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V. Vuitton, M.-C. Gazeau, Y. Bénilan, F. Raulin (L.I.S.A, University Paris XII, France)
To interpret the concentrations of the products observed in Titan's atmosphere and to better understand its methane chemistry, theoretical models have been developed. But, as all photochemical models, those of Titan's suffer from the lack of adequate experimental data. This lack is particularly enhanced for polyynes since these compounds are highly thermally unstable and therefore very difficult to study in the laboratory. In particular, kinetic rate constants of the reactions of C2nH radicals with polyynes have not yet been measured: the data used in the models are estimated from the reaction of C2H with C2H2. Also, the quantum yields of dissociation and production of metastable triplets states (C2H2*, C4H2*) present large uncertainties. Yet, if the radiative lifetimes of these states are long enough, their reactions with polyynes could be as important as the ones of C2nH radicals in the low stratosphere. Finally, absorption cross sections of long chain polyynes have not been studied in gas phase, although their photodissociation is supposed to control the formation of the haze particles present in the high atmosphere. A review of the current knowledge on polyyne photochemistry will be presented and we will emphasise the experimental data necessary to improve the models.