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
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
Chemical Physics
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][1]
[1]: /lookup/doi/10.1126/science.aay9531
Rapid progress in atomic, molecular, and optical (AMO) physics techniques enabled the creation of ultracold samples of molecular species and opened opportunities to explore chemistry in the ultralow temperature regime. In particular, both the external and internal quantum degrees of freedom of the reactant atoms and molecules are controlled, allowing studies that explored the role of the long-range potential in ultracold reactions. The kinetics of these reactions have typically been determined using the loss of reactants as proxies. To extend such studies into the short-range, we developed an experimental apparatus that combines the production of quantum-state-selected ultracold KRb molecules with ion mass and kinetic energy spectrometry, and directly observed KRb + KRb reaction intermediates and products [M.-G. Hu and Y. Liu, et al., Science, 2019, 366, 1111]. Here, we present the apparatus in detail. For future studies that aim for detecting the quantum states of the reaction products, we demonstrate a photodissociation based scheme to calibrate the ion kinetic energy spectrometer at low energies.
Advances in AMO techniques lead to the creation of ultracold samples of molecular species and opened opportunities to explore chemistry in the ultralow temperature regime. While many prior studies investigated how long-range forces influence ultracold reactions, we aim to extend the exploration into the short-range where chemistry actually takes place. To this end, we developed a new experimental apparatus that combines the production of quantum-state-selected ultracold molecules with ion mass and kinetic energy spectrometry. Using such an apparatus, we were able to probe the exchange reaction between ultracold potassium-rubidium (KRb) molecules in its entirety, detecting both the reaction intermediates and the products. Such direct signals allowed us to further investigate this reaction, and eventually gain control over its various aspects. This thesis will describe the identification of an unusually long-lived reaction intermediate and a direct measurement of its lifetime, steering the pathway of the reaction with light, and controlling the quantum state distribution of the reaction products via an external magnetic field. The techniques we have developed here open up the exciting possibility to understand and manipulate state-to-state chemistry in the ultracold regime.
Advances in atomic, molecular, and optical physics techniques allowed the cooling of simple molecules down to the ultracold regime ([Formula: see text]1 mK) and opened opportunities to study chemical reactions with unprecedented levels of control. This review covers recent developments in studying bimolecular chemistry at ultralow temperatures. We begin with a brief overview of methods for producing, manipulating, and detecting ultracold molecules. We then survey experimental works that exploit the controllability of ultracold molecules to probe and modify their long-range interactions. Further combining the use of physical chemistry techniques such as mass spectrometry and ion imaging significantly improved the detection of ultracold reactions and enabled explorations of their dynamics in the short range. We discuss a series of studies on the reaction KRb + KRb → K 2 + Rb 2 initiated below 1 μK, including the direct observation of a long-lived complex, the demonstration of product rotational state control via conserved nuclear spins, and a test of the statistical model using the complete quantum state distribution of the products.
We consider ultracold, chemically reactive scattering collisions of the diatomic molecules KRb. When two such molecules collide in an ultracold gas, we find that they are energetically forbidden from reacting to form the trimer species K2Rb or Rb2K, and hence can only react via the bond-swapping reaction $2\mathrm{KRb}\ensuremath{\rightarrow}{\mathrm{K}}_{2}+{\mathrm{Rb}}_{2}$. Moreover, the tiny energy released in this reaction can in principle be set to zero by applying electric or microwave fields, implying a means of controlling the available reaction channels in a chemical reaction.
Studying ultracold chemical reactions with quantum state resolution reveals details of the reaction processes, especially with single quantum state preparation of the reactants and state-selective detection of reaction products. In this thesis, we investigate two main processes: KRb + KRb → K2 + Rb2 reactions, and Rb and KRb atom-molecule collisions. In the KRb bimolecular reaction, we demonstrate the preservation of quantum coherence in bimolecular reactions for the first time, which is surprising in a reaction considered to be largely chaotic. Additionally, we proposed a coherent control scheme to manipulate product yields across different product channels. These results represent a critical step toward probing quantum coherence and entanglement in the chemical reaction. In atom-molecule collisions between Rb and KRb, we discovered an exceptionally long-lived intermediate complex, KRb2, which can be photo-excited by trapping light. The observed complex lifetime was orders of magnitude longer than theoretical predictions, underscoring the limitations of the current theory. We further explored the dependence of complex lifetimes on the initial quantum states and external electric and magnetic fields, providing additional experimental benchmarks to guide future theoretical work. Separately, we found that inelastic collisions between hyperfine-excited Rb atoms and KRb molecules can lead to rotation excitation of KRb post-collision. By probing the product state distribution, the result suggests that mechanical angular momentum is coupled to the spins. Such couplings are too weak in the current theory models to explain the result. Moreover, our result contradicts state-of-the-art coupled-channel calculations. These suggest that some subtle effects, such as molecular vibration and conical intersections, play a critical role in the reaction dynamics. The final piece of the thesis describes the observation of resonant interaction between a Rydberg atom and an ensemble of polar molecules. Such hybrid quantum systems have been proposed for a wide range of applications. Our work provides an experimental demonstration of resonant dipolar interactions between Rydberg atoms and ultracold polar molecules, paving the way for the realization of hybrid systems for quantum computation and simulation.