters are summarised and the effect of temperature and pressure are discussed, particularly with reference to identifying phase transformations and characterising the electronic and structural environment of the Mossbauer nuclei. Instrumentation is summarised, particularly as it relates to in situ measurements of phase transformations, and a brief survey of applications is given. The appendix includes a worked example that illustrates the methodology of investigating a phase transformation using in situ Mossbauer spectroscopy. Numerous textbooks and review chapters have been written on Mossbauer spectroscopy, and a selection of the most relevant ones as well as some useful resources are listed in Table 1. 

Absorber thickness, which is the amount of sample present in the gamma-ray flux (usually expressed as the number of Mossbauer nuclei per centimeters squared), must always be carefully considered in Mossbauer experiments. This subject has been dealt with in detail by Chandra and Lokanathan (1977), Shimony, (1965), Blarney (1977),... 

Out of all conjugated polymers studied, polypyrrole is the host matrix to which the largest number of complex anions have been introduced, some of them containing Mossbauer nuclei. 

The last contribution (Chapter 7) deaHng with the role of Mossbauer spectroscopy in the science of molecular sieves was provided by Lovat V.C. Rees, one of the pioneers in this field. Although Mossbauer spectroscopy is appHcable in zeolite research only to a small extent because of the limited number of suitable Mossbauer nuclei, we are indebted to this technique for valuable knowledge of and a deeper insight into some special groups of zeoHtes and zeolite/guest systems. This is particularly true of molecular sieves, which contain the most important Mossbauer nucleus Fe in their framework and/or extra-framework guests (cations, adsorbates, encapsulated complexes, and so on). 

Mossbauer spectroscopy is a powerful technique that may give information on the occurrence of tin and organotin polymeric structures. This is made possible through the investigation of molecular dynamics of " Sn nuclei embedded in molecular fragments and in particular through the well established correlation between the recoil free fraction of yrays (Debye-Waller-Mossbauer or Lamb-Mossbauer factor) and the mean square displacement of the nuclei (or of the masses bound to the Mossbauer nuclei). This correlation appears to be a linear dependence of the logarithm of the resonant peaks area, InA (proportional to Lamb Mossbauer factor,/ ) and the absolute temperature, T. 

Generally, from Mossbauer spectra one can obtain useful scientific information about the oxidation state of the Mossbauer atom, the high-spin or low-spin nature of the electronic structure of the Mossbauer atom, the degree of covalent bonding, the symmetry of the immediate environment, the magnetic interaction behavior between the Mossbauer nuclei as well as the rigidity of the crystal lattice containing the Mossbauer atom. 

The microenvironment (see Sect. 25.1.5.5) is of basic importance for analytical purpose. Since there is a one-to-one correspondence between the microenvironment and the hyperfine interactions, a simple Mossbauer pattern is always associated with a microenvironment characteristic of one particular population of Mossbauer nuclei in the studied sample. For brevity, such a population of like Mossbauer nuclei is referred to as a Mossbauer species. 

---