We investigate the lifetimes of complexes formed in ultracold molecule collisions. Employing both transition-state-theory and an optical model approach we examine processes that can extend the lifetime of complexes beyond that predicted by Rice-Ramsperger-Kassel-Marcus theory. We focus on complexes that possess only one open channel, and find that the extreme distribution of widths for this case favors low decay rates. Thus decay from a complex into a single energetically available channel can be anomalously slow, and moreover nonexponential in time. We apply the theory to two systems of current experimental interest, RbCs and NaRb, finding qualitatively that the empirical time scales can be accounted for in the theory.
We investigate the lifetimes of complexes formed in ultracold molecule collisions. Employing both transition-state-theory and an optical model approach we examine processes that can extend the lifetime of complexes beyond that predicted by Rice-Ramsperger-Kassel-Marcus theory. We focus on complexes that possess only one open channel, and find that the extreme distribution of widths for this case favors low decay rates. Thus decay from a complex into a single energetically available channel can be anomalously slow, and moreover nonexponential in time. We apply the theory to two systems of current experimental interest, RbCs and NaRb, finding qualitatively that the empirical time scales can be accounted for in the theory.
We investigate the lifetimes of complexes formed in ultracold molecule collisions. Employing both transition-state theory and a non-Hermitian microscopic model, we examine processes that can extend the lifetime of complexes beyond that predicted by Rice-Ramsperger-Kassel-Marcus (RRKM) theory. We focus on complexes that possess only one open channel, and find that the extreme distribution of widths for this case favors low decay rates. Thus, decay from a complex into a single energetically available channel can be anomalously slow, even orders of magnitude slower than the RRKM rate, and moreover the decay may be nonexponential in time.
A thorough understanding of molecular scattering in the ultralow temperature regime is crucial for realizing long coherence times and enabling tunable interactions in molecular gases, systems which offer many opportunities in quantum simulation, quantum information, and precision measurement.Of particular importance is the nature of the long-lived intermediate complexes which may be formed in ultracold molecular collisions, as such complexes can dramatically affect the stability of molecular gases, even when exothermic reaction channels are not present.Here, we investigate collisional loss in an ultracold mixture of 40 K 87 Rb molecules and 87 Rb atoms, where chemical reactions between the two species are energetically forbidden.Through direct detection of the KRb 2 Ã intermediate complexes formed from atom-molecule collisions, we show that a 1064 nm laser source used for optical trapping of the sample can efficiently deplete the complex population via photoexcitation, an effect which can explain the strong twobody loss observed in the mixture.By monitoring the time evolution of the KRb 2 Ã population after a sudden reduction in the 1064 nm laser intensity, we measure the lifetime of the complex [0.39(6) ms], as well as the photoexcitation rate for 1064 nm light [0.50ð3Þμs -1 ðkW=cm 2 Þ -1 ].The observed lifetime, which is ∼10 5 times longer than recent estimates based on the Rice-Ramsperger-Kassel-Marcus statistical theory, calls for new theoretical insight to explain its origin.
The puzzle of "sticky collisions," in which molecular collision complexes exhibit unexpectedly long lifetimes, remains an unresolved mystery. A central challenge to solving this mystery is that traditional close-coupling calculations remain limited by the vast computational cost needed to take into account all the degrees of freedom involved in the collision. In this work, we propose a statistical model designed to simulate the result of full close-coupling calculations, with the goal of collecting statistics about reasonable lifetimes of collision complexes. To do so, we numerically sample resonances using random matrix theory and utilize results from quantum defect theory to calculate scattering properties and lifetimes. We find that in the limit of dense resonances, our theory agrees well with the Rice-Ramsperger-Kassel-Markus (RRKM) prediction, whereas in the limit of sparse resonances, the physics is governed by threshold behavior rather than resonant effects. By comparing these predictions to experimental results in two limits, we argue that close-coupling calculations alone may be insufficient to resolve the issue of long lifetimes.
Lifetimes of complexes formed during ultracold collisions are of current experimental interest as a possible cause of trap loss in ultracold gases of alkali-metal dimers. Microsecond lifetimes for complexes formed during ultracold elastic collisions of K2 with Rb are reported, from numerically exact quantum-scattering calculations. Thermally averaged lifetimes are compared with those calculated using a simple density-of-states approach, which are shown to be reasonable. This validates the density-of-states approach and suggests that the formation of long-lived complexes is indeed the cause of observed experimental trap loss in ultracold alkali-metal-dimer systems. Long-lived complexes correspond to narrow scattering resonances which we examine for the statistical signatures of quantum chaos, finding that the positions and widths of the resonances are in good agreement with the Wigner-Dyson and Porter-Thomas distributions, respectively.
Collisional complexes, which are formed as intermediate states in molecular collisions, are typically short-lived and decay within picoseconds. However, in ultracold collisions involving bialkali molecules, complexes can live for milliseconds, completely changing the collision dynamics. This can lead to unexpected two-body loss in samples of nonreactive molecules. During the past decade, such "sticky" collisions have been a major hindrance in the preparation of dense and stable molecular samples, especially in the quantum-degenerate regime. Currently, the behavior of the complexes is not fully understood. For example, in some cases, their lifetime has been measured to be many orders of magnitude longer than recent models predict. This is not only an intriguing problem in itself but also practically relevant, since understanding molecular complexes may help to mitigate their detrimental effects. Here, we review the recent experimental and theoretical progress in this field. We treat the case of molecule-molecule as well as molecule-atom collisions.
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.