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

Characterization of a DC glow discharge in N2-H2 with electrical measurements and neutral and ion mass spectrometry

The addition of small amounts of H2 were investigated in a DC glow discharge in N2, at low pressure (~1 mbar) and low power (0.05 to 0.2 W.cm-3). We quantified the electric field, the electron density, the ammonia production and the formation of positive ions for amounts of H2 varying between 0 and 5%, pressure values between 0.5 and 4 mbar, and currents between 10 and 40 mA. The addition of less than 1% H2 has a strong effect on the N2 plasma discharges. Hydrogen quenches the (higher) vibrational levels of N2 and some of its highly energetic metastable states. This leads to the increase of the discharge electric field and consequently of the average electron energy. As a result, higher quantities of radical and excited species are suspected to be produced. The addition of hydrogen also leads to the formation of new species. In particular, ammonia and hydrogen-bearing ions have been observed: N2H+ and NH4+ being the major ones, and also H3+, NH+, NH2+, NH3+, N3H+ and N3H3+. The comparison to a radiofrequency capacitively coupled plasma (RF CCP) discharge in similar experimental conditions shows that both discharges led to similar observations. The study of N2-H2 discharges in the laboratory in the adequate ionization conditions then gives some insights on which plasma species made of nitrogen and hydrogen could be present in the ionosphere of Titan. Here, we identified some protonated ions, which are reactive species that could participate to the erosion of organic aerosols on Titan.Comment: Paper accepted in Plasma Sources Science and Technology in March 2023. The current version on arXiv is the submitted versio

Gaseous chemistry for a Titan's atmospheric plasma experimental simulation

We present the first study of gaseous composition monitoring for the PAMPRE experiment, which simulates Titan's atmospheric chemistry by radio-frequency N 2-CH 4 plasma. Methane consumption is quantified for various N 2-CH 4 gas mixtures. Moreover in situ mass spectrometry (MS) and ex-situ gas chromatography coupled with mass spectrometry (GC-MS) analyses reveal a large dominance of nitrile species in the gas phase chemistry

N2-H2 capacitively coupled radio-frequency discharges at low pressure: II. Modeling results: The relevance of plasma-surface interaction

In this work, we present the results of simulations carried out for N2-H2 capacitively coupled radio-frequency discharges, running at low pressure (0.3-0.9 mbar), low power (5-20 W), and for amounts of H2 up to 5%. Simulations are performed using a hybrid code that couples a two-dimensional time-dependent fluid module, describing the dynamics of the charged particles in the discharge, to a zero-dimensional kinetic module, that solves the Boltzmann equation and describes the production and destruction of neutral species. The model accounts for the production of several vibrationally and electronic excited states, and contains a detailed surface chemistry that includes recombination processes and the production of NH x molecules. The results obtained highlight the relevance of the interactions between plasma and surface, given the role of the secondary electron emission in the electrical parameters of the discharge and the critical importance of the surface production of ammonia to the neutral and ionic chemistry of the discharge.The Portuguese Foundation sponsored this research for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2019

N2-H2 capacitively coupled radio-frequency discharges at low pressure. Part I. Experimental results: Effect of the H2 amount on electrons, positive ions and ammonia formation

The mixing of N2 with H2 leads to very different plasmas from pure N2 and H2 plasma discharges. Numerous issues are therefore raised involving the processes leading to ammonia (NH3) formation. The aim of this work is to better characterize capacitively-coupled radiofrequency plasma discharges in N2 with few percents of H2 (up to 5%), at low pressure (0.3-1 mbar) and low coupled power (3-13 W). Both experimental measurements and numerical simulations are performed. For clarity, we separated the results in two complementary parts. The actual one (first part), presents the details on the experimental measurements, while the second focuses on the simulation, a hybrid model combining a 2D fluid module and a 0D kinetic module. Electron density is measured by a resonant cavity method. It varies from 0.4 to 5 109 cm-3, corresponding to ionization degrees from 2 10-8 to 4 10-7. Ammonia density is quantified by combining IR absorption and mass spectrometry. It increases linearly with the amount of H2 (up to 3 1013 cm-3 at 5% H2). On the contrary, it is constant with pressure, which suggests the dominance of surface processes on the formation of ammonia. Positive ions are measured by mass spectrometry. Nitrogen-bearing ions are hydrogenated by the injection of H2, N2H+ being the major ion as soon as the amount of H2 is >1%. The increase of pressure leads to an increase of secondary ions formed by ion/radical-neutral collisions (ex: N2H+, NH4 +, H3 +), while an increase of the coupled power favours ions formed by direct ionization (ex: N2 +, NH3 +, H2 +).N. Carrasco acknowledges the financial support of the European Research Council (ERC Starting Grant PRIMCHEM, Grant agreement no. 636829). A. Chatain acknowledges ENS Paris-Saclay Doctoral Program. A. Chatain is grateful to Gilles Cartry and Thomas Gautier for fruitful discussions on the MS calibration. L.L. Alves acknowledges the financial support of the Portuguese Foundation for Science and Technology (FCT) through the project UID/FIS/50010/2019. L. Marques and M. J. Redondo acknowledge the financial support of the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UIDB/04650/2019

Simulation de la Physico chimie de l'atmosphère de Titan par plasma Radio Fréquence

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Production d'équivalents d'aérosols de l'atmosphère de Titan par plasma radio fréquence

L'expérience pampre permet de produire des équivalents des aérosols de l'atmosphère de Titan grâce à un plasma radio fréquence à basse pression fonctionnant dans un mélange N2-CH4 représentatif de l'atmosphere Titan fonctionnant en mode permanent ou en mode pulsé. Le premier objectif de cette thèse est d'étudier le plasma. Les mesures de densité éléctronique par cavité micor onde montrent que celle-ci est plus faible dans les mélanges N2-CH4 que dans l'azote pur. Par spectroscopie optique d'émission nous montrons que cette diminution de densité est compensée par une augmentation de l'énergie des électrons. Le second objectif est une étude de la croissance des aérosols. Des observations au meb mettent en évidence l'influence des paramètres du plasma sur la taille des aérosols : proportion de CH4, débit de gaz, durée des impulsions. En régime permanent, les aérosols ont des diamètres compris entre 300NM et 1 M. Avec des impulsions de plasma de 10S les aérosols ont un diamètre moyen de 90 NM représentatif de ceux de Titan. Le mode pulsé montre que le temps de formation des aérosols est de quelques dizaines de secondes. Ce temps est réduit à pression élevée ou à faible concentration de CH4. Enfin, par mesure de polarisation de la lumière d'un laser à 532NM diffusée par les aérosols, nous étudions leur croissance in situ et en temps réel. Par comparaison des variations du degré de polarisation mesurées avec un code de diffusion de mie, nous mesurons l'évolution au cours du temps de la taille moyenne des aérosols ainsi que leur distribution en taille. Les vitesses de croissance du rayon des aérosols sont de l'ordre de 4NM/SThe experiment prampre produces equivalents of aerosols of Titan's atmosphere thanks to a low pressure radio frequency plasma in a N2-CH4 gaseous mixture, representative of Titan's atmosphere, working in continuous or pulsed mode. The first goal of this work is to study the plasma. Electron density measurements, by microwave resonant cavity method, show this latter is lower in N2-CH4 mixtures than in pure N2. By optical emission spectroscopy, we show that this decrease in electron density is compensated by an increase in their energy. The second goal is a study of aerosols growth. Sem observations point out the influence of plasma parameters on the aerosols mean diameter between 300NM and 1 M. With 10S pulses, it is about 90NM, representative of Titan's aerosols. The pulsed mode shows that the formation time of aerosols is a few tens of seconds. This time is reduced at high pressure or at low amounts of CH4. At last, by measuring the polarization degree of the light of a 532NM laser beam scattered by aerosols, we can study their growth in situ and in real time. By comparing the measured polarization degree variations withe a mie-scattering model, we measure the evolution of aerosols mean radius and size distribution withe time. The obtained growth rates in radius of aerosols are about 4NM/SVERSAILLES-BU Sciences et IUT (786462101) / SudocSudocFranceF

Experimental study of the temporal evolution of N2(C3?u) and N2(B3?g) in a nitrogen pulsed discharge

The N2 (C 3IIu) and N2 (B 3IIg) triplet states are studied in the positive column of a nitrogen dc pulsed discharge by optical emission spectroscopy. Time variations of all vibrational levels are observed during a 700 µs pulse duration in a pressure range from 0.05 to 4 Torr. Even if the current remains constant during the pulse, the bands intensities emitted from the triplet states change during the pulse. According to the state and the vibrational level, we observe an increase or a decrease of line intensities as a function of pressure. It is pointed out that for N2 (B 3IIg) time variations are not similar for low and high vibrational levels. From the observation of individual vibrational levels, time evolution of vibrational distributions (VDs) are measured for N2 (B 3IIg) and N2 (C 3IIu). From these VDs a vibrational temperature is deduced. For a 4 Torr pressure, the deduced vibrational temperature increases from 5000 to 9000 K for N2 (B 3IIg) and from 3000 to 9000 K for N2 (C 3IIu). This vibrational temperature is representative of the temporal evolution of the plasma and, even after 700 µs, a steady state is not reached

Atomic oxygen recombination on fused silica: modelling and comparison to low-temperature experiments (300 K)

International audienceThis work is devoted to the study of atomic oxygen recombination on a glass surface, mainly in connection with atomic sources development. In this paper we present a non-stationary model for atomic oxygen recombination on a fused silica surface. Kinetics equations for oxygen atoms, taking into account heterogeneous reactions between gaseous atoms and the surface (Eley-Rideal mechanisms), as well as homogeneous processes involving surface migration of adsorbed species (Langmuir-Hinshelwood mechanisms), are solved. Surface reaction coefficients are calculated, and the choice of numerical values for surface parameters is discussed. The solution to the equations is compared to our previous experiments concerning the influence of the surface state on atomic recombination. An estimation is made of surface reaction coefficient values

Titan-like reactors to simulate globally the chemistry in Titan's atmosphere

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