Electric Quadrupole Effects

Abstract

So far we have considered only the magnetic interactions of the nucleus with its surroundings. To be sure, by implication we have considered the effect of the nuclear charge, since it determines the electron orbits and where the nucleus sits in a molecule. However, we have not considered any electrical effects on the energy required to reorient the nucleus. That such effects do exist can be seen by considering a nonspherical nucleus. Suppose it is somewhat elongated and is acted on by the charges shown in Fig. 10.1. We see that Fig. 10.1b will correspond to a lower energy, since it has put the tips of the positive nuclear charge closer to the negative external charges. There is, therefore, an electrostatic energy that varies with the nuclear orientation. Of course,¹ turning the nucleus end for end does not affect the electrostatic energy. Consequently, for spin ½ nuclei the electrostatic energy does not split the m _I degeneracy.