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V. Vuitton, M.-C. Gazeau (L.I.S.A., Universités Paris VII et XII, France), C. Gée (Université de Cergy-Pontoise, France), C. Crépin (L.P.P.M., Université Paris-Sud, France), Y. Bénilan (L.I.S.A., Universités Paris VII et XII, France)
Diacetylene has been detected in the atmosphere of Titan and can lead to the formation of long-chain polyynes that are invoked as the main link between the gas phase and the haze particles visible at high altitude. However, the current photochemical models fail to reproduce the observed concentrations of C4H2. This is due to the lack of kinetic data with regard to these polyynes. The formation of a metastable state, referred as C4H2*, has been suggested to explain its high photochemical quantum yield well below its dissociation threshold. The characterisation of C4H2* and the determination of its lifetime are of crucial importance since radiative quenching of C4H2* determines the steady-state number density of C4H2* at the highest altitudes. A longer radiative lifetime would directly increase the importance of C4H2* chemistry. We used a standard matrix isolation technique where C4H2 is trapped at low temperature in rare gas matrices. The C4H2 molecule is excited in a singlet state by a near-UV laser beam at 249 nm. The relaxation to metastable triplet states is enhanced by the rare gas via a spin-orbit coupling. This technique allows time resolved spectroscopy on the emission from the metastable state and lifetime measurements can be performed. We report here the first determination of the radiative lifetime of C4H2* trapped in different rare gas matrices (Ar, Kr,...). We will emphasize the consequences of this new result for the concentrations of diacetylene and of higher molecular weight compounds in Titan's atmosphere.