Abstract
Understanding ultracold collisions involving molecules is of fundamental importance for current experiments, where inelastic collisions typically limit the lifetime of molecular ensembles in optical traps. Here we present a broad study of optically trapped ultracold RbCs molecules in collisions with one another, in reactive collisions with Rb atoms, and in nonreactive collisions with Cs atoms. For experiments with RbCs alone, we show that by modulating the intensity of the optical trap, such that the molecules spend 75% of each modulation cycle in the dark, we partially suppress collisional loss of the molecules. This is evidence for optical excitation of molecule pairs mediated via sticky collisions. We find that the suppression is less effective for molecules not prepared in the spin-stretched hyperfine ground state. This may be due either to longer lifetimes for complexes in the dark or to laser-free decay pathways. For atom–molecule mixtures, RbCs + Rb and RbCs + Cs, we demonstrate that the rate of collisional loss of molecules scales linearly with the density of atoms. This indicates that, in both cases, the loss of molecules is rate-limited by two-body atom–molecule processes. For both mixtures, we measure loss rates that are below the thermally averaged universal limit.
Highlights
We demonstrated that loss of RbCs molecules from an optical trap is ratelimited by a two-body process [60], consistent with the ‘sticky collision’ hypothesis that the loss is mediated by the formation of long-lived two-molecule collision complexes
We observed a suppression of the collisional loss by applying square-wave modulation to the intensity of the optical trap to form a time-averaged potential where 75% of each modulation cycle is dark
We find no significant change in the two-body rate coefficient when the optical trap is modulated and interpret this observation using our rate-equation model for the optical excitation of collision complexes
Summary
We begin by examining molecule-molecule collisions in an optical trap, where the intensity of the light is modulated as a square-wave such that the molecules spend. When the trap light is off, complexes can form and break apart without the risk of destructive optical excitation This leads to a reduction in the loss rate, with the maximum fractional reduction in loss equal to the duty cycle of the modulation. In this case we expect a slower rate of molecule loss in the modulated trap (dashed line) when compared to the CW trap (solid line). Measurements in modulated traps allow one to distinguish optical loss from other loss mechanisms only if the lifetime of the complex is significantly shorter than the dark time and the optical loss dominates over any laser-free loss
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