Session X02: Ultracold Chemical Reactions of Molecules

Controlling chemistry with magnetic fields: Feshbach resonances for NaLi molecules

NaLi molecules have special properties among ultracold molecules. Due to their small van der Waals length, cross sections for inelastic collisions are small, although the molecule is very reactive. A longstanding goal of chemistry at the quantum level is the control of chemical reactions with external fields. This has motivated the search for Feshbach resonances in molecular systems. However, it is possible that many systems don’t have any resonances due to short lifetimes of collisional complexes or high density of states, preventing resonances from being resolved. So far, only a single molecule-molecule resonance has been observed, in collisions between triplet NaLi molecules. For atom-molecule collisions, resonances have been seen only for NaLi + Na and NaK + K. I will report on our studies of Feshbach resonances with NaLi. A rich spectrum of 25 resonances in spin-stretched NaLi + Na collisions could be explained using state-of-the-art quantum-chemistry calculations. They involve collisional complexes of size 30 – 40 Bohr radii and are due to spin-rotation and spin-spin couplings in combination with the anisotropic atom-molecule interactions. In contrast, our observations of inelastic collisions between NaLi molecules are not well understood and imply that even reactive molecules without reaction barrier can support long-lived collisional complexes.   

Quantum-chemical insights into molecular few-body complexity

I will present examples of how molecular electronic structure and quantum scattering calculations can support and explain ultracold quantum physics experiments. Quantum-chemical calculations of potential energy curves, permanent and transition electric dipole moments, and fine and hyperfine coupling constants provide parameters for effective Hamiltonians describing nuclear dynamics. Multichannel quantum scattering calculations give scattering lengths and elastic, inelastic, and reactive rate constants. I will discuss the capabilities and limits of state-of-the-art methods applied to systems based on alkali-metal and alkaline-earth-metal atoms and present our recent results for ongoing experimental efforts, including chemical reactions between reactive alkaline-earth-metal dimers [1] and collisional losses of non-reactive molecules due to intermediate complex formation [2], as well as a detailed understanding of collision dynamics in atom-molecules systems [3].   

Harnessing Chemical Reactions for Quantum Science?

Coherence and entanglement are key features in quantum mechanics, although they are susceptible to environmental perturbations. Chemical reactions, which involve the breaking and forming of bonds, are inherently quantum but are also highly dynamical processes. A fundamental question arises: Can coherence be preserved during chemical reactions and subsequently harnessed to produce entangled products? To address this question, we conduct investigations within the context of an atom-exchange chemical reaction (2KRb -> K2 + Rb2) at a temperature of 500nK. I will share our research findings. Additionally, I will also discuss another approach to entangle molecules using precisely controlled dipolar interactions for an array of individually trapped NaCs molecules in optical tweezers.   

Numerically exact quantum dynamics and control of ultracold atom-molecule collisions: Magnetic Feshbach resonances

Despite its relevance to ultracold controlled chemistry and quantum information science, the quantum dynamics of ultracold molecular collisions is far from completely understood due to the rapidly proliferating rotational, vibrational, fine, and hyperfine states coupled by highly anisotropic intermolecular interactions and external electromagnetic fields. I will review our efforts aimed at better understanding the physics of ultracold atom-molecule collisions, which include (i) an efficient total rotational angular momentum (TRAM) basis for incorporating the effects of hyperfine structure and external magnetic fields in molecular quantum scattering calculations and (ii) multichannel quantum defect theory with a frame transformation (MQDT-FT) [2]. These improvements allow for a substantial reduction in basis size and computational effort, making it possible to obtain the first numerically converged spectra of magnetic Feshbach resonances in highly anisotropic Rb + SrF collisions in the rigid-rotor approximation. Applications to quantum interference-based coherent control of ultracold atom-molecule collisions will be discussed.

[1] T. V. Tscherbul and J. P. D’Incao, Phys. Rev. A 108, 053317 (2023); [2] M. Morita, P. Brumer, and T. V. Tscherbul, arXiv:2309.00263.