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.