Nonequilibrium Scattering/Evaporation Dynamics at the Gas-Liquid Interface: Wetted Wheels, Self-Assembled Monolayers, and Liquid Microjets

CONSPECTUS: We often teach or are taught in our freshman courses that there are three phases of matter -gas, liquid and solid -where the ordering reflects increasing complexity and strength of interaction between the molecular constituents. But arguably there is also a fascinating additional phase of matter associated with the microscopically thin interface (<10 molecules thick) between the gas and liquid, which is still poorly understood and yet plays a crucial role in fields ranging from chemistry of the marine boundary layer and atmospheric chemistry of aerosols to the passage of O2 and CO2 through alveolar sacs in our lungs. The work in this Account provides insights into three challenging new directions for the field, each embracing a rovibronically quantum -state-resolved perspective. Specifically, we exploit the powerful tools of chemical physics and laser spectroscopy to pose two fundamental questions. (i) At the microscopic level, do molecules in all internal quantum-states (e.g., vibrational, rotational, electronic) colliding with the interface stick with unit probability? (ii) Can reactive, scattering, and/or evaporating molecules at the gas-liquid interface avoid collisions with other species and thereby be observed in a truly nascent collision-free distribution of internal degrees of freedom? To help address these questions, we present studies in three different areas: (i) reactive scattering dynamics of F atoms with wetted-wheel gas-liquid interfaces, (ii) inelastic scattering of HCl from self-assembled monolayers (SAMs) via resonance-enhanced photoionization (REMPI)/velocity map imaging (VMI) methods, and (iii) quantum-state-resolved evaporation dynamics of NO at the gas-water interface. As a recurring theme, we find that molecular projectiles reactively, inelastically, or evaporatively scatter from the gas-liquid interface into internal quantum-state distributions substantially out of equilibrium with respect to the bulk liquid temperatures (TS). By detailed balance considerations, the data unambiguously indicate that even simple molecules exhibit rovibronic state dependences to how they stick to and eventually solvate into the gas-liquid interface. Such results serve to underscore the importance of quantum mechanics and nonequilibrium thermodynamics in energy transfer and chemical reactions at the gas-liquid interface. This nonequilibrium behavior may well make this rapidly emergent field of chemical dynamics at gas-liquid interfaces more complicated but even more interesting targets for further experimental/ theoretical exploration.

Automated summary

We often learn in our freshman courses that there are three phases of matter, but there is an additional phase associated with the microscopically thin interface between gas and liquid. In this Account, we investigate three new directions for the field, using quantum-state-resolved perspectives and tools of chemical physics and laser spectroscopy. Our studies reveal rovibronic state dependences in the sticking and solvation of molecules at the gas-liquid interface, indicating the importance of quantum mechanics and nonequilibrium thermodynamics in energy transfer and chemical reactions. This rapidly emergent field of chemical dynamics at gas-liquid interfaces is complex but intriguing for further exploration.