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[20.08] Atmospheric Chemistry of Titan: Total Rate Constant for the Reaction of CH₃ + C₂H₅ + M arrow C₃H₈ + M at Low Temperatures and Pressures

R. J. Cody, A. S. Pimentel, W. A. Payne (NASA / GSFC), F. L. Nesbitt (Catholic University of America)

The recent infrared observation by Roe et al (2003, ApJ 597, L65) has unambiguously detected the hydrocarbon propane in the stratosphere of Titan. Photochemical models predict that the reaction CH₃ + C₂H₅ + M arrow C₃H₈ + M (1) is the source of C₃H₈ in the atmospheres of Titan and the Outer Planets and is expected to be one of the important loss processes for C₂H₅. There have been four direct studies of the rate constant for Reaction 1 but none at the lower temperatures and pressures needed for the atmospheric modeling studies. Therefore, we have measured the total rate constant at T = 295 and 202 K, and P = 0.4, 1 and 2 Torr. The measurements are performed in a discharge - fast flow system with C₂H₅ in excess. Pre-mixed gases of CH₄ and C₂H₆ in helium are added through a movable injector, and the radicals CH₃ and C₂H₅ are formed by the reactions of fluorine atoms with CH₄ and C₂H₆, respectively. We monitor the decay of the CH₃ radical via low-energy (10 ev) electron impact mass spectrometry. Because of complications from secondary chemistry, the rate constants for Reaction 1 are derived by a one-parameter fitting of the CH₃ decay curve to a numerical simulation of the reaction system using the Facsimile program. The results show a dependence of the rate constant upon pressure but no or very small negative temperature dependence. The total rate constants for the reaction CH₃+C₂H₅ are k(295K, 0.4Torr) = 1.0x10^-11, k(295K, 1Torr) = 2.1x10^-11, k(295K, 2Torr) = 3.0x10^-11, k(202K, 0.4Torr) = 1.1x10^-11, k(202K, 1Torr) = 2.6x10^-11, and k(202K, 2Torr) = 3.3x10^-11, all in units of cm³ molecules^-1 s^-1.

Acknowledgements. The NASA Planetary Atmospheres Program supports this research. ASP thanks the National Academy of Science for the award of a research associateship.

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Bulletin of the American Astronomical Society, 36 #4
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