Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002dps....34.3906c&link_type=abstract
American Astronomical Society, DPS Meeting #34, #39.06; Bulletin of the American Astronomical Society, Vol. 34, p.911
Astronomy and Astrophysics
Astronomy
Scientific paper
Cyanoacetylene (HC3N) is one of the few nitrile compounds observed by Voyager in the atmosphere of Titan. The three body reaction H + HC3N + M -> products (1) constitutes one of the loss processes for cyanoacetylene in the photochemical models of the Titan atmosphere; e.g. the 1984 model of Yung, Allen and Pinto. The rate constant for this reaction has never been measured. The models use a value equal to that for the reaction H + C2H2 + M. We have measured the rate constant for Reaction 1 at T = 298 K over a pressure range of 0.5 - 2.0 Torr with helium as the bath gas. The experimental technique is discharge fast flow with mass spectrometric detection and monitoring of the first order decay of HC3N. The H atom, which is the excess reactant by a factor of 100-660, is generated via the fast reaction F + H2 -> H + HF. For each pressure, first order rate constants (kfirst) were measured for [H]= 1.2 - 13 x 1013 molecule cm-3. The bimolecular rate constants (kbi) were derived from the slopes of the plots of kfirst versus [H]. Within our experimental uncertainty, we did not see a pressure dependence of the bimolecular rate constant for the limited pressure range of 0.5 - 2 Torr at T = 298K. The results at T=298K are kbi(0.5 Torr)=2.0x10-13, kbi(1.0 Torr)=2.2x10-13 and kbi(2.0 Torr)=1.8x10-13, all in units of cm3 molecule-1 s-1. These measured rate constants are about a factor of 50 faster than those estimated by analogy with the H + C2H2 reaction. These results could render Reaction 1 a more important loss process than previously estimated. Measurements of the rate constants for reaction 1 are continuing at T = 250K and 200K. The Planetary Atmospheres Program of NASA Headquarters is providing the funding for this research.
Cody Regina J.
Parker J. K.
Payne Walter A.
Stief Louis J.
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