Chemistry under EUV Irradiation of H2_2-CO-N2_2 Gas Mixtures: Implications for Photochemistry in the Outer CSE of Evolved Stars

{CircumStellar Envelopes (CSEs) of stars are complex chemical objects for which theoretical models encounter difficulties in elaborating a comprehensive overview of the occurring chemical processes. Along with photodissociation, ion-neutral reactions and dissociative recombination might play an important role in controlling molecular growth in outer CSEs. The aim of this work is to provide experimental insights into pathways of photochemistry-driven molecular growth within outer CSEs to draw a more complete picture of the chemical processes occurring within these molecule-rich environments. A simplified CSE environment was therefore reproduced in the laboratory through gas-phase experiments exposing relevant gas mixtures to an Extreme UltraViolet (EUV) photon source. This photochemical reactor should ultimately allow us to investigate chemical processes and their resulting products occurring under conditions akin to outer CSEs. We used a recently developed EUV lamp coupled to the APSIS photochemical cell to irradiate CSE relevant gas mixtures of H$_2$, CO and N$_2$, at one wavelength, 73.6 nm. The detection and identification of chemical species in the photochemical reactor was achieved through in-situ mass spectrometry analysis of neutral and cationic molecules. We find that exposing CO-N$_2$-H$_2$ gas mixtures to EUV photons at 73.6 nm induces photochemical reactions that yield the formation of complex, neutral and ionic species. Our work shows that N$_2$H$^+$ can be formed through photochemistry along with highly oxygenated ion molecules like HCO$^+$ in CSE environments. We also observe neutral N-rich organic species including triazole and aromatic molecules. These results confirm the suitability of our experimental setting to investigate photochemical reactions and provide fundamental insights into the mechanisms of molecular growth in the outer CSEs

The C(3P) + NH3 reaction in interstellar chemistry: II. Low temperature rate constants and modeling of NH, NH2 and NH3 abundances in dense interstellar clouds

A continuous supersonic flow reactor has been used to measure rate constants for the C + NH3 reaction over the temperature range 50 to 296 K. C atoms were created by the pulsed laser photolysis of CBr4. The kinetics of the title reaction were followed directly by vacuum ultra-violet laser induced fluorescence (VUV LIF) of C loss and through H formation. The experiments show unambiguously that the reaction is rapid at 296 K, becoming faster at lower temperatures, reaching a value of 1.8 10-10 cm3 molecule-1 s-1 at 50 K. As this reaction is not currently included in astrochemical networks, its influence on interstellar nitrogen hydride abundances is tested through a dense cloud model including gas-grain interactions. In particular, the effect of the ortho-to-para ratio of H2 which plays a crucial role in interstellar NH3 synthesis is examined

Flow tube studies of the C(3P) reactions with ethylene and propylene

International audienceProduct detection studies of C(3P) atom reactions with ethylene, C2H4(X1Ag) and propylene, C3H6(X1A′) are carried out in a flow tube reactor at 332 K and 4 Torr (553.3 Pa) under multiple collision conditions. Ground state carbon atoms are generated by 193 nm laser photolysis of carbon suboxide, C3O2 in a buffer of helium. Thermalized reaction products are detected using tunable VUV photoionization and time of flight mass spectrometry. For C(3P) + ethylene, propargyl (C3H3) is detected as the only molecular product in agreement with previous studies on this reaction. The temporal profiles of the detected ions are used to discriminate C(3P) reaction products from side reaction products. For C(3P) + propylene, two reaction channels are identified through the detection of methyl (CH3) and propargyl (C3H3) radicals for the first channel and C4H5 for the second one. Franck–Condon Factor simulations are employed to infer the C4H5-isomer distribution. The measured 1:4 ratio for the i-C4H5 isomer relative to the methylpropargyl isomers is similar to the C4H5 isomer distribution observed in low-pressure flames and differs from crossed molecular beams data. The accuracy of these isomer distributions is discussed in view of large uncertainties on the photoionization spectra of the pure C4H5 isomer

Accounting for Molecular Flexibility in Photoionization: Case of tert-Butyl Hydroperoxide

tert-butyl hydroperoxide (tBuOOH) is a common intermediate in the oxidation of organic compounds that needs to be accurately quantified in complex gas mixtures for the development of chemical kinetic models of low temperature combustion. This work presents a combined theoretical and experimental investigation on the synchrotron-based VUV single photon ionization of gas-phase tBuOOH in the 9.0 - 11.0 eV energy range, including dissociative ionization processes. Computations consist of the determination of the structures, vibrational frequencies and the energetics of neutral and ionic tBuOOH. The Franck-Condon spectrum for the tBuOOH+ (X+) + e- tBuOOH (X) +h transition is computed, where special treatment is undertaken because of the flexibility of tBuOOH, in particular regarding the OOH group. Through comparison of the experimental mass-selected threshold photoelectron spectra with explicitly correlated coupled cluster calculations and Franck-Condon simulations, thermochemical values are obtained as the adiabatic ionization energy and the appearance energy of the only fragment observed within the above-mentioned energy range, identified as the tert-butyl C4H9+. Finally, the signal branching ratio between the parent and the fragment ions is provided as a function of photon energy, essential to quantify tBuOOH in gas-phase oxidation/combustion experiments via advance mass spectrometry techniques

Enabling planetary science across light-years. Ariel Definition Study Report

Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, was adopted as the fourth medium-class mission in ESA's Cosmic Vision programme to be launched in 2029. During its 4-year mission, Ariel will study what exoplanets are made of, how they formed and how they evolve, by surveying a diverse sample of about 1000 extrasolar planets, simultaneously in visible and infrared wavelengths. It is the first mission dedicated to measuring the chemical composition and thermal structures of hundreds of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System. The payload consists of an off-axis Cassegrain telescope (primary mirror 1100 mm x 730 mm ellipse) and two separate instruments (FGS and AIRS) covering simultaneously 0.5-7.8 micron spectral range. The satellite is best placed into an L2 orbit to maximise the thermal stability and the field of regard. The payload module is passively cooled via a series of V-Groove radiators; the detectors for the AIRS are the only items that require active cooling via an active Ne JT cooler. The Ariel payload is developed by a consortium of more than 50 institutes from 16 ESA countries, which include the UK, France, Italy, Belgium, Poland, Spain, Austria, Denmark, Ireland, Portugal, Czech Republic, Hungary, the Netherlands, Sweden, Norway, Estonia, and a NASA contribution

kinetic of formation of van der Waals clusters and products detection of carbon atoms reaction of interest for combustion and astrophysical and atmospheric environments

Cette thèse à été réalisée au sein de l'équipe d'astrophysique de laboratoire du département de physique moléculaire de l'Institut de Physique de Rennes. Dans ces travaux, une première partie présente l'application de la technique CRESU à l'étude d'agrégats de Van der Waals d'eau et de propane. Nous avons observé expérimentalement la formations d'agrégats d'eau sur une gamme de températures allant de 22.9 à 69.4 K, puis modélisé les premières étapes de l'agrégation en nous appuyant sur la détermination théorique de coefficients de vitesse. Nous avons également effectué les premières mesures du coefficient de formation du dimère de propane sur une gamme de température allant de 22.9 à 49.1 K. La seconde partie de ces travaux porte sur la détection de les produits de réactions impliquant les atomes de carbone et diverses molécules (C2H4, C2H6, C4H8 et NH3) à une température ambiante. Ces études ont été menées au synchrotron de l'Adavanced Light Source de Berkeley. Nous avons également étudié la cinétique et la formation des produits de la réaction entre atomes de carbone et ammoniac sur une gamme de température allant de 50 à 296 K. Pour cela nous avons utilisé le dispositif CRESU de l'Institut des Sciences Moléculaires de Bordeaux. Ces données ont été incrémentées dans un modèle de nuage interstellaire dense afin de voir leur influence sur l'abondance des hydrures azotées. Les travaux de cette thèse contribuent à mieux comprendre les mécanismes de réactions élémentaires menant à la formation et à la croissance de molécules et d'agrégats de Van der Waals en phase gazeuse dans des conditions physiques variées. Ils fournissent des données importantes pour la modélisation d'atmosphères planétaires, de nuages interstellaires et de processus de combustion.This thesis has been carried out in the team of astrophysics laboratory in the Molecular Physics Department of the Physics Institute of Rennes. In this work, a first part presents the application of the CRESU technique to aggregation of van der Waals clusters of water and propane. We experimentally observed water aggregation on a range of temperatures from 22.9 to 69.4 K, then modeled the early stages of aggregation building on the theoretical determination of rate coefficients. We also made the first measurements of propane dimer formation coefficient over a temperature range of 22.9 to 49.1 K. The second part of this work concerns the detection of the products reactions involving carbon atoms and various molecules (C2H4 , C2H6 , C4H8 and NH3) at room temperature. These studies were conducted at the synchrotron Adavanced Light Source of Berkeley . We also studied the kinetics and products formation of the reaction between carbon atoms and ammonia over a temperature range of 50-296 K. To do this we used the device CRESU of the molecular science institute of Bordeaux. This data was incremented in a dense interstellar cloud model to see their influence on the abundance of nitrogen hydrides. The work of this thesis contribute to better understanding the mechanisms of reactions leading to the formation and growth of molecules and van der Waals clusters in the gas phase in various physical conditions. They provide important data for modeling planetary atmospheres , interstellar clouds and combustion process

Sujet de thèse en cours : Cinétique de formation d'agrégats de van der Waals et détection de produits de réactions d'atomes de carbone d'intérêt pour la combustion et les environnements astrophysiques et atmosphériques [Soutenance 23.09.2016]

supervisors Ludovic Biennier and Sébastien Le Picard (department of molecular physics)sous la direction de Ludovic Biennier et de Sébastien Le Picard dans le département de physique moléculair

Gas-Phase Positive Ion Insights during Laboratory EUV Irradiation of Titan's Relevant Gas Mixtures

International audienceIn the atmosphere of Saturn's moon Titan, the chemical growth is believed to occur through an efficient chemical coupling between radicals and reac-tive charged species. However, the role of positively charged particles in haze formation chemistry is far from being understood. Here, we present a mass spec-trometry investigation with a relatively low pressure interface to chemically characterize the positive ions formed in the pre-haze formation phase starting from the EUV photolysis of N2 /CH4 gas mixtures

Low-Pressure EUV Photochemical Experiments: Insight on the Ion Chemistry Occurring in Titan's Atmosphere

International audienceThanks to the Cassini spacecraft onboard instruments, it has been known that Titan's ionospheric chemistry is complex and the molecular growth is initiated through the photolysis of the most abundant species directly in the upper atmosphere. Among the pool of chemical compounds formed by the photolysis, N‐bearing species are involved in the haze formation but the chemical incorporation pathways need to be better constrained.In this work, we performed low‐pressure EUV photochemical laboratory experiments. The APSIS reactor was filled with a N2/CH4 (90/10%) gas mixture relevant to the upper atmosphere of Titan. The cell was irradiated by using a EUV photon source at 73.6 nm which has been difficult to produce in the laboratory for previous studies. The photoproducts (both neutral and ionic species) were monitored in situ with a quadrupole mass spectrometer. The chemical pathways are explained by confronting experimental observations and numerical predictions of the photoproducts.The most interesting result in this work is that methanimine was the only stable N‐bearing neutral molecule detected during the experiments and it relies on N2+ production. This experimental result is in agreement with the relatively high abundance predicted by 1D‐photochemical models of Titan;s atmosphere and comforts methanimine as an intermediate towards the formation of complex N‐bearing organic molecules.This experiment is only testing one part of the overall chemical scheme for Titan's upper atmosphere due to the selective wavelength but demonstrates the capability to probe the chemical pathways occurring in Titan's atmosphere by minimizing bias coming from wall surface reactions