Direct observation of bimolecular reactions of ultracold KRb molecules

Glimpsing an exchange of partners When two diatomic molecules collide, they can sometimes swap partners. For instance, two potassium-rubidium (KRb) molecules can produce K 2 and Rb 2 . The four-atom intermediate formed upon collision is typically too scarce and short-lived to spot, even using ultrafast techniques. Hu et al. circumvented this problem by studying the reaction at temperatures approaching 0 kelvin. Using a combination of mass spectrometry and velocity-map imaging, the authors directly characterized the ionized K 2 Rb 2 complex as well as the reactant and product populations. Science , this issue p. 1111