Direct observation of bimolecular reactions of ultracold KRb molecules

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Science  29 Nov 2019:
Vol. 366, Issue 6469, pp. 1111-1115
DOI: 10.1126/science.aay9531

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Glimpsing an exchange of partners

When two diatomic molecules collide, they can sometimes swap partners. For instance, two potassium-rubidium (KRb) molecules can produce K2 and Rb2. 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 K2Rb2 complex as well as the reactant and product populations.

Science, this issue p. 1111


Femtochemistry techniques have been instrumental in accessing the short time scales necessary to probe transient intermediates in chemical reactions. In this study, we took the contrasting approach of prolonging the lifetime of an intermediate by preparing reactant molecules in their lowest rovibronic quantum state at ultralow temperatures, thereby markedly reducing the number of exit channels accessible upon their mutual collision. Using ionization spectroscopy and velocity-map imaging of a trapped gas of potassium-rubidium (KRb) molecules at a temperature of 500 nanokelvin, we directly observed reactants, intermediates, and products of the reaction 40K87Rb + 40K87Rb → K2Rb2* → K2 + Rb2. Beyond observation of a long-lived, energy-rich intermediate complex, this technique opens the door to further studies of quantum-state–resolved reaction dynamics in the ultracold regime.

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