36th DPS Meeting, 8-12 November 2004
Session 20 Titan
Poster I, Tuesday, November 9, 2004, 4:00-7:00pm, Exhibition Hall 1A

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[20.04] IR and UV Spectroscopic Studies of C2N2, HC3N, DC3N and HC315N : Implication for Photochemical Models and Observations of Titan's Atmosphere

Y. Benilan, N. Fray, A. Jolly, F. Raulin (LISA), C. Crepin (LPPM), R. Kolos (IPC), J.-C. Guillemin (ENCR)

The photochemistry of N2 and CH4 at high altitudes in Titan's atmosphere leads to the production of numerous saturated and unsaturated hydrocarbons and nitriles. HC3N, the first member of the cyanopolyyne's family (with formula HC2n+1N, n>1) has been detected but the question remains concerning the heavier species. Those are of special interest since they are predicted by photochemical models to be one of the main link between the gas phase and the solid phase visible as high altitude haze. C2N2 is the first member of the dicyanopolyynes (with formula C2nN2, n>1). It has been observed at Titanís north pole during Voyager encounter and the second member of this family, C4N2, is the only condensate clearly identify in this atmosphere.

Since several years we have conducted in our laboratory systematic spectroscopic studies of organic compounds in the infrared and ultraviolet wavelength ranges. Our latest studies include C2N2, HC3N and two isotopomers: DC3N and HC315N.. Infrared absorption coefficients have been measured for all molecules. For cyanogen, the absorption coefficient of the n4+n5 combination mode at 732 cm-1 has been precisely measured . The position and intensities of the vibrational bands of three isotopes of HC3N will be compared. Finally, the implications of actual results on the possible detection from observations of the Cassini/Huygens mission will be discussed.

The photochemistry of cyanoacetylene is still not well understood. For example, the quantum yield of C3N formation from the dissociation of HC3N is still controversial and the presence of an reactive metastable state, like the one of C4H2, has sometime been proposed. We will present the emissions spectra obtained after UV excitation (193 and 248 nm) of HC3N and its isotopomers using matrix isolation techniques. The implications of the obtained results on photochemical models will then be discussed.

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