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[15.02] Rate Constant for the Recombination Reaction of CH₃ Free Radicals: A Loss Process in Outer Planet Atmospheres

R.J. Cody, W.A. Payne, R.P. Thorn, L.J. Stief (NASA/GSFC), F.L. Nesbitt (Coppin State College), M.A. Iannone (Millersville University)

The methyl free radical (CH₃) has been observed recently in the atmosphere of both Saturn and Neptune. Atmospheric models predict much higher abundances of CH₃ than observed, especially in the case of Saturn. A major, but not exclusive, cause of the disagreement has been identified as the underestimation of the loss process for CH₃ via self-reaction at the low temperatures and pressures prevailing in these atmospheric systems.

Although the reaction CH₃ + CH₃ + M arrow C₂H₆ + M has been extensively studied both theoretically and experimentally, the laboratory conditions have been, with only a few exceptions, higher temperatures (T\geq 298K), higher pressures (P\geq 10 Torr - 13.3 mbar) or M=Ar rather than H₂ or He as the bath gas. We are measuring the rate constant of this reaction under physical conditions more suitable to the outer planet atmospheres, i.e. P=0.6 - 2.0 Torr of He and T= 202 - 160K. The experimental technique is discharge fast flow with mass spectrometric detection and monitoring of the CH₃ decay. The methyl radical is generated via the fast reaction F + CH₄ arrow CH₃ + HF.

Experiments at T=298K and P=1 Torr He yield k=2.5x10^-11 cm³molecule^-1s^-1 which is consistent with previous measurements allowing for extrapolation to lower pressure or differences in bath gas M. Our experiments at T=202K and P=1 Torr yield a slightly faster rate of k=4.0x10^-11 cm³molecule^-1s^-1. The pressure dependence in the range of 0.6 to 2.0 Torr is negligible. Experiments at T=160K, a temperature more relevant for the outer planet atmospheres, will be initiated in the near future. The experimental results will be compared with very recent theoretical calculations of the rate constant at low temperatures and pressures. The impact of these results upon the atmospheric models will be briefly discussed.

The Planetary Atmospheres Program of NASA Headquarters has provided funding for this reseach.