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Y. B\'{e}nilan, A. Jolly, F. Raulin (LISA, Univ. Paris 7 et 12, France), LISA Team
Methylacetylene, CH3C2H, has been detected in Titan atmosphere during the Voyager encounter but not its isomer, allene (CH2CCH2). This result has been hardly explained by photochemical models since both isomers are supposed to chemically react in the same way. Consequently, it has been proposed that the isomerisation of allene into methylacetylene occurs on Titan. We have measured the infrared and ultraviolet spectra of these two molecules. A special care has been taken to obtained samples as pure as possible. First, absolute band intensities have been determined from IR measurements on both compounds. These data allow to set an upper limit on the relative abundance of allene as compared to methylacetylene in Titan atmosphere. Then, the ultraviolet spectra of these molecules have been measured in the 185-240 nm range at several temperatures, and specially low temperatures. The absorption coefficients determined from these measurements consist in a continuum above 185 nm. This imply that both methylacetylene and allene will be hardly detected by remote sensing observations in the UV range. Nevertheless, the comparison of the dissociative coefficient of allene and methylacetylene, calculated for wavelength greater than 185 nm, seems to explain the results deduced from IR observations. We conclude that no isomerisation processes need to be introduced in photochemical models to explain the relative abundance of allene and methylacetylene in Titan atmosphere.