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

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${}_2^{\ast }$ 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${}_2^{\ast }$ 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.

9 pages, 4 figures