Reaction Dynamics of Polyatomic Molecules: Low Temperature Reactions and Coherent Photodissociation

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In this project funded by the Chemical Structure, Dynamics, and Mechanism-A (CSDM-A) program of the Chemistry Division, Professor Arthur Suits and his research team at the University of Missouri, Columbia are developing new tools for studying how chemical reactions occur, in other words, how the atoms within the molecules move, how bonds are broken and new ones formed. The Suits laboratory is developing reaction chambers where the starting materials are injected as uniform supersonic flows of gas. After a pulse of laser light is applied, the changes in molecular structure caused by the laser are observed at different downstream distances. Since the flow rate is known precisely, the downstream distances correspond to time since the laser pulse, and the rates at which the molecules change structure can be calculated. The use of supersonic flows allows very fast chemical reactions to be studied. The downstream events are captured using other laser or microwave spectroscopy techniques. Microwave light causes some molecules to rotate in very specific ways, which in turn reveals information about their structure. The reactions being studied include oxygen atoms reacting with hydrocarbons with double carbon-carbon bonds (C=C) and triple bonds (C≡C), and the light-induced dissociation of ethylene sulfide (C2H4S) and nitrogen dioxide (NO2). These studies are providing new insights into how chemical reactions occur in general. The specific reactions being studied are also important for more accurate modeling of chemical reactions in the atmosphere and in astronomical environments. This project is providing training for a graduate student and a post-doctoral fellow. Data analysis software developed during the research is made available to other research groups worldwide.The research takes advantage of two unique new instruments to probe the reaction dynamics, spectroscopy, and low temperature kinetics of polyatomic molecules. The first of these instruments combines the revolutionary Chirped-pulse Fourier-transform microwave (CPTMW) technique with intense low-temperature pulsed uniform supersonic flows for kinetics and dynamics studies. The research also involves an instrument that combines the same uniform supersonic flows with high-resolution ultra-sensitive continuous-wave cavity ring-down spectroscopy (CRDS), providing capabilities complementary to those of the microwave-based system. The latter instrument has recently demonstrated Simultaneous Kinetics and Ringdown (SKaR) in a supersonic expansion for the first time. In addition to the studies in low temperature flows, the Suits team is also investigating how coherent orbital polarization reveals effects of the so-called geometric phae, wherein the system wavefunction must undergo a sign change following a transit around a conical intersection (CI). The current studies extend investigations of the geometric phase to NO2 and ethylene sulfide, promising novel probes and insights into nonadiabatic dynamics. In terms of scientific Broader Impacts, this project extends capabilities into the mid-IR spectrum, which in turn permits the application of the SKaR kinetics method to many systems of astrochemical and astronomical interest.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.