36th DPS Meeting, 8-12 November 2004
Session 20 Titan
Poster I, Tuesday, November 9, 2004, 4:00-7:00pm, Exhibition Hall 1A

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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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