Detection of long-lived complexes in ultracold atom-molecule collisions

Though intermediate complexes formed through molecular collisions are integral to understanding chemical reactions and making stable molecular gasses, their short lifetimes at even cryogenic temperatures preclude detailed study. In the ultracold regime, however, such complexes have been observed to live substantially longer, providing new opportunities to study molecular scattering and reactions. Here, we investigate ultracold collisions between ^{40}K^{87}Rb molecules and ^{87}Rb atoms, a collision for which chemical reactions are energetically forbidden. The KRb_2^* intermediate complexes formed through atom-molecule collisions are detected directly via ionization imaging, and we show that a 1064 nm laser source used for optical trapping of the sample can efficiently deplete the complex population via photo-excitation. Our measured complex lifetime is ~10^5 times longer than recent estimates based on Rice-Ramsperger-Kassel-Marcus statistical theory. These experimental results call for new insight to explain such a dramatic discrepancy.