Detection of Long-Lived Complexes in Ultracold Atom-Molecule Collisions

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 40K87Rb molecules and 87Rb atoms, where chemical reactions between the two species are energetically forbidden. Through direct detection of the KRb2* 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 KRb2* 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 [0.50(3) mu s(-1) (kW=cm(2))(-1)]. The observed lifetime, which is similar to 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.