Laboratory Formation of Carbonaceous Particles in Dilute Gas Mixtures of Organic Molecules

Details Laboratory Formation of Carbonaceous Particles in Dilute Gas Mixtures of Organic Molecules Laboratory Formation of Carbonaceous Particles in Dilute Gas Mixtures of Organic Molecules

Dec 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998baas...30.1453c&link_type=abstract

American Astronomical Society, DPS meeting #30, #55.P30; Bulletin of the American Astronomical Society, Vol. 30, p.1453

Astronomy and Astrophysics

Astrophysics

Scientific paper

Particles of various organic compositions have been observed in the atmospheres of the outer planets, their satellites, and comets. Infrared (IR) spectral features reveal the presence of these particles in the outer planetary system, especially Titan. ``CHON" particles in the coma of Comet Halley were analyzed chemically by the Giotto spacecraft. Organic molecules in the atmospheres of these bodies are present in gas concentrations of parts per million by volume (ppmv). A recently developed laboratory technique can generate small carbonaceous particles from gas mixtures containing organic molecules in the ppmv range either in the presence or absence of water. The particles are formed by the photolysis of a gas mixture of ppmv of small organic molecules (C2 to C8) in a carrier gas at atmospheric pressure. The photolysis source is an excimer laser at 193 nm. Since the flow area of the gas is larger than that of the laser beam, a mixture of irradiated and unirradiated gases are producing the particles. A visible laser (He-Ne at 633 nm) is used to monitor the particles by scattering. Nitrogen, Argon, or Helium are the carrier gases in slowly flowing gas streams. The gas flows counter to the laser beam propagation direction. Since the laser beam penetrates 10-40 cm of the 150 cm long flow tube, a particle formation front is established, and samples are collected upstream of this front. The particles are collected on ZnS or ZnSe substrates, and their IR spectra from 2.5-14 (or 20) microns is measured with an FTIR instrument. The samples are then heated at low temperatures, and the IR spectra are remeasured. Since the most extensive set of experiments has consisted of photolyzing a gas mixture of several ppmv of small aromatic molecules in each of the carrier gases with saturated water vapor, these conditions are more representative of cometary atmospheres. Aromatic gas mixtures without water have been photolyzed to be more representative of outer planetary atmospheres. Comparisons are made between the IR spectra of the particles formed from the two types of gas mixtures. This research is being supported by the Laboratory Astrophysics Program of NASA Headquarters.

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