Molecule–molecule and atom–molecule collisions with ultracold RbCs molecules

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 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.

Anomalous Lifetimes of Ultracold Complexes Decaying into a Single Channel: What's Taking So Long in There?

“…Nevertheless, molecules in experiments certainly behave as if they are governed by a statistical theory. Molecular loss is convincingly modeled by theories in which the molecules, upon approaching a certain relative distance, vanish with a probability that usually sits somewhere between 0.5 and 1 [41]. It is assumed that these molecules vanish into complexes, as there is no place else for them to go.…”

Complexes formed in collisions between ultracold alkali-metal diatomic molecules and atoms

“…Christianen et al [61] proposed that the complexes are instead excited by the trapping laser, and showed that this can occur fast enough to account for the observed trap loss. This proposal is supported by experiments on collisions of RbCs [62,63] and 40 KRb [64], though recent experiments on Na 40 K [65] and on Na 39 K and Na 87 Rb [66] suggest that the complexes have longer lifetimes than predicted in the absence of the trapping laser.…”

“…In parallel with the work on molecule-molecule collisions, experiments have been carried out on atommolecule collisions. The systems studied experimentally include 40 K 87 Rb with 40 K and Rb [27,67], 87 RbCs with 87 Rb and Cs [63], Na 39 K with Na and 39 K [42,68] and Na 40 K with 40 K [50,69]. For each molecule, reaction is energetically allowed in a collision with the lighter atom but forbidden in a collision with the heavier one.…”

Quantum control of reactions and collisions at ultralow temperatures

“…Later, they found that the suppression of photo-excitation by the intensity modulation is less effective when 87 Rb 133 Cs molecules are not prepared in the spin-stretched hyperfine state. 105 In the KRb experiment, Liu et al directly observed the depletion of the reaction products and the reaction complex due to the trap laser, using the VMI technique. They measured the lifetime of the complex in the intensity-modulated dipole trap by employing the trap laser as a killing beam, and measured the increase in the number of the collision complex during the dark time.…”

“…Gregory et al studied collisions between 87 Rb 133 Cs and 133 Cs and observed a loss rate coefficient smaller than the universal value. 105…”