Session K06: DCP Award Session

Vibrations of Correlated Electrons and Spins

Oscillations can reveal secrets of nature, from the vibrational signature of a chemical bond to the gravitational wave recording the final waltz of two black holes. Here, we aim to understand 2D quantum phases of electrons and spins from their coherent oscillations. We apply pump-probe spectroscopy to 2D materials and interfaces where increasing number of ordered electron and spin phases have been discovered. In the first example, we probe coherent oscillation of ordered spins, i.e., spin waves, in the 2D layered magnetic semiconductor, CrSBr with intralayer ferromagnetic and interlayer antiferromagnetic order and with a semiconducting bandgap at ~ 1.5 eV. The presence of both semiconducting and magnetic properties led to our discovery of coupling between interlayer electronic hybridization to magnetic order. We take advantage of this to show the strong coupling of excitons to coherent magnons, opening the door to optically accessible spin information and quantum interconnects. In the second example, we take advantage of recently discovered band flattening at transition metal dichalcogenides (TMDs) moiré interfaces. In these systems, the much-reduced electron kinetic energy (bandwidth) results in the dominance of many-body and on-site Coulomb repulsion and the formation of ordered phases of electrons. We show the formation of ordered electronic phases from excitonic sensing at the WSe₂/WS₂ moiré interface. We successfully obtained collective oscillation frequencies represent the first quantitative probes of the many body potential energy landscape for 2D electron ordering.

    

Herbert P. Broida Award Winner: Lai-Sheng WangProbing Dipole-Bound States Using High-Resolution Resonant Photoelectron Imaging of Cryogenically-Cooled Anions

Negative ions do not possess Rydberg states, but polar anions may have diffuse dipole-bound states just below the detachment threshold, analogous to Rydberg states of neutral molecules. Excitation to vibrational levels of the dipole-bound state can induce autodetachment via vibronic coupling. The resulting resonant photoelectron spectrum is highly non-Franck-Condon and yields much richer vibrational information than conventional photoelectron spectroscopy. We developed an experimental apparatus integrating an electrospray ionization source with photoelectron spectroscopy, which allowed negative ions from solution samples to be studied in the gas phase. Subsequent development of a cryogenically-cooled Paul trap to create cold anions from electrospray has allowed high-resolution photoelectron imaging to be conducted for complex molecular anions, opening opportunities to probe dipole-bound excited states using photodetachment spectroscopy and resonant photoelectron imaging. I will present recent advances in our investigation of dipole-bound excited states, including the observation of p-type dipole-bound states and electron correlation induced by the electric field of the diffuse dipole-bound electron.   

Irving Langmuir Award in Chemical Physics Winner: Valeria Molinero

The Most Potent Snowmakers

The crystallization of water in clouds plays a key role in weather and climate through its effect on albedo and precipitation. The unassisted, homogeneous nucleation of ice occurs at temperatures lower than -32^oC, which are only achieved in high altitude clouds. Ice formation in lower lying, warmer clouds is promoted by atmospheric aerosols. Although the majority of atmospheric aerosols are minerals, the most efficient at forming ice are of biological origin. Ice nucleating bacteria are the most potent ice-nucleating agents in the biosphere and the atmosphere, contributing to cloud glaciation and precipitation, and routinely used for the synthetic production of snow. These bacteria have proteins in their outer membrane that are able to nucleate ice at temperatures as high as -1^oC. This presentation will discuss our quest to elucidate the molecular mechanisms by which bacterial proteins and other potent ice nucleants promote water crystallization, what makes them so outstanding, and how we can use that knowledge to design synthetic nucleants for applications ranging from cryopreservation to cloud seeding.   

Asymmetric Top Molecules for Quantum Science: From Bent Triatomics to Functionalized Aromatic Species

The diversity of electronic structures, geometries, and atomic constituents present in polyatomic molecules potentially makes them powerful building blocks for next-generation experiments in quantum science and precision measurement. Using complex molecules for these applications requires us to confront one of their most defining characteristics: molecular asymmetry. Until recently, the lack of strict selection rules was thought to make infeasible the laser-based control of asymmetric top molecules (ATMs; molecules with three distinct moments of inertia). We have shown that this is likely not the case and have put forward an experimental toolbox for optical cycling and laser cooling of ATMs. In addition, we have proposed theoretical design principles that allow pushing beyond the M-O-R motif that is characteristic of the polyatomic molecules that have so far been laser cooled—opening up a wider world of unusual geometries and ro-vibrational modes for scientific use. Experimental studies of the molecules CaSH, CaNH$_2$, and CaOC₆H₅ validate key findings of our theoretical results and demonstrate initial progress toward optical cycling and laser cooling of ATMs.   

Nonequilibrium effects of vibrational strong coupling on chemical reactions

Vibrational strong coupling (VSC), or the strong interaction between molecular vibrations and an optical cavity photon, gives rise to hybrid light-matter states called polaritons. Experiments find that VSC can drastically alter the kinetics of organic reactions. However, transition-state theory (TST), the most commonly used reaction-rate theory, has been unsuccessful at explaining such "VSC catalysis." One assumption of TST is that internal thermal equilibrium (i.e., within the states of each chemical species) is maintained throughout the reaction. Here, we explore how VSC affects chemical reactions in the limit where the equilibrium assumption breaks down.   

Statistical Mechanical Paradigms for Next-Generation Coarse-Grained Modeling

Since nature is intrinsically multiscale, a successful multiscale model must be able to deliver a consistent description across different length and time scales. In particular, molecular soft matter is relevant to many interdisciplinary fields in terms of multiscale modeling from materials to biology. Yet, it is challenging to develop a consistent multiscale approach to explain the chemical and physical changes that span the molecular to the macroscopic, which are often tightly coupled. This talk presents the new paradigms for understanding the principles of statistical mechanics underlying bottom-up coarse-grained modeling and for the faithful construction of multiscale models of molecular soft matter. Here, a systematic approach is presented to tackle the multiscale challenge from three microscopic-driven perspectives: design principles, model universality, and model fidelity. By constructing a multiscale model from microscopic-driven design principles, I will demonstrate that systematic bottom-up coarse-grained models derived from molecular-scale information can deliver consistent physical descriptions at different scales (universality) with accurate recapitulation of structure and dynamics (fidelity). These findings illustrate how rigorous, physics-driven coarse-grained modeling can facilitate the efficient simulation of complex molecular soft matter and guide the future era of multiscale modeling.