Formation of ultra-cold ⁷Li⁴He molecules in a seeded supersonic jet

Van der Waals molecules containing helium have exceptionally low binding energies. For example, the ⁷Li⁴He dimer, which has been observed previously in buffer gas experiments,¹ has a binding energy of the order of tens of milliKelvin. Here, we report on the observation of ⁷Li⁴He dimers in a seeded supersonic jet of helium. These are formed by three-body collisions ⁷Li + ⁴He + ⁴He → ⁷Li⁴He + ⁴He, and detected with laser-induced fluorescence on the A²Π(v') ← X²Σ⁺(v''=0) transitions, with v' = 5 and 6. In contrast to previous work, the ⁷Li⁴He dimers are ultracold, with a temperature of 10 mK. As such, this method opens up a possible route to studies of molecular formation and collisions in the 1 mK to 100 mK temperature range, intermediate between the sub-mK or above 100 mK temperatures of most previous work. We are also investigating the origin of additional spectral features that are observed to the red of the bound-bound transitions, and carrying out an analysis of the formation rate of the dimers in the helium jet.

1. N. Tariq et al., Phys. Rev. Lett. 110, 153201 (2013)