Ralf Kaiser (University of Hawai'i), Alexander Mebel (Florida International University) and Arthur Suits (Wayne State University) are jointly supported for a research project exploring the formation and growth mechanisms of unsaturated hydrocarbons in low temperature environments. In collaboration with Ian Sims, University of Rennes (France) they will use an array of techniques including electronic structure theory, multimass ion imaging, crossed beam experiments, and kinetic studies to unravel ethynyl radical reactions under conditions that will give insight into the atmospheric chemistry on Titan, proto Earth, and hydrocarbon-rich planets and their satellites in the outer Solar System. The multi-faceted research goals include 1) to investigate the collision energy dependent reaction dynamics leading to hydrocarbon growth under single collision conditions employing a hydrocarbon-free crossed molecular beams machine; 2) to elucidate the photodissociation dynamics of hydrocarbon molecules under collision-free conditions, identifying all products and branching fractions; 3) to reveal the low temperature kinetics of ethynyl radical reactions with unsaturated hydrocarbon molecules by carrying out the reactions in a collimated flow of ultra-cold gas; 4) to use ab initio/statistical theory calculations to investigate ethynyl radical reactions and photodissociation processes over a broad range of collision energies, temperatures, pressures, and wavelengths, and in those systems where non-statistical effects are observed, to pursue additional dynamics studies in collaboration with Joel Bowman (Emory University); and 5) to apply these findings to chemical reaction networks in collaboration with Yuk Yung (Caltech), modeling the hydrocarbon growth in the atmosphere of Titan and comparing the model predictions with publicly available data from the Cassini-Huygens mission to Titan and with astronomical observations provided by collaborator Alan Tokunaga (University of Hawai'i). The models can then be refined until an agreement between predicted and observed concentrations of hydrocarbon molecules in Titan's atmosphere is reached and a coherent picture of the underlying chemistry emerges.

Hydrocarbon radicals such as ethynyl, C2H, are important, highly reactive intermediates in hydrocarbon-enriched atmospheres of planets and their moons such as Titan. By understanding the chemical dynamics and reactions of these radicals in a laboratory setting, and by investigating the photochemistry of hydrocarbon molecules, insight will be gained into the chemistry of the organic haze layers on Titan (a moon of Saturn with a dense cold, nitrogen and methane-based atmosphere) and the evolution of that atmosphere. Other broader impacts of this project include interdisciplinary training in reaction dynamics and astrochemistry, scientific workshops, and training of a diverse group of junior researchers. A public web site will offer virtual laboratory tours as well as an introduction to the fields of chemical reaction dynamics and astrochemistry. This project is funded by the Collaborative Research in Chemistry (CRC) Program and the Office of International Science and Engineering.

Project Report

The goal of our NSF-CRC project has been to understand the underlying me­cha­ni­s­ms on the synthesis of complex, hy­­­dro­gen-deficient hy­dro­­car­bons on the mo­­le­­cu­lar level in low temperature environ­ments, and to apply these findings to bet­ter com­pre­hend the che­­mical processes leading to the organic, hydrocarbon-based haze layers in the atmosphere of Sa­turn’s mo­on Ti­tan. To achieve these goals, we developed a tightly integrated collaborative network by com­bi­ning funda­mental studies in experimental physical chemistry exploiting the power of single col­li­si­on con­di­tions with electronic structure theory and chemical modeling of Titan’s atmos­phe­re. Our pro­ject has been extre­mely pro­duc­tive not only scientifically, but also from the standpoints of hu­­man re­sour­ce development and edu­ca­ti­o­nal outreach. We revealed the underlying reaction mecha­ni­sms, dynamics, and kinetics of se­veral key reaction classes forming complex (≥ C4) molecules relevant to the chemical processing of Ti­tan’s organic aerosol layers: polyynes, (polycyclic) aromatic hydrocarbons (PAHs), resonantly stabilized free radicals (RSFRs), cyanopolyynes, and nitrogen-bearing (polycyclic) aromatic hydro­car­bons (N-PAHs) thus effectively coupling Titan’s hydrocarbon and nitrogen chemistry. We further trained 22 scientists (12 un­d­e­r­­­­­gra­duate students,9 graduate students, and 12 postdoctoral fellows, of which 6 belong to underrepre­sen­­ted gro­u­ps) in the fiel­ds of chemical dynamics, che­­­­mi­cal kinetics, theoretical chemistry, and planetary che­­mis­try. Our co-mentoring, collaborative trai­ning, and (international) mobility of the network researchers resulted in 63 vi­sits to la­bo­ra­to­ri­es, work­shops, and confe­ren­ces. We feel that these data demonstrate our sincere efforts to contribute sig­ni­fi­can­t­ly to the training and education of scientists capable of assu­mi­ng le­ad­ing roles in US science in the fu­ture. To disseminate the results to the community, the PI and CoIs have been or­ga­ni­zing annual, three-day long interdisciplinary workshops on ‘Titan’s Che­­mistry – Observations, Experiments, Computations, and Modeling’ held across the US and in Europe in Honolulu (2007), Miami (2008), San Juan (2009), Abbaye de St Jacut de la Mer (France) (2010), and Kauai (2011) with typically 50 to 60 participants per workshop. These meetings brou­ght to­gether experimental and the­o­re­ti­cal chemists with planetary scientists, astronomers and mo­del­ers. Note that the third Titan workshop (2009) led to a special issue of JPCA featuring a collection of contributions arising from the workshop; the fourth workshop was organized primarily by the Rennes team following the Faraday Discussion 147 ‘Chemistry of the Planets’ chaired by IRS. This enabled us to attract a particularly wide range of participants to the NSF workshop. All Titan mee­tings were immensely suc­ces­sful at bridging the dis­ci­pli­na­ry boundaries that se­parate the­se com­munities, with the result that a high­er level of science was achi­eved across all of the­se fields. Research results from our network were also disseminated to te­achers, high schools, and to the public. For example, PI presented lectures in Astrochemistry and As­tro­biology (2007-) at teacher workshops at the University of Hawaii. At these occasions, lab tours were also conducted so that the instructors could ‘see’ how modern research in chemistry and astrobiology is being conducted. This in turn enables educators to incorporate research into school and college teaching. Further, our paper by X. Gu et al. on the possible formation of building blocks of the organic aerosol lay­ers on Titan entitled Chemical dynamics of triacetylene formation and application to Titan's atmosphere pub­lished in The Proceedings of the National Academy of Sciences U.S.A. [106, 16078-16083 (2009)] generated substantial public interest. It was featured, for instance by Chemistry Wor­ld (RSC, UK), CEN (ACS) as well as national and international newspapers. AGS presented lectures on "Chemistry in Space" to elementary schools (Ann Arbor Open School), to amateur astronomy groups (University Lowbrow Astronomers) and to an adult education class (Chelsea Adult Learners).

National Science Foundation (NSF)
Division of Chemistry (CHE)
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Charles D. Pibel
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University of Hawaii
United States
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