Hyperfine Interactions

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In Chapter 2 we have, for the sake of simplicity, dealt with transitions between unperturbed energy levels of “bare” nuclei and have called the mean transition energy E0. In reality, however, we deal with atoms and molecules, with gases, liquids, and solids. Nuclei are therefore generally embedded in electric and magnetic fields, which may be created by shell electrons and charges in the near neighborhood. We also have to keep in mind that nuclei are positively charged and may possess various kinds of nuclear moments. These generally interact with the electric and magnetic fields in the nuclear region and perturb the nuclear energy levels. The perturbation, called nuclear hyperfine interactions, may be such that it only shifts the nuclear energy levels, as is. the case in electric monopole interaction (e0), or such that it splits degenerate nuclear levels into sublevels without shifting the centroid of the multiplet, as is observed in electric quadrupole interaction (e2) and magnetic dipole interaction (m1). Only these three kinds of interactions have to be considered in practical Mö;ssbauer spectroscopy. Electric dipole interaction (e1) does not exist because of symmetry arguments (invariance of nuclear forces relative to change in sign of coordinates). Interactions of higher order (m3, e4, etc.) are negligible; their energy effects are so small that they cannot be resolved in a Mö;ssbauer spectrum.