Contrary to the popular view that an electron is a ball-like particle orbiting the nucleus, quantum mechanics dictates that it ought to be described in terms of a spatial probability distribution – a charge cloud, rather than a rigid ball. The shape of such a charge cloud is uniquely defined by its orbital angular momentum, and can be conveniently decomposed into different multipoles (e.g., monopole, dipole, quadrupole, …).
In general, electron clouds around different atoms are not correlated, but in certain ordered compounds they are. As neutron scattering relies on the constructive interference of rays scattered from an ensemble of many atoms, only such compounds are amenable to study. Furthermore, the shape of a given electron cloud can also change with time, and its temporal evolution can be probed via inelastic neutron scattering. The accompanying figure shows data from a quadrupole-ordered UPd3 crystal measured on the Merlin spectrometer. From an analysis of the energy- and momentum-transfer dependence of the scattered intensity, it is possible to deduce how electron clouds around particular atoms evolve in time.