DPS 34th Meeting, October 2002
Session 39. Laboratory Investigations
Oral, Chair(s): J. Allen and R.A. Baragiola, Friday, October 11, 2002, 8:45-10:15 and 10:45-11:15am, Ballroom

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[39.07] Selecting, Predicting, and Measuring Key Hydrocarbon Photochemistry Reactions: CH3 + CH3 + H2

G. P. Smith, D. L. Huestis, D. Nash (SRI International)

Understanding hydrocarbon species profiles and transport in outer planet atmospheres measured from probes such as Cassini or telescopes such as ISO requires photochemical models with accurate low temperature and pressure kinetics. We apply a sensitivity analysis technique to identify the key controlling reactions for various species, which require evaluation, theoretical extrapolation, and/or additional measurement.

The methyl radical recombination reaction to form ethane is most prominent. The sparsity of low temperature measurements and lack of a theoretical basis for extrapolations to planetary conditions has presented a wide choice and uncertainty in pressure dependent expressions for this rate in modeling. We apply 3 levels of RRKM absolute rate theory to this reaction to account for existing laboratory data and produce more reliable recommended values for low temperatures and pressures. The best theory for low pressure recombination rates at low temperature, a common class in Saturn models, is also a fundamental issue.

Remaining uncertainty in this procedure lies in the low pressure rate constant values at temperatures of 300K and below, and in determining the relative third body efficiencies of H2 and He relative to the Ar used in most laboratory work. The relevant low pressure limit regimes are difficult to access experimentally. Two related experimental approaches will be described, in which 248-266 nm laser photolysis of CH3I is used to generate known quantities of CH3. A Knudsen cell reactor measures the steady state yield of ethane product using a mass spectrometer, at pressures of 2-25 mtorr and quarter second residence times. The second laser experiment uses time-resolved, in situ, resonant multiphoton ionization detection of the CH3 reactive decay, at somewhat higher pressure.

Supported by NASA Planetary Atmospheres and NSF Planetary Astronomy Programs. DN is a participant in an NSF Research Experiences for Undergraduates program.

If the author provided an email address or URL for general inquiries, it is as follows:

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.