New Tools and New Approaches for Spectroscopy, Kinetics and Dynamics in Uniform Supersonic Flows

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In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Arthur Suits of the University of Missouri, Columbia, is developing new laser and microwave spectroscopy techniques to study the detailed interactions and properties a range of reactive molecules (for example, organic molecules that contain multiple bonds, like butadiene, allene and propyne) and molecular clusters (for example water). These new methods provide insight into the structures of these molecules, and how these molecules react to form different product molecules when they absorb energy. In addition to new insights about the structure and reactivity of the molecules considered in this project, the research is also continuing the development of laser and spectroscopy techniques from which other research groups can benefit. In particular, the Suits group is continuing to develop and improve a data analysis software package (Finite Slice Analysis, or "FinA"). The group makes the software available to other researchers free of charge. In addition to the training received in cutting-edge laser and spectroscopic techniques, the students involved in this project also gain experience in ab initio electronic structure calculations so they have a solid foundation and some practical experience with these methods. These studies take advantage of two new instruments being developed to probe the reaction dynamics, spectroscopy, and low temperature kinetics of polyatomic molecules. Particular attention is directed to radical species important in atmospheric chemistry and astrochemistry. The first of these instruments combines Chirped-pulse Fourier-transform microwave technique with intense low-temperature pulsed uniform supersonic flows, allowing broadband rotational spectroscopy to be used as a nearly universal tool in the study of photodissociation, product branching, spectroscopy and low temperature kinetics. The second instrument 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. Both of these instruments are the first of their kind. These studies are applied 1) to probe detailed product branching in photodissociation reactions; 2) to develop microwave-infrared double resonance methods for application to vibrational spectroscopy of radicals and reaction intermediates; 3) to probe reaction kinetics at low temperature with new detail including vibrational dependence and isomer- and conformer-specificity, and for transient radical species in low abundance; 4) to characterize the thermochemistry of water clusters such as the water hexamer and other non-covalently bound systems; and 5) to adapt sensitive CRDS methods to detect and characterize important atmospheric and astrochemical radicals and reaction intermediates kinetically trapped in local energy minima at low temperature in the flow.