Imaging the structure of the trimer systems 4He3 and 3He4 He2

Helium shows fascinating quantum phenomena unseen in any other element. In its liquid phase, it is the only known superfluid. The smallest aggregates of helium, the dimer (He2) and the trimer (He3) are, in their predicted structure, unique natural quantum objects. While one might intuitively expect the structure of 4He3 to be an equilateral triangle, a manifold of predictions on its shape have yielded an ongoing dispute for more than 20 years. These predictions range from 4He3 being mainly linear to being mainly an equilateral triangle. Here we show experimental images of the wave functions of 4He3 and 3He4 He2 obtained by Coulomb explosion imaging of mass-selected clusters. We propose that 4He3 is a structureless random cloud and that 3He4 He2 exists as a quantum halo state.