Synopsis

# Excited Molecules Escape Their Trap

Physics 13, s57
The loss of ultracold molecules from an optical trap comes from optical excitation of the complexes that form when two of the molecules collide.

Researchers study the behavior of molecules in dipolar gases by cooling them to ultracold temperatures and confining them in an optical trap. However, these experiments are plagued by the mysterious loss of molecules—always two at a time—from the trap. Now Philip Gregory and his colleagues at Durham University, UK, pinpoint the source of that loss. They demonstrate that complexes can temporarily form from two-molecule collisions and that those complexes can absorb enough energy from the optical trap’s lasers to escape the trap.

In many ultracold gases, colliding molecules temporarily pair up to form a two-molecule complex. Recent experiments with nonreactive molecules, such as rubidium-cesium (RbCs), have found that these complexes quickly vanish from the optical trap.

To explore the mechanism for that disappearance, Gregory and his colleagues confined 3000 RbCs molecules in an optical trap formed from a single laser beam. They cyclically ramped the beam’s intensity up and down, such that the molecules spent up to 75% of each cycle in the dark, and they measured the number of molecules in the trap over time.

When the trap was off (the molecules were in the dark), the team found that the ${\left(\text{RbCs}\right)}_{2}$ complex existed for 0.53 ms, after which the molecules separated and remained in the trap. The lifetime decreased to $<100$ ns when the trap was on, with the complexes disappearing from the trap after that time. The team says that their finding indicates that RbCs loss is caused by the trap’s laser, which excites the complexes, giving them the energy kick they need to escape. Now that they have identified the loss mechanism, the team says that steps can be taken to mitigate it, for example by varying the intensity of the optical trap so that the molecules spend some of the time in the dark.

This research is published in Physical Review Letters.

–Rachel Berkowitz

Rachel Berkowitz is a Corresponding Editor for Physics based in Seattle, Washington, and Vancouver, Canada.

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