Hydrogen bonding is a ubiquitous molecular interaction that plays a key role in atmospheric chemistry. Indeed, the configurations of molecular clusters offer exquisite insights not only into the nature of hydrogen bonding forces that influence their stability, but they also serve as essential tools for understanding and modelling condensed-phase environments. In this view, hydrogen-bonded molecular clusters act as a bridge between the gas and liquid phases. Hydrogen-bonded molecular clusters occupy an interesting size regime large enough to exhibit significant complexity, but small enough that their potential energy surfaces can still be explored in some detail by experimental methods and modern ab initio theory. We hypothesize that the complexity of the potential energy surfaces are sufficient to manifest properties that will carry over to larger systems.

Aerosols are large particles that impact public health, climate and energy balance in the atmosphere. A longstanding gap in our understanding of atmospheric aerosols is the fundamental processes underpinning secondary organic aerosol (SOA) formation and growth, which hinge signficantly on hydrogen-bonding intermolecular interactions as shown in the figure below. Volatile organic compounds (VOCs) are emitted in abundance from biogenic sources and may undergo oxidation, yielding multifunctional highly-oxygenated molecules that may condense into a pre-existing aerosol particle. Hydrogen bonding between the oxidized products and the particle interface is facilitated by hydrogen bonding and increases molecular transport across the gas-particle interface. A substantial fraction of aerosols are estimated to arise from SOA that grow to cloud condensation nuclei, which change cloud albedo and energy balance. We are investigating the intermolecular interactions of selected molecular clusters to obtain a molecular description of SOA.

As the figure below illustrates, new particle formation (NPF) is another pathway linked to aerosol formation, whereby nucleation of gas-phase precursors form hydrogen-bonded molecular clusters that subsequently grow to larger particles. Current knowledge of molecular clusters implicated in NPF is limited to molecular masses, thus relying on theoretical calculations and modeling to infer hydrogen-bonding functional groups. However, theoretical models are hard-pressed to describe the flexible structures of suspected molecular clusters. Therefore, this uncertainty severly hampers the development of detailed NPF mechanisms. Experimental benchmarks are needed to characterize the major functional groups, configurations, and dynamics of model NPF molecular clusters to reduce vast computational searches and streamline modeling parameterization.

Open-shell (or radical) species such as nitric oxide (NO) are important oxidizing agents in the atmosphere and they are also prevalent in combustion processes. In order to quantify their abundance in these environments, fluorescence methods are typically employed to obtain quantum yields. However, correction for electronic quenching (nonradiative collisional de-excitation of NO in its excited state with atomic or molecular partners) is necessary to account for in order to determine its effect on fluorescence quantum yields and thus NO concentrations. We seek to characterize the fate of electronically quenched NO with molecular partners and the pathways by which nonadiabatic processes occur. These systems are amenable to rigorous theoretical methods to generate accurate potential energy surfaces. With direct experimental input, the computed surfaces may be refined to yield insights into the basic forces acting upon nuclei during electronic quenching. Therefore, laboratory measurements are needed to provide stringent tests of theoretical methods and to improve predictive modeling of atmospheric and combustion processes.

As the figure belows shows, collisional quenching of NO in its excited electronic state with ambient atoms or molecules may involve nonreactive or reactive quenching. Nonreactive quenching returns NO population to rovibrational states in its ground electronic state, while also resulting in energy transfer to the quenching partner (e.g., O2). Reactive quenching results in chemical transformation including reaction products such as nitric dioxide (NO2) and oxygen atoms (O). Prior to either pathway, there is a finite probability of undergoing nonadiabatic curve crossing through a conical intersection which couples the ground and excited electronic states of NO. Furthermore, collisional complexes such as NO3 may also be generated in the exit channel prior to dissociation. Our goals include characterizing the product branching between nonreactive and reactive quenching as a function of the quenching partner, in addition to identifying the signatures from the quenching dynamics that are imprinted on the products.