State-Resolved Spectroscopy and Dynamics of Chemical Transients

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In this project funded by the Chemical Structure Dynamics and Mechanisms A (CSDM-A) program of the Chemistry Division, Professor David Nesbitt of the University of Colorado uses sophisticated laser spectroscopy and molecular beam techniques to probe reactive chemical intermediate species at the gas-liquid interface. The curious paradox of chemistry is that the chemical intermediates of greatest relevance are necessarily present in vanishingly low concentrations and therefore require higher and higher sensitivity detection methods to observe. Nowhere are these sensitivity needs more pronounced than at the gas-liquid interface, whereby an ultrathin layer (10-7 cm) serves as the “doorway” between molecules in the gas phase and deep inside the bulk liquid. The proposed research is directed towards systematic exploration of such interfaces, exploiting high resolution laser spectroscopy and molecular beam tools to “bounce” molecules from liquids and probing the chemical composition, vibrational, rotational, speeds, and directions in which the product scatters from the surface. Prof. Nesbitt is continuing his 31st-33rd years as director of the CU Wizards Science Outreach program, which entails organizing 10 demo-filled science presentations each academic year (from September to June) for children in lower and middle school, in areas of science ranging from Black Holes to the Physics of Cooking to the What's Behind Climate Change? In particular, the proposed studies are exploiting supersonic expansions of simple molecules (e.g., DCl, HCl, CO, CO2, OCS, NO) moving at up to 2000 m/s (2-10x faster than the speed of sound) colliding with simple liquids (e.g., water, salty water, glycerin, hydrocarbon oils, molten salts, and even molten gold at 1100 C!). Nesbitt’s students are utilizing high resolution laser spectroscopy to explore how cold molecular projectiles “warm up to” and even significantly exceed the temperature of the bulk liquid. These results turn out to depend sensitively on and therefore quantitatively probe the nature of this thin interfacial layer (e.g., is it “rough” vs “smooth”, “soft” vs “hard”, “acidic” or “basic”, or “water-hating” (hydrophobic) vs. “water-loving” (hydrophilic), etc.), thereby providing detailed insights into the collision dynamics and chemistry of molecules at interfaces. This is relevant to how, for example, molecules such as O2 and CO2 cross through the lungs, dissolve into the blood stream, and eventually enter/leave the cells in any living organism. The gas-liquid interface is arguably the least well understood phase of matter and yet represents the sole “portal” through which gas molecules can dissolve into (or be expelled out of) a liquid. The gas-liquid interface thus represents a new scientific frontier about which remarkably little is known, but which nevertheless has impacts in areas as disparate as pulmonary medicine, ocean surface layer chemistry, atmospheric aerosol chemistry, ozone hole formation, and global climate change.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.