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Low-Temperature Hydrocarbon Photochemistry:
CH3 + CH3 Recombination in Giant Planet Atmospheres

Planetary emissions of the methyl radical CH3 were observed for the first time in 1998 on Saturn [1] and Neptune [2] by the ISO (Infrared Space Observatory) mission satellite [3]. CH3 is produced by VUV photolysis of CH4 and is the key photochemical intermediate leading complex organic molecules on the giant planets and moons. The CH3 emissions from Saturn were unexpectedly weak [1]. A suggested remedy is to increase the rate of the recombination reaction

CH3 + CH3 + H2 -> C2H6 + H2

at 140 K to a value at least 10 times that measured at room temperature in rare gases, but within the range of disagreeing theoretical expressions when extrapolated to low temperature [4-6].

We are performing laboratory experiments at low temperature and very low pressure. The experiments are supported by RRKM theoretical modeling that is calibrated using the extensive combustion literature. The distinction between "high" and "low" pressure is a significant one [7]. In the so called "low pressure limit" the rate of recombination is limited by the rate of stabilization or energy removal by the third body called "M" (really H2), and the overall recombination rate coefficient is written as

krecomb(M->0) ~ k0[M]

In the "high pressure limit" the buffer gas pressure is sufficiently high to stabilize every collision complex, and the overall recombination rate coefficient becomes pressure independent:

krecomb(M->infinity) ~ kinfinity

Our recent calculations indicate that k0 rises with decreasing temperature much faster than does kinfinity. These results mean that low temperature laboratory experiments need to be performed at quite low pressures, say 0.01 mbar or less in order to extrapolate to the 0.001 mbar and below characteristic of the relevant regions of the giant planet atmospheres [8]. This is consistent with the recent work in Stief's laboratory [9,10], in which no pressure dependence was observed at 202 and 155 K for pressures from 0.6 to 2.6 mbar.

References

  1. S. K. Atreya, S. G. Edgington, Th. Encrenaz, and H. Feuchtgruber, "ISO Observations of C2H2 on Uranus and CH3 on Saturn," in The Universe as Seen by ISO, P. Cox and M. F. Kessler, Eds, (ESA-SP 427 1999).
  2. B. Bezard, P. N. Romani, H. Feuchtgruber, and T. Encrenaz, "Detection of Methy Radicals in Neptune," Astrophys. J. 515, 868-872 (1999).
  3. Th. Encrenaz, "Observation of Solar System Objects with the ISO Satellite," Bull. Am. Astron. Soc. 30, 1059 (1998).
  4. M. T. Macpherson, M. J. Pilling, and M.J.C. Smith, "The Pressure and Temperature Dependence of the Rate Constant for Methyl Radical Recombination over the Temperature Range 296-577 K," Chem. Phys. Lett. 94, 430-433 (1983).
  5. I. R. Slagle, D. Gutman, J. W. Davies, and M. J. Pilling, "Study of the Recombination Reaction CH3 + CH3 -> C2H6. 1. Experiment," J. Phys. Chem. 92, 2455-2462 (1988).
  6. D. Walter, H.-H. Grotheer, J. W. Davies, M. J. Pilling, and A. F. Wagner, "Experimental and Theoretical Study of the Recombination Reaction CH3 + CH3 -> C2H6," Proc. Combust Inst. 23, 107-114 (1990).
  7. G. P. Smith and D. L. Huestis, "Molecular Recombination in Laser Media. I. Theoretical Study of NF2 + F + Ne -> NF3 + Ne," J. Appl. Phys. 52, 6041-6045 (1981).
  8. J. I. Moses, B. Bezard, E. Lellouch, H. Feuchtgruber, and G. R. Gladstone, and M. Allen, "Photochemical Models of Saturn's Atmosphere: I. Hydrocarbon Chemistry and Comparisons with ISO Observations," Icarus 143, 244-298 (2000).
  9. R. J. Cody, W. A. Payne, R. P. Thorn, and L. J. Stief, "Rate Constant for the Recombination of CH3 Free Radicals: A Loss Process in Outer Planet Atmospheres," Bull. AAS 32, 1020 (2000).
  10. R. J. Cody, L. J. Stief, F. L. Nesbitt, and M. A. Iannone, "Rate Constant for the CH3 Recombination Reaction at 155 K: A Loss Process in Outer Planet Atmospheres," Bull. AAS 33, 1080 (2001).

Acknowledgements

This research is supported by the NSF Planetary Astronomy and NASA Planetary Atmospheres programs.

Principal Investigators

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