Direct Observation of Ultracold Bimolecular Reactions

Chemical reactions result in the conversion of reactants to products through an intermediate state or a transition state. Unambiguous identifications of and investigations of chemical reactions are thus most complete when detecting all of the involved relevant species: reactants, products, and any transient intermediate states. Although direct observation of an intermediate collision complex is challenging because it is generally short-lived, it holds many of the key insights to understand the reaction bond breaking and forming processes. Femtochemistry techniques have been instrumental in resolving the short time scales necessary to directly probe transient intermediates for unimolecular reactions. In this work, we take the contrasting new approach of prolonging the lifetime of the intermediate by preparing reactant molecules in their lowest ro-vibronic quantum state at ultralow temperatures, thereby drastically reducing the number of exit channels accessible upon the collision of a pair of molecules. Using ionization spectroscopy and velocity-map imaging (VMI) of a trapped gas of potassium-rubidium molecules at a temperature of 500~nK, we directly observe reactants, intermediates, and products of the four-center reaction $^{40}$K$^{87}$Rb + $^{40}$K$^{87}$Rb $\rightarrow$ K$_2$Rb$^*_2$ $\rightarrow$ K$_2$ + Rb$_2$. This work reveals a long-lived energy-rich intermediate complex and opens the door for studies of quantum-state resolved reaction dynamics in the ultracold regime.