High vibrational temperatures in plasmas can result in a significant, or even a majority presence of vibrationally excited reactants. For small polyatomic molecules, such a methane, more than half of all molecules are vibrationally excited when the vibrational temperature reaches 1000K. For larger molecules, this fraction is reached at even lower vibrational temperatures. For catalytic reactions in which the reactant's initial dissociation on a catalyst surface is rate limiting, vibrational energy (Evib) could play a significant role in promoting transition state access and enhancing reaction rate. To isolate and assess how Evib impacts reactivity, we use a supersonic molecular beam to control precisely the incident kinetic energy of molecules approaching a well-characterized catalytically active surface. We then tune an infrared laser to resonantly excite a single vibrational state of the reagent molecules in the beam and measure their reaction probability, S0, for dissociation and chemisorption on the catalyst surface. We can tune the laser to excite different vibrational states and reveal how the Evib and vibrational motion (e.g. stretch vs. bend) impact S0. Experiments without laser excitation provide a tight upper limit on the reactivity of molecules in the ground vibrational state and serve as a point of comparison for our excited state reactivity measurements. We find that vibrational energy can, in some cases, be even more effective than translational energy in promoting C-H bond activation. Furthermore, reaction probabilities for vibrationally excited states can be orders of magnitude higher than those for the vibrational ground state at the translational energies most prevalent in a processing environment. We find that impact of Evib on S0 is often related to the extent of geometric deformation required for transition state access. This insight provides guidance for identifying those reactions most likely to be enhanced by Evib. Taken together, these studies show that vibrationally excited molecules could play an important, or even dominant role in reaction kinetics, particularly under non-thermal conditions where vibrational temperatures are particularly high, as is the case in many plasma processing environments.

Role of vibrationally excited states in heterogeneously catalyzed reactions: Lessons from reactivity measurements of molecules in single vibrational states

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.