Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997dps....29.3305s&link_type=abstract
American Astronomical Society, DPS meeting #29, #33.05; Bulletin of the American Astronomical Society, Vol. 29, p.1037
Astronomy and Astrophysics
Astronomy
Scientific paper
Reactions of the C2H3 radical play a prominent role in the hydrocarbon chemistry of the atmospheres of Titan and all the outer planets. An important reaction of C2H3 in these atmospheric systems is CH3 + C2H3 (1). The self reaction C2H3 + C2H3 (2), while of minor importance in atmospheric models, is a complicating secondary reaction in the study of CH3 + C2H3. We have determined the product branching ratios at low pressure (1.3 mbar He) and at T = 298 and 200 K for the C2H3 + C2H3 and the CH3 + C2H3 reactions. The only data available for both of these reactions is at T = 298 K and at rather high pressure (130 mbar). The measurements were performed in a discharge flow system coupled with collision-free sampling to a mass spectrometer operated at low electron energies. For the vinyl self reaction we measured the following product channel yields: C2H3 + C2H3 = C2H2 + C2H4, Gamma_ {2a} = 1.00; C2H3 + C2H3 = C4H6 (1,3-butadiene), Gamma_ {2b} < 0.01 at both T = 298 K and 200 K. For the methyl + vinyl reaction we measure : CH3 + C2H3 = C2H2 + CH4, Gamma_ {1a} = 0.90 at T= 298 K and 0.68 at T = 200 K; CH3 + C2H3 = C3H6 (propylene), Gamma_ {2b} = 0.10 at T = 298 K and 0.32 at T = 200 K. Future photochemical models of the outer planets and satellites should use the more appropriate low temperature/low pressure data. The disproportionation channel (Gamma_ {a}) for all three reactions reforms C2H2 while the combination reaction (Gamma_ {b}) leads to the larger hydrocarbon species C2H4, C3H6 and C4H6. The competition between the disproportionation and combination channels as a function of temperature and pressure has a pronounced effect on the chemical composition of planetary atmospheres as a function of altitude.
Chillier X. F.
Nesbitt Fred L.
Payne Walter A.
Stief Louis J.
Thorn P. Jr. R.
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