Abstract
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$_{2}^{*}$ 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 photo-excitation, an effect which can explain the universal two-body loss observed in the mixture. By monitoring the time-evolution of the KRb$_{2}^{*}$ 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 photo-excitation rate for $1064$ nm light ($0.50(3)$ $\mu$s$^{-1}($kW/cm$^{2})^{-1}$). The observed lifetime is ${\sim}10^{5}$ times longer than recent estimates based on the Rice-Ramsperger-Kassel-Marcus statistical theory, which calls for new insight to explain such a dramatic discrepancy.
Highlights
IntroductionChemical reactions are quantum mechanical transformations from one chemical species to another
At the microscopic level, chemical reactions are quantum mechanical transformations from one chemical species to another
The corresponding RRKM complex lifetime is given by τc 1⁄4 2πħρc=N, where ρc represents the density of states (DOS) of the complex (Fig. 1), and N denotes the number of available dissociation channels
Summary
Chemical reactions are quantum mechanical transformations from one chemical species to another. In the particular instance of ultracold collisions between bialkali dimers, one prominent feature of the corresponding short-range collision dynamics is the formation of longlived intermediate complexes [20] These complexes can live for millions of molecular vibrations due to the limited number of dissociation channels that are available when the reactants are prepared in their lowest energy quantum states [21,22]. They impact the quantum state distribution of reaction products, as they can redistribute energy among the various modes of motion [23], but they can dramatically affect the stability of molecular gases where exothermic reaction channels do not exist This can happen, for instance, if the complexes are excited by photons from external laser sources [24–26], or if they undergo collisions with other atoms or molecules [21,22,27]. Because of their extremely long lifetimes, a full theoretical description of the resulting
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