Cold bimolecular collisions have a central role in various processes, including quantum chemistry, preparation of states of quantum matter, detailed examination of fundamental physics in two-body interactions and may enable the formation of quantum degenerate molecular matter by evaporative cooling. However, collisions between trapped, naturally occurring molecules have not been directly observed so far owing to the low collision rates of dilute samples.

Here we report the direct observation of collisions between cold trapped molecules, without the need for laser cooling. Loading molecular oxygen at densities of 10^10 cm^-3 into an 800-millikelvin-deep superconducting magnetic trap enabled us to observe a clear deviation from exponential decay of the captured molecules, indicating two-body losses in the trap. The ability to distinct the trap center from the edge and measuring different decay rates at these points set bounds on the ratio between the elastic- and inelastic-scattering rates - the key parameter determining the feasibility of evaporative cooling.

Two-body collisions between trapped species was independently identified as the main loss mechanism by adding lower density lithium atoms and observing fast non-exponential decay of the atoms from the trap, dependent on the molecules’ density, thus paving the way for studies of cold interspecies collisions in a magnetic trap.

Ref:

Y. Segev*, M. Pitzer*, M. Karpov*, N. Akerman, J. Narevicius & E. Narevicius, Nature 572, 189-193 (2019)