2-molecule bound states in shielded collisions of ultracold molecules

Ultracold molecules may be shielded from destructive collisions with either static or microwave fields. In both cases the shielding produces a repulsive potential at moderately long range (100s of bohr). Outside this there is a long-range attractive potential produced by dipole-dipole interactions. The resulting attractive well may support 2-molecule bound states [1], which have now been observed for microwave shielding [2].

The strength of the attraction is weakly dependent on field, but the repulsion is strongly dependent on it. As a result, the well depth and the number of bound states can be controlled. For some molecules, the real part α of the scattering length can be tuned to large or small positive or negative values, or through zero, as required.

We have carried out coupled-channel scattering and bound-state calculations on shielding with static electric fields. We have investigated CaF [3] and a variety of alkali-dimer molecules [4]: RbCs, NaK, NaRb, NaCs, together with KAg and CsAg. Shielding is effective in all cases. For RbCs, the scattering length remains positive at all fields and no bound state is formed. For NaK, the scattering length reaches negative values, but there is still no bound state. For NaRb and NaCs, there is 1 bound state, with poles in α where the state crosses threshold. For KAg and KCs there are multiple bound states, with multiple poles.

We have calculated the binding energies of the states as a function of static field, and also their lifetimes for decay to pairs of molecules in lower rotational states. The lifetimes are very long.

The bound states are both a blessing and a curse. They offer the opportunity to form and study 2-molecule (4-atom) bound states of ultracold molecules. They exist at very long range (1000s of bohr), but may provide a gateway to shorter-range states. However, their existence also promotes 3-body recombination, which may hinder evaporative cooling [5].

1. G. Quéméner et al., PRL 131, 043402 (2023)

2. X.-Y. Chen et al., arXiv:2306.00962

3. B. Mukherjee et al., Phys Rev Res 5, 033097 (2023)

4. B. Mukherjee & J. M. Hutson, arXiv:2311.08301

5. N. Bigagli et al., arXiv:2312.10965