Helium Droplet Spectroscopy of Atmospherically Significant Reaction Intermediates

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In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) and Environmental Chemical Sciences (ECS) programs of the Chemistry Division, Professor Gary Douberly of the University of Georgia is using liquid helium droplets to isolate and characterize the intermediates of chemical reactions. Reaction intermediates are difficult to study because they are only present for a short period of time before they react to form more stable molecules. Professor Douberly and his students overcome this long-standing experimental challenge by sequentially adding the individual reactants to liquid helium droplets. In the ultra-cold environment of the helium droplet, which is about 452 degrees Fahrenheit below zero, the reaction intermediates are "frozen out", allowing time for their molecular structures to be observed using laser spectroscopy. This project targets two fundamental chemical reactions that play key roles in the atmosphere, and is thus impacting our understanding of the complex web of chemical reactions associated with air pollution and the formation of small liquid droplets called aerosols.The first chemical reaction targeted by the project is the oxidation of isoprene, a small molecule that accounts for half of the organic compounds emitted into the atmosphere by plants, and is thus important to our understanding of the troposphere, the lowest portion of the Earth's atmosphere. Isoprene is oxidized by hydroxyl radicals, which are added to the helium droplets, and infrared spectroscopy is used to probe the branching of the reaction into the various intermediates. The intermediates are then reacted with oxygen to produce the associated hydroxyperoxy radicals, which are also studied using infrared techniques. The second aim of the project centers on identifying the minimum number of water molecules that are needed to observed the spontaneous transfer of a proton from sulfuric acid to water, a process that is central to the nucleation of atmospheric aerosol particles. This process is studied through the sequential addition of water molecules to helium droplets containing a single sulfuric acid molecule. Species selective spectroscopy is then used to determine the structures of these small acid-water clusters and identify the onset of proton loss from the acid. Analogous measurements probe the effect of ammonia (and other amines) on reducing the critical cluster size necessary for spontaneous proton transfer. In addition to providing important insight into fundamental atmospheric processes, the project provides opportunities for student training, mentoring, and the integration of laboratory research with undergraduate-centered educational activities.