Freezing out chemical reactions to have a closer look in the quantum realm

Catching a glimpse of the breaking and formation of chemical bonds in ultracold chemical reactions.

Image courtesy of Ming-Guang Hu, Department of Chemistry and Chemical Biology, Harvard University
By “freezing out” the rotation, vibration, and motion of potassium-rubidium molecules to a temperature of 500 nanokelvin, scientists “trapped” the reaction in the intermediate stage for a longer time.

The Science

Chemical reactions transform reactants to products through intermediate states. In these states, electrons that break and form bonds rearrange themselves. However, such intermediates are often short-lived, making them hard to study. But by bringing a molecule to a temperature barely above absolute zero, scientists can “trap” the reaction in the intermediate stage for a much longer time. In this study, scientists examined an ultracold gas made of potassium-rubidium molecules with two atoms. Using photoionization, where a photon interacts with an atom or molecule to form an ion, researchers directly observed the reaction’s reactants and products. They were also able to watch the transient, intermediate complexes of this bimolecular chemical reaction where molecules exchange nuclei.

The Impact

This work is the first time scientists have directly observed transient intermediate complexes formed during an ultracold bimolecular chemical reaction. This research provides scientists access to the moment where chemical bonds break and new molecular species form. This opens the door to new fundamental studies of controlled chemical reactions in the quantum regime. These studies will shed new light on the underlying mechanisms which drive reactions. Understanding these mechanisms will help scientists determine how occupied quantum states will be distributed in the resulting chemical products.


For the first time scientists have directly observed an intermediate, K2Rb2*, in the ultracold chemical reaction: KRb + KRb à K2 + Rb2. These ultracold temperatures afford sufficient quantum level detail to benchmark theoretical calculations.

Chemical reactions between heteronuclear diatomic molecules (the reactants) can result in the exchange of nuclear partners via the breaking and formation of chemical bonds. Such an exchange produces new molecular species (the products) which are distinct from the reactants. For this to occur, the reactants must come together for a brief moment, where they form an intermediate, four-atom complex. In conventional reactions near room temperature, this complex exists for such a short time that it is highly challenging to observe directly, even with ultrafast laser techniques. At ultracold temperatures, on the other hand, the reactant molecules can become trapped for an extended period in this state, making it possible to detect.

Here,scientists produced a gas of chemically reactive KRb molecules in their electronic, vibrational, and rotational ground state, at an ultralow temperature of 500 nK. They then utilized photoionization to perform mass spectrometry and velocity-map imaging of the various species involved in the reaction, KRb + KRb à K2Rb2à K2 + Rb2. Using this novel approach, the authors were able to monitor the reaction in real time and to directly observe both the reactants and products, and, for the first time, the intermediate K2Rb2 complex.


Kang-Kuen Ni
Department of Chemistry and Chemical Biology, Harvard University


This work was supported by funding from the Department of Energy Office of Science, the David and Lucile Packard Foundation, and the National Science Foundation through the Harvard-MIT Center for Ultracold Atoms. The 40K isotope used in this work was provided by the DOE Office of Science, specifically by the Isotope Program in the Office of Nuclear Physics.


Hu, M.G. et al., “Direct Observation of Bimolecular Reactions of Ultracold KRb Molecules.” Science 366, 1111-1115 (2019). [DOI: 10.1126/science.aay9531]

Related Links

The Coldest Reaction: With ultracold chemistry, researchers get a first look at exactly what happens during a chemical reaction: Harvard University press release

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