New Tools and Applications for Kinetics and Dynamics with Broadband Rotational Spectroscopy

With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) program in the Division of Chemistry, Professor Arthur Suits of the University of Missouri-Columbia is developing new tools to permit applications of Chirped-Pulse Fourier-Transform Micro/Millimeter Wave Spectroscopy to study low temperature kinetics of radical-radical reactions. The first reactions to be studied include formyl and acetyl radicals with the stable open shell molecules NO and O2 as well as reactions involving hydrogen atoms and amino radicals.

The radical-radical reactions that will be the subject of these studies are important in a wide range of extreme environments. However, they are very challenging to investigate experimentally, and are not readily treated by the “gold standard” theoretical approaches. These studies could lead to a better understanding of chemical reactivity in complex systems, benchmark the theoretical characterization of inherently multireference systems that are the subject of active investigation by electronic structure theorists, and have implications for astrochemistry, atmospheric chemistry, and combustion.

Students and post-docs will be trained in state-of-the-art instrument development methods, in reaction dynamics and photochemistry, in kinetics and kinetic modeling, in data analysis, and issues in astrochemistry and atmospheric chemistry. Techniques developed will be freely shared with the broader community.

Broadband rotational spectroscopy is a powerful, near-universal probe technique but there are challenges in coupling it to the low temperature flow environment for kinetics measurements. Sampling the flow to a separate detection region is possible but this can introduce shocks and produce nonequilibrium conditions that interfere with the measurements.

Professor Suits and his students will instead develop an extended nozzle in which the reactions take place, which is then followed by a shock-free second expansion to low pressure ideal for detection by rotational spectroscopy. Professor Suits and his students will also couple Resonance Enhanced Multiphoton Ionization (REMPI) to the low temperature flow environment to probe low temperature kinetics, product branching, and dynamics for these challenging radical-radical reactions.

This strategy will be used to detect hydrogen atoms and other atomic species for kinetics measurements which are otherwise difficult or impossible to probe in the flow environment. They will also adapt the REMPI technique to determine the conditions inside the extended nozzle which will be essential to validate the conditions used for the rate determinations.

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.