Hyungmok Son, a recent Harvard Physics Ph.D graduate, and his colleague at Harvard-MIT Center for Ultracold Atoms (CUA) have developed a new approach to control the outcome of chemical reactions at the quantum level. Traditionally, chemical reactions are controlled with temperature and chemical catalysts, but more recently, with external electromagnetic fields. For instance, researchers of Doyle group at Harvard University suppressed reactive collisions between CaF molecules by applying microwaves.
In the new study by Son et al., an external magnetic field was used to control the reactive collisions between ultracold sodium-lithium (NaLi) molecules and sodium (Na) atoms. Son and his colleagues showed that the phase of wavefunction of a colliding molecule and atom can be tuned across a Feshbach resonance and this induced an interference of the wavefunction. Depending on the phase of the wavefunction, the interference can be constructive or destructive, which enhances or suppresses the probability of inelastic scattering or reactions at short range. From the decay of molecules, researchers observed the tunability of reactive or inelastic collisions which was unprecedently large as 100-fold.
Fig. 1: Quantum interference and inelastic or reactive collisions can be understood by an optical analogy. On the right side, a laser beam is partially transmitted and reflected between two mirrors, M1 and M2. How much light reaches the left side depends on how the multiple reflections interfere. Thus, a change in the phase (which can be controlled through the separation between the two mirrors) will control where the light goes by tuning the interference from destructive to constructive. Similarly, in the atom-molecule collision studied at Harvard-MIT CUA, the wavefunction (green line) of a Na atom (yellow sphere) and a NaLi molecule (yellow and red spheres) can be reflected (right arrow) and transmitted (left arrow) at long distances and at close distances due to the atom-molecule interaction potential (purple line). In the figure, R is the inter-particle distance. As in the optical analogy, a phase shift of the wavefunction can determine the outcome of the molecular collision, including reaction pathways. The phase is controlled via magnetically tunable Feshbach resonances. The wavefunction transmitted to the short range (less than 1 nanometer distance) results in chemical reactions, which, for instance, can create diatomic Na molecules (two yellow sphere) and Li atoms (red sphere). Constructive or destructive interference gives suppressed (slower) or enhanced (faster) reactive/inelastic collisions at short range, respectively.
Interestingly, another Feshbach resonance studied by Son et al. showed much less dynamic range tunability of reactive or inelastic process: only a factor of 3 or so. Researchers found that such weaker resonance populated a more chemically reactive intermediate complex which hindered an interference or a buildup of the wavefunction, whereas the stronger resonance populated a long-lived intermediate complex that was likely to lead reactions at short range with a strong interference.
Researchers explained all the observations using a few simple equations that were based on the optical analogy: an interference of photons in a Fabry-Perot resonator consisting of two mirrors (see Figure 1). The study manifests that in a system like a NaLi-Na mixture, the molecular collisions at short range, which are mostly considered to be too complicated, can be controlled and understood with a simple physical picture.The study was published in Science on March 4, 2022: Hyungmok Son et al, "Control of reactive collisions by quantum interference," Science (2022). DOI: 10.1126/science.abl7257.
Fig. 2: The Harvard-MIT CUA team in their lab (from left): Yu-Kun Lu, Juliana Park, Alan Jamison, Wolfgang Ketterle (PI), and Hyumgmok Son (lead author).