Mossbauer spectroscopy electric field gradient interactions

Spiering, H. The Electric Field Gradient and Quadmpole Interaction. In Long, G. (ed.) Mossbauer Spectroscopy Applied to Inorganic Chemistry, p. 79. Plenum, New York (1984)... 

Figure 14 shows three Fe case studies of the time behavior of the photons reemitted in the forward direction and a comparison with the typical spectra obtained in Mossbauer spectroscopy. Figure 14a corresponds to the case for which there is no hyperfine interaction. The nuclear levels are not split, and only one transition between ground and excited state is possible. In that case, the Mossbauer spectrum shows a single-absorption line and contains only y-quanta of equal energy. In the presence of an electric field gradient (Fig. 14b), the splitting of the excited state is... 

One less-well-known technique, which has many experimental aspects in common with Mossbauer spectroscopy, deserves special attention at this point, since it gives valuable information about the electric-field gradients and the magnetic hyperfine interactions of radioactive nuclei in solids at ambient conditions and under pressure. In this technique, two y-rays with different energies from two different transitions of an individual nucleus in a radioactive-decay cascade are recorded consecutively. The spatial and temporal perturbation of the emission probability by the hyperfine fields is registered in the corresponding perturbed angular correlation (PAC) spectra. 

The existence of an electric quadrupole interaction is one of the most useful features of Mossbauer spectroscopy. The energy levels in the presence of an electric field gradient (e.f.g.), q, are ... 

Spieting H. (1984) The Electric Field Gradient and the Quadrupole Interaction, In Long GJ (ed), Mossbauer spectroscopy applied to inorganic chemistry, Vol. 1. Plenum Press, New York, p 77. 

Orbital degrees of freedom are one of the important parameters to discuss the ordered ground states in materials. Orbital occupancy reflects electric field gradient (EFG) at nuclei. Then, Mossbauer spectroscopy can detect orbital occupancy of electrons, especially d or f electron cases. Electronic orbits in f electrons are usually called electronic quadrupole moments. Electronic quadrupole moments directly interact with nuclear quadrupole moments, which are observed as nuclear quadrupole interactions. Since the nuclear quadrupole moment in the Mossbauer transition is relatively large, 3.1 barn, the nuclear quadrupole interaction is relatively easy to detect as asymmetry of spectra even when hyperfine fields are observed. 

In contrast to magnetic properties, the theory of electric-field-like properties is much easier to cast into a set of working equations. One of them has attracted particular interest, and that is the electric field gradient (EFG). This property is of decisive importance to Mossbauer spectroscopy, i.e., to the spectroscopy of excited nuclear states whose energies are modulated by the molecular structure (the chemical environment ). In order to see how this property arises, we study the electrostatic electron-nucleus interaction of extended, not spherically symmetric charge distributions. For this we apply a multipole expansion in order to generate the properties term by term. 

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