Intrinsic lifetime of metastable excited C4H2: implications for the photochemistry of C4H2 in Titanʹs atmosphere

A better understanding of the complex organic chemistry occurring in the methane-rich atmosphere of Titan can be achieved by comparing observational data with the results of photochemical models. Until now, these models failed to reproduce the observed concentrations of C4H2. This may be due to the lack of kinetic data with regard to polyynes chemistry. In particular, no data were available for the intrinsic lifetime of the metastable excited state of C4H2. This state, probably a triplet excited state, seems to be a precursor in the photochemical reactions occurring in the atmosphere of Titan. Standard matrix isolation technique associated with time resolved spectroscopy is well adapted to characterize metastable states. Absorption and emission spectra (phosphorescence) of C4H2 in rare gas matrices are presented and a vibrational analysis is conducted. We also report the first experimental intrinsic lifetime of C4H2∗, measured in Ar and Kr matrices. A value of about 100 ms in the gas phase is deduced from matrices results (∼45 ms in Ar and ∼20 ms in Kr). The determined lifetime of C4H2, despite being two orders of magnitude higher than the value currently used in the models, cannot explain the discrepancy between models and observations. Nevertheless, it can account for an increase in the production of heavier compounds.