Aromatic Formation Promoted by Ion-Driven Radical Pathways in EUV Photochemical Experiments Simulating Titan's Atmospheric Chemistry.

In the atmosphere of Titan, Saturn's main satellite, molecular growth is initiated by 85.6 nm extreme ultraviolet (EUV) photons triggering a chemistry with charged and free-radical species. However, the respective contribution of these species to the complexification of matter is far from being known. This work presents a chemical analysis in order to contribute to a better understanding of aromatic formation pathways. A gas mixture of N2/CH4 (90/10%) within the closed SURFACAT reactor was irradiated at a relatively low pressure (0.1 mbar) and room temperature for 6 h by EUV photons (∼85.6 nm). The neutral molecules formed at the end of the irradiation were condensed in a cryogenic trap and analyzed by electron ionization mass spectrometry. An analysis of the dominant chemical pathways highlights the identification of benzene and toluene and underlies the importance of small ion and radical reactions. On the basis of the experimental results, a speculative mechanism based on sequential H-elimination/CH3-addition reactions is proposed for the growth of aromatics in Titan's atmosphere. Elementary reactions to be studied are given to instill future updates of photochemical models of Titan's atmosphere.

On an EUV Atmospheric Simulation Chamber to Study the Photochemical Processes of Titan's Atmosphere.

The in situ exploration of Titan's atmosphere requires the development of laboratory experiments to understand the molecular growth pathways initiated by photochemistry in the upper layers of the atmosphere. Key species and dominant reaction pathways are used to feed chemical network models that reproduce the chemical and physical processes of this complex environment. Energetic UV photons initiate highly efficient chemistry by forming reactive species in the ionospheres of the satellite. We present here a laboratory experiment based on a new closed and removable photoreactor coupled here to an Extreme Ultraviolet (EUV) irradiation beam produced by the high-order harmonic generation of a femtosecond laser. This type of EUV stable source allow long-term irradiation experiments in which a plethora of individual reactions can take place. In order to demonstrate the validity of our approach, we irradiated for 7 hours at 89.2 nm, a gas mixture based on N2/CH4 (5%). Using only one wavelength, products of the reaction reveal an efficient photochemistry with the formation of large hydrocarbons but especially organic compounds rich in nitrogen similar to Titan. Among these nitrogen compounds, new species had never before been identified in the mass spectra obtained in situ in Titan's atmosphere. Their production in this experiment, on the opposite, corroborates previous experimental measurements in the literature on the chemical composition of aerosol analogues produced in the laboratory. Diazo-compounds such as dimethyldiazene (C2H6N2), have been observed and are consistent with the large nitrogen incorporation observed by the aerosols collector pyrolysis instrument of the Huygens probe. This work represents an important step forward in the use of a closed cell chamber irradiated by the innovative EUV source for the generation of photochemical analogues of Titan aerosols. This approach allows to better constrain and understand the growth pathways of nitrogen incorporation into organic aerosols in Titan's atmosphere.