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[40.25] Loss Process for the C₂H₅ Radical in the Atmospheres of Jupiter and Saturn: First Direct, Absolute Measurement of the Rate Constant for the Reaction H + C₂H₅ at Low Pressure and Temperature

L.J. Stief, A.S. Pimentel, W.A. Payne, F.L. Nesbitt, R.J. Cody (NASA/Goddard Space Flight Center)

Photochemical models of the atmospheres of Jupiter and Saturn predict the reaction H + C₂H₅ to be the most important loss process for C₂H₅ in these atmospheres. In addition, the reaction channel H + C₂H₅ arrow 2 CH₃ is a significant source of the methyl radical. There are only two relatively modern studies of the H + C₂H₅ reaction, both of which depend on extensive modeling involving eight elementary reactions. The motivation for the present study is the lack of direct, absolute measurements of the rate constant for the H + C₂H₅ reaction at low pressures and temperatures appropriate for outer planet models.

In the present experiments the reactants H and C₂H₅ are rapidly and simultaneously generated by reaction of F with appropriate mixtures of H₂ and C₂H₆. Using the technique of discharge-flow with collision-free sampling to a mass spectrometer, we monitor the decay of C₂H₅ in excess H. In contrast to previous studies of this reaction, the primary H + C₂H₅ reaction is isolated and the radical decays only by reaction with H and by loss at the wall. Secondary reactions such as the self-reaction of C₂H₅ are negligible. At P = 1 Torr He we measure k (298K) = 1.13 x 10^-10 cm³ molecule^-1 s^-1 and k (202K) = 1.18 x 10^-10 cm³ molecule^-1 s^-1. Experiments at T = 155 K are in progress. The reaction is temperature independent as expected based on studies of other atom-radical reactions. Our result at T = 298 K lies between those of the two relatively modern but complex studies of this reaction. The present total rate constant data and planned product yield studies at low pressures and temperatures will then be available for use in future photochemical models of the atmospheres of the outer planets.

The Planetary Atmospheres Program of NASA Headquarters is supporting this research.

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.