Observation of Mixed Fermionic-Bosonic Helium Clusters by Transmission Grating Diffraction

Small weakly bound boson-fermion ⁴Hem³Hen clusters formed in a free jet expansion are identified using nondestructive transmission grating diffraction. The observations confirm the existence of more than 11 very tenuous complexes including the three-body halo molecule ⁴He₂³He and the pseudo-Borromean complex ⁴He₂³He₂. Effective cluster formation temperatures, extracted from a sudden freeze model for cluster growth using theoretical binding energies, increase smoothly with cluster size, thereby confirming the calculations with the possible exception of ⁴He₂³He₂