Ultracold molecules, with their extraordinary controllability, offer a host of applications in both physics and chemistry, including quantum computation and simulation, test of fundamental physics theories, precise determination of molecular constants, and state-to-state reaction dynamics. Yet, because the creation of these molecules require highly specialized techniques and fortuitous molecular properties, the number of species which can be made ultracold in labs remains very limited (and chemistry is less fun with only a few at the party!).

Here in the Liu lab, we are working to improve the chemical diversity of the ultracold world using a hybrid trap that enables the confinement and quantum control of a variety of atoms and molecules, both ionic and neutral. Our approach exploits the fact that, unlike neutral molecules, molecular ions can be cooled, trapped, and controlled using generalized techniques. Any charged molecular species may be incorporated into a Coulomb crystal of laser-cooled atomic ions of similar charge-to-mass ratio, and enjoy sympathetic cooling of their translational motion. The internal molecular motion (e.g., rotation, vibration) may be cooled via radiative thermalization to a cryogenic environment, collisions with a cold buffer gas, or, in the case of single molecular ions, quantum-logic spectroscopy. From the ultracold molecular ions, desired neutral fragments may be generated using near-threshold photodissociation, and captured into a magnetic or optical dipole trap. This will bring many neutral species which cannot be laser-cooled into the ultracold regime.

Our new platform will enable the spectroscopy and reaction studies of a variety of chemical species with unprecedent resolution. Below are a few potential scientific directions.

Production of ultracold atomic hydrogen

As the first element, hydrogen’s unique theoretical simplicity makes it an attractive subject in many fundamental studies. For example, precision spectroscopy of hydrogen provides stringent tests of quantum electrodynamics (QED) theory, while studies of chemical reactions involving hydrogen (e.g., H + H₂) provides crucial benchmarks for the development of reaction dynamics theories. For further improvements in the resolution of hydrogen experiments, there has been considerable interest and effort in bringing atomic hydrogen into the ultracold regime, but the light mass of hydrogen and the lack of accessible laser cooling transitions makes the task challenging. In our hybrid trap, we will investigate the possibility of generate ultracold atomic hydrogen through near-threshold photodissociation of hydride ions (e.g., CaH⁺).

Ion-neutral reaction dynamics

Much of the chemistry in space is driven by reactions between neutral and ionic molecules. Our hybrid trap is a natural platform for studying this important class of reactions. Investigating key reactions (e.g., H₃⁺ + O → OH⁺ + H₂, CH₃⁺ + N → H₂CN⁺ + H) at the state-to-state level will provide detailed insights into their dynamics, and ultimately improve our understanding of astrochemical processes.