Characterization of aromaticity in analogues of titan's atmospheric aerosols with two-step laser desorption ionization mass spectrometry

The role of polycyclic aromatic hydrocarbons (PAH) and Nitrogen containing PAH (PANH) as intermediates of aerosol production in the atmosphere of Titan has been a subject of controversy for a long time. An analysis of the atmospheric emission band observed by the Visible and Infrared Mapping Spectrometer (VIMS) at 3.28 micrometer suggests the presence of neutral polycyclic aromatic species in the upper atmosphere of Titan. These molecules are seen as the counter part of negative and positive aromatics ions suspected by the Plasma Spectrometer onboard the Cassini spacecraft, but the low resolution of the instrument hinders any molecular speciation. In this work we investigate the specific aromatic content of Titan's atmospheric aerosols through laboratory simulations. We report here the selective detection of aromatic compounds in tholins, Titan's aerosol analogues, produced with a capacitively coupled plasma in a N2:CH4 95:5 gas mixture. For this purpose, Two-Step Laser Desorption Ionization Time-of-Flight Mass Spectrometry (L2DI-TOF-MS) technique is used to analyze the so produced analogues. This analytical technique is based on the ionization of molecules by Resonance Enhanced Multi-Photon Ionization (REMPI) using a {\lambda}=248 nm wavelength laser which is selective for aromatic species. This allows for the selective identification of compounds having at least one aromatic ring. Our experiments show that tholins contain a trace amount of small PAHs with one to three aromatic rings. Nitrogen containing PAHs (PANHs) are also detected as constituents of tholins. Molecules relevant to astrobiology are detected as is the case of the substituted DNA base adenine

A new infrared band in the Interstellar and Circumstellar Clouds: C_4 or C_4H?

We report on the detection with the Infrared Space Observatory (ISO) of a molecular band at 57.5 microns (174 cm^{-1}) in carbon-rich evolved stars and in Sgr B2. Taking into account the chemistry of these objects the most likelihood carrier is a carbon chain. We tentatively assign the band to the nu_5 bending mode of C_4 for which a wavenumber of 170-172.4 cm^{-1} has been derived in matrix experiments (Withey et al. 1991). An alternate carrier might be C_4H, although the frequency of its lowest energy vibrational bending mode, nu_7, is poorly known (130-226 cm^{-1}). If the carrier is C_4, the derived maximum abundance is nearly similar to that found for C_3 in the interstellar and circumstellar media by Cernicharo, Goicoechea & Caux (2000). Hence, tetra-atomic carbon could be one of the most abundant carbon chain molecules in these media.Comment: 11 pages, 1 figure, accepted in ApJ Letter

The Need for Laboratory Measurements and Ab Initio Studies to Aid Understanding of Exoplanetary Atmospheres

We are now on a clear trajectory for improvements in exoplanet observations that will revolutionize our ability to characterize their atmospheric structure, composition, and circulation, from gas giants to rocky planets. However, exoplanet atmospheric models capable of interpreting the upcoming observations are often limited by insufficiencies in the laboratory and theoretical data that serve as critical inputs to atmospheric physical and chemical tools. Here we provide an up-to-date and condensed description of areas where laboratory and/or ab initio investigations could fill critical gaps in our ability to model exoplanet atmospheric opacities, clouds, and chemistry, building off a larger 2016 white paper, and endorsed by the NAS Exoplanet Science Strategy report. Now is the ideal time for progress in these areas, but this progress requires better access to, understanding of, and training in the production of spectroscopic data as well as a better insight into chemical reaction kinetics both thermal and radiation-induced at a broad range of temperatures. Given that most published efforts have emphasized relatively Earth-like conditions, we can expect significant and enlightening discoveries as emphasis moves to the exotic atmospheres of exoplanets.Comment: Submitted as an Astro2020 Science White Pape

Étude expérimentale et théorique de la contribution de la composante organique réfractaire à la phase gazeuse dans l'environnement cométaire

Certaines espèces gazeuses observées dans la coma, telles que CO et pourraient provenir de la degradation de composes solides presents dans les grams cometaires. J'ai montré que le HÎv est stable par irradiation UV et qu'il se sublime lorsqu'il est chauffé, il n'est donc pas un bon candidat pour une source de radicaux GN. J'ai mis en évidence, par spectroscopie IR, la production de HGN, Œ1 et C par irradiation UV des polymères de HGN et celles de NH et HGN par chauffage entre 430 et 580 K, la production de l'ensemble de ces espèces gazeuses a été quantifiée. Ensuite, j'ai modélisé la production de GN dans la coma. Bien que la densité de GN ne puisse pas être reproduite en prenant en compte la dégradation des polymères de HEN, cette hypothèse ne peut pas être infirmée. Dans Hale-Bopp, l'évolution héliocentrique des taux de production de H est rapide. J'ai montré que cette évolution est due la prédominance de la production de H par dégradation thermique du POM jusqu'à 3,5 UA.PARIS12-CRETEIL BU Multidisc. (940282102) / SudocSudocFranceF

Experimental Investigation of the Photochemical Production of Hydrocarbons in Warm Gas Giant Exoplanet Atmospheres

International audienceIn warm (equilibrium temperature <1000 K) gas giant exoplanet atmospheres, the observation of trace species in abundances deviating from thermochemical equilibrium predictions could be used as an indicator of disequilibrium chemical processes, such as photochemistry. To predict which compounds could be used as such tracers, it is therefore essential to study how photochemical processes affect their abundances. For this purpose, we investigated experimentally the efficiency of the photochemical formation of hydrocarbons in gas mixtures representative of warm gas giant atmospheres as a function of the gas temperature at millibar pressures. We find that, compared to thermal reactions alone, photochemistry efficiently promotes, under the studied conditions, the formation of hydrocarbons, with the detection of acetylene, ethane, and propane, as well as carbon monoxide. Therefore, our results confirm the importance of photochemistry in exoplanet atmospheres as a disequilibrium process. Ethane is the major hydrocarbon formed in our experiments, in apparent contradiction with the prediction by thermo-photochemical models that acetylene should be the main hydrocarbon product. We also observe an evolution of the hydrocarbon production efficiency as a function of the temperature, a behavior not reproduced by a 0D thermo-photochemical model. Additional studies are necessary to definitively understand the origin of the differences between the experimental and modeling results and to infer the importance of our results for understanding hydrocarbon formation in warm gas giant exoplanet atmospheres. Finally, our work demonstrates the importance of experimental studies together with modeling studies to accurately interpret, understand, and predict observations of exoplanet atmospheres

Intrinsic lifetime of metastable excited C4H2: implications for the photochemistry of C4H2 in Titan's atmosphere

International audienceA 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 , measured in Ar and Kr matrices. A value of about in the gas phase is deduced from matrices results ( in Ar and 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

Experimental Investigation of the Photochemical Production of Hydrocarbons in Warm Giant Exoplanet Atmospheres

International audienc

Temperature dependency of CO2 and H2O absorption cross sections in the VUV: Implications for thermo-photochemistry of Hot Jupiters

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SPECTROSCOPIC STUDY OF CYANOACETYLENE CATION: SLOW PHOTO-ELECTRON SPECTROSCOPY AND AB-INITIO INVESTIGATIONS

Author Institution: Univ. Paris Est Creteil & Univ. Paris Diderot, Institut Pierre Simon Laplace,61 Ave du General de Gaulle, 94010, Creteil, France; Synchrotron SOLEIL, LÕOrme des Merisiers, St.Aubin, B.P. 48, 91192, Gif-sur-Yvette Cedex, France; Observatoire de Paris-Meudon, 5 place Jules-Jansen, 92195, Meudon, FranceCyanoacetylene is one of the key minor constituents in the atmosphere of Titan. The ion HCCCN+ has been detected in this atmosphere and it is supposed to be formed by the reaction between C3H2+ and atomic nitrogen\footnote{C.~Barrientos, P.~Redondo and A.~Largo \textit{J.~Chem.~Phys. A} \underline{\textbf{104}}(49), 11541-11548. 2000}. We present here a spectroscopic investigation of the cyanoacetylene cation using photoexcitation of the neutral by vacuum-ultraviolet (VUV) synchrotron radiation coupled to a velocity map imaging electron/ion coincidence spectrometer\footnote{L.~Nahon, N.~De Oliveria,J.~F.~Gil,B.~Pilette,O.~MarcouillÌ©, B.~La garde and F~Polack \textit{Journal~of Synchrotron~Radiation} \underline{\textbf{19}}(4), 508-520; 2012}. The cation spectroscopy is studied by the Slow Photoelectron Spectroscopy technique (SPES) (figure below) and the Total Ion Yield (TIY). The TIY has been calibrated to absolute units using the known propane absolute cross-section. Quantum chemical calculations are performed in order to interpret these spectra. These calculations deal with the equilibrium geometries, electronic-state patterns and evolutions, and harmonic and anharmonic wavenumbers