In one of the previous chapters, you were pointed as optional reading to the first few sections of Rose (2012) (toy model paper). As an optional task, you are encouraged to read now sections 4 and 5, and comment on them on Perusall (you’ll find the paper there as well). These sections show how electron penetration in the nucleus affects the quadrupole interaction: the quadrupole shift. This effect is similar in nature to the monopole shift, but shows itself in a higher multipole order. In contrast to the monopole shift, it is observable only in a way that is similar to the Bohr-Weisskopf effect. Moreover, the existence of the quadrupole shift has not yet been convincingly demonstrated experimentally yet (see Table 2).