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Supporting data for Commentary Regarding the CRESU-SIS Experiment: Concerns About the Uniform Supersonic Flow Reactor, 2023 Jupyter Python Notebook -> regressions and plots kinetics traces data, from Mortada 2022 Figure 3 pseudo-first order coefficient data, from Mortada 2022 Figure 3

In this thesis we present the design and the development of two novel instruments to optimize the experimental conditions for laboratory astrophysics applications to study gas phase collisional processes of astrochemical interest at low temperatures using the CPUF (chirped pulse in uniform flow) technique. CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus coupled with two Chirped-Pulse Fourier Transform mm-Wave (CP-FTmmW) spectrometers with the aim of the determination of branching ratios of multi-channel reactions relevant to astrochemistry. Although CP-FTmmW spectroscopy is very well suited for low temperature applications, its sensitivity is challenged by many technical difficulties, the most notorious is pressure broadening, a phenomenon induced by collisions that attenuates the molecular signal of the species studied. A series of spectroscopic measurements with a Ka band spectrometer were carried out at room temperature to assess its impact using two molecules of astrochemical interest, vinyl cyanide and benzonitrile colliding with a helium buffer. The results illustrate the need to optimize the CRESU environment to allow sensitive detection of reaction products. In this work we present the development of the SKISURF (SKImmed uniform SUpeRsonic Flow) apparatus, where the CRESU flow is sampled via a skimmer in a secondary expansion process, into a much lower temperature and pressure environment creating favorable conditions for the detection of the molecular signal. An E band spectrometer is used for the characterization and conducting the first measurements of the branching ratio of the CN+ ethylene reaction at 35 K, the probing has been carried out at a temperature of 5 K. A pulsed Laval nozzle system has been designed, built and characterized as a second technique to increase the sensitivity of CPUF. It relies on pulsing the CRESU flows at a synchronized frequency with the use of an aerodynamic chopper upstream of the Laval nozzle which helps to increase the pumping efficiency and reduce the pressure effect. It also allows to generate a much lower temperature flows than the classic CRESU apparatus down to ~ 6 K. In addition, the system presents an economical advantage as it reduces the amount of gas and species consumed. The design aspects and the characterization using both numerical and experimental methods of the newly developed instruments are discussed.; Dans cette thèse, nous présentons la conception et le développement de deux nouveaux instruments afin d’optimiser les conditions expérimentales pour des applications d'astrophysique de laboratoire. L’objectif est d’étudier les processus collisionnels en phase gazeuse d'intérêt astrochimique à basse température en utilisant la technique CPUF (chirped pulse in uniform flow). Ce travail s'inscrit dans le cadre du projet CRESUCHIRP, un programme pluriannuel visant à construire un nouvel appareil CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) couplé à deux spectromètres CP-FTmmW (Chirped-Pulse Fourier Transform mm-Wave) dans le but de déterminer les rapports de branchement des réactions d’intérêt astrochimique. Bien que la spectroscopie CP-FTmmW soit très bien adaptée aux applications à basse température, son efficacité est affectée par de nombreuses difficultés techniques, la plus notable étant l'élargissement collisionnel, un phénomène induit par les collisions qui atténuent le signal moléculaire des espèces étudiées. Une série de mesures spectroscopiques avec un spectromètre en bande Ka a été réalisée à température ambiante pour évaluer son impact en utilisant deux molécules d'intérêt astrochimique, l’acrylonitrile et le benzonitrile sont mis en collisions avec l’hélium. Les résultats illustrent la nécessité d'optimiser l'environnement du CRESU pour permettre une détection quantitative des produits de réaction. Dans ce travail, nous présentons le développement de l'appareil SKISURF (SKImmed uniform SUpeRsonic Flow), où le flux CRESU est échantillonné via un skimmer dans un processus d'expansion secondaire, ayant lieu dans un environnement à température et pression beaucoup plus basses en créant des conditions favorables pour la détection du signal moléculaire. Un spectromètre en bande E est utilisé pour la caractérisation et la réalisation des premières mesures du rapport de branchement de la réaction CN+ éthylène à 35 K, le sondage a été effectué à une température de ~ 5 K. Un système d’une tuyère de Laval pulsée a été conçu, construit et caractérisé comme deuxième technique pour augmenter la sensibilité du CPUF. Il repose sur la pulsation des jets du CRESU à une fréquence synchronisée à l’aide d'un hacheur aérodynamique placé en amont de la tuyère de Laval ce qui permet d'augmenter l'efficacité du pompage et de réduire l'effet de pression. Il permet également de générer des flux à des températures beaucoup plus basses que celles de l'appareil CRESU classique, jusqu'à ~ 6 K. Le système présente aussi un avantage économique car il réduit la quantité de gaz et d'espèces consommés. Les aspects de la conception et la caractérisation à l'aide de méthodes numériques et expérimentales des instruments nouvellement développés sont évoqués.

International audience; The present chapter highlights the main discoveries and breakthroughs described in the previous chapters of this book that uniform supersonic flows have allowed during the last forty years in the field of physical chemistry. The accumulation of experimental data in the domain of very-low-temperature reaction kinetics has stimulated new developments in theoretical quantum chemistry and astrochemical modeling, some of which will be further emphasized. Although a considerable body of knowledge has been explored during these four decades, we will comment about the many remaining challenges and open questions that could be tackled further using CRESU environments. Exploring these fields should be facilitated in the near future thanks to the significant number of new instrumental diagnostic tools that are in development in several research groups worldwide. © 2022 by World Scientific Publishing Europe Ltd.

Understanding the mechanisms governing the formation of complex organic molecules in the interstellar medium and cold planetary atmospheres relies in particular on the exploration of ionic processes under extreme conditions. The CRESU-SIS experimental setup, developed to study ion-neutral reactions in the gas phase at low temperatures, is able to meet this challenge. This unique instrument, which combines a selective ion source (SIS) with the CRESU reactor which generates a uniform supersonic flow at very low temperature, makes it possible to measure the rate coefficients of the reactions between ions and neutral molecules and to determine the branching ratios between the different exit channels. The reactivity of N2+, Ar+, and H2CN+ ions with hydrocarbons (e.g CH4, C2H4 ...) has been studied experimentally and analyzed using quantum calculations. The reaction between N2+ and the linear isomers of C3H4 (allene and propyne) has been included into a photochemical model for the atmosphere of Titan., La compréhension des mécanismes régissant la formation des molécules organiques complexes dans le milieu interstellaire et les atmosphères planétaires froides repose notamment sur l’exploration de processus ioniques en conditions extrêmes. Le dispositif expérimental CRESU-SIS, développé pour étudier les réactions ion-neutres en phase gazeuse à basse températures, est en mesure de répondre à ce défi. Cet instrument unique, qui combine une source d’ions sélective (SIS) au réacteur CRESU qui génère un écoulement supersonique uniforme de très basse température, permet de mesurer les coefficients de vitesse de réactions entre des ions et des molécules neutres et de déterminer les rapports de branchement entre les différentes voies de sortie. La réactivité des ions N2+, Ar+, et CH2CN+ avec des hydrocarbures (e.g CH4, C2H4…) a été étudiée expérimentalement et analysée à l’aide de calculs quantiques. La réaction entre N2+ et les isomères linéaires de C3H4 (allene et propyne) a été incluse dans un modèle photochimique pour l’atmosphère de Titan.

International audience; An overview is given of collisional energy transfer measurements made in uniform supersonic flows. The fundamentals of molecular collisions and electronic, vibrational, and rotational energy transfer are briefly described, followed by a summary of the collisional systems that have been studied to date. The measurement of total and state-to-state collisional rate constants and cross sections is described and illustrated by reference to case studies for rotational, vibrational, and electronic energy transfer. These experimental data are critical for radiative transfer modeling of astrophysical environments as well as for fundamental tests of theoretical calculations. © 2022 by World Scientific Publishing Europe Ltd.

Nitrogen-bearing molecules, like methylamine (CH3NH2), can be the building blocks of amino acids in the interstellar medium (ISM). At the ultralow temperatures of the ISM, it is important to know its gas-phase reactivity towards interstellar radicals and the products formed. In this work, the kinetics of the OH + CH3NH2 reaction was experimentally and theoretically investigated at low- and high-pressure limits (LPL and HPL) between 10 and 1000 K. Moreover, the CH2NH2 and CH3NH yields were computed in the same temperature range for both pressure regimes. A pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) apparatus was employed to determine the rate coefficient, k(T), in the 11.7–177.5 K range. A drastic increase of k(T) when the temperature is lowered was observed in agreement with theoretical calculations, evaluated by the competitive canonical unified statistical (CCUS) theory, below 300 K in the LPL regime. The same trend was observed in the HPL regime below 350 K, but the theoretical k(T) values were higher than the experimental ones. Above 200 K, the calculated rate coefficients are improved with respect to previous computational studies and are in excellent agreement with the experimental literature data. In the LPL, the formation of CH3NH becomes largely dominant below ca. 100 K. Conversely, in the HPL regime, CH2NH2 is the only product below 100 K, whereas CH3NH becomes dominant at 298 K with a branching ratio similar to the one found in the LPL regime (≈70%). At T > 300 K, both reaction channels are competitive independently of the pressure regime, Las moléculas que contienen nitrógeno, como la metilamina (CH 3 NH 2 ), pueden ser los componentes básicos de los aminoácidos en el medio interestelar (ISM). A las temperaturas ultrabajas del ISM, es importante conocer su reactividad en fase gaseosa hacia los radicales interestelares y los productos formados. En este trabajo se investigó experimental y teóricamente la cinética de la reacción OH + CH 3 NH 2 a límites de baja y alta presión (LPL y HPL) entre 10 y 1000 K. Además, las reacciones CH 2 NH 2 y CH 3Los rendimientos de NH se calcularon en el mismo rango de temperatura para ambos regímenes de presión. Se empleó un aparato CRESU (acrónimo francés de Reaction Kinetics in a Uniform Supersonic Flow) pulsado para determinar el coeficiente de velocidad, k ( T ), en el rango de 11,7–177,5 K. De acuerdo con los cálculos teóricos, evaluados por la teoría de la estadística unificada canónica competitiva (CCUS), se observó un aumento drástico de k ( T ) cuando se baja la temperatura, por debajo de 300 K en el régimen LPL. La misma tendencia se observó en el régimen HPL por debajo de 350 K, pero el k teórico ( T) los valores fueron superiores a los experimentales. Por encima de 200 K, los coeficientes de velocidad calculados mejoran con respecto a estudios computacionales previos y están en excelente acuerdo con los datos de la literatura experimental. En el LPL, la formación de CH 3 NH se vuelve mayoritariamente dominante por debajo de ca. 100 K. Por el contrario, en el régimen HPL, CH 2 NH 2 es el único producto por debajo de 100 K, mientras que CH 3 NH se vuelve dominante a 298 K con una relación de ramificación similar a la encontrada en el régimen LPL (≈70%). A T > 300 K, ambos canales de reacción son competitivos independientemente del régimen de presión.

International audience; The present book deals with more than thirty years of research performed in Chemical Physics with uniform supersonic flows and CRESU-like apparatuses. As such, it is linked to processes occurring at temperature close to absolute zero, studied for their theoretical interest or for their key role in ultracold media. The goal of this first chapter is to introduce a few concepts and equations that are ubiquitous within this context. The leading thread is the question of Local Thermodynamic Equilibrium (LTE) in dilute gases, complete or partial, and how it is implied in kinetic processes (Section 1.2), cold astrophysical media (Section 1.3), laboratory methods (Section 1.4), and the derivation of hydrodynamic equations (Section 1.5). It has been chosen to use the Boltzmann equations to derive the link between microscopic quantities (as state-to-state cross sections) and macroscopic quantities (as rate coefficients). How they are used in the derivation of hydrodynamic equations is also highlighted. The key role of distribution functions for translational or internal energies either in astrophysical media (Section 1.3) or laboratory methods (Section 1.4) is discussed. Note that the concept of temperature is itself subtle and is discussed at length in Section 1.2. © 2022 by World Scientific Publishing Europe Ltd.

This thesis presents research work on the construction and development of a new instrument and its utilization in the framework of the CPUF technique (Chirped Pulse in Uniform Flow) to study low-temperature collisional processes of astrochemical interest. The thesis details the characteristics of the CRESUCHIRP apparatus and the technical development related to its utilization as a CPUF instrument. The new CRESU (Cinétique de Réaction en Écoulement supersonique Uniforme) apparatus and the supporting instrumentation are described. The characterization of an E-band (60-90 GHz) CP-FTmmW spectrometer (Chirped Pulse-Fourier Transform millimeter Wave) is presented with its coupling to the rest of the instrument. The capability of the spectrometer as a spectroscopic tool is presented with the measurement of pressure broadening coefficients at room temperature for the system OCS + He. Experiments using the CPUF technique are presented with the measurement of the difference in collisional interaction of isomers at low temperatures: HCN and HNC with He. Studies of the potential of the instrument for application to kinetics measurement at low temperature are presented for the reactions CN + ethane at 10 K and CN + acetylene at 30 K. Tentative calibrations of these two reactions are presented. The strengths and weaknesses of the new instrument are discussed, as well as potential improvements to the technique.; Cette thèse présente la construction et le développement d'un nouvel instrument et son utilisation via la technique CPUF technique (Chirped Pulse in Uniform Flow) pour étudier des phénomènes collisionnels à basse température d'intérêt astrophysique. La thèse détaille les caractéristique de l’instrument CRESUCHIRP ainsi que les développement technique lié a son utilisation pour la technique CPUF. Le nouveau réacteur CRESU (Cinétique de Réaction en Écoulement supersonique Uniforme) ainsi que les instruments qui y sont liés sont présentés. La caractérisation du nouveau spectromètre CP-FTmmW (Chirped Pulse-Fourier Transform millimeter Wave) en bande E (60-90 GHz) est détaillé ainsi que le couplage avec le reacteur CRESU. Les performances du spectromètre en tant qu'instrument spectrocopique sont présentées avec la mesure de l'élargissement collisionnel à température ambiante pour le système OCS + He. Les expériences utilisant la technique CPUF sont présentées avec la mesure de l'évolution de l'élargissement collisionnel des systèmes HCN + He et HNC + He en fonction de la température (10-70 K). Une étude du potentiel de l'instrument pour son application sur des mesures de cinétique à basse température est présentée pour les reactions entre le radical CN et l' éthane à 10 K et l'acétylène à 30 K. Une tentative de calibration des produits est détaillée. Les forces et faiblesses du nouvel instrument sont discutés ainsi que de potentielles améliorations à la technique.

International audience; The rate coefficient, k(T), for the gas-phase reaction between OH radicals and acetone CH3C(O)CH3, has been measured using the pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique (T = 11.7-64.4 K). The temperature dependence of k(T = 10-300 K) has also been computed using a RRKM-Master equation analysis after partial revision of the potential energy surface. In agreement with previous studies we found that the reaction proceeds via initial formation of two prereactive complexes both leading to H2O + CH3C(O)CH2 by H-abstraction tunneling. The experimental k(T) was found to increase as temperature was lowered. The measured values have been found to be several orders of magnitude higher than k(300 K). This trend is reproduced by calculations, with an especially good agreement with experiments below 25 K. The effect of total gas density on k(T) has been explored. Experimentally, no pressure dependence of k(20 K) and k(64 K) was observed, while k(50 K) at the largest gas density 4.47 × 1017 cm-3 is twice higher than the average values found at lower densities. The computed k(T) is also reported for 103 cm-3 of He (representative of the interstellar medium). The predicted rate coefficients at 10 K surround the experimental value which appears to be very close to that of the low pressure regime prevailing in the interstellar medium. For gas-phase model chemistry of interstellar molecular clouds, we suggest using the calculated value of 1.8 × 10-10 cm3 molecule-1 s-1 at 10 K, and the reaction products are water and CH3C(O)CH2 radicals. © 2019 American Chemical Society.

International audience; A small dimension Laval nozzle connected to a compact high enthalpy source equipped with cavity ringdown spectroscopy (CRDS) is used to produce vibrationally hot and rotationally cold high-resolution infrared spectra of polyatomic molecules in the 1.67 mu m region. The Laval nozzle was machined in isostatic graphite, which is capable of withstanding high stagnation temperatures. It is characterized by a throat diameter of 2 mm and an exit diameter of 24 mm. It was designed to operate with argon heated up to 2000 K and to produce a quasi-unidirectional flow to reduce the Doppler effect responsible for line broadening. The hypersonic flow was characterized using computational fluid dynamics simulations, Pitot measurements, and CRDS. A Mach number evolving from 10 at the nozzle exit up to 18.3 before the occurrence of a first oblique shock wave was measured. Two different gases, carbon monoxide (CO) and methane (CH4), were used as test molecules. Vibrational (T-vib) and rotational (T-rot) temperatures were extracted from the recorded infrared spectrum, leading to T-vib = 1346 +/- 52 K and T-rot = 12 +/- 1 K for CO. A rotational temperature of 30 +/- 3 K was measured for CH4, while two vibrational temperatures were necessary to reproduce the observed intensities. The population distribution between vibrational polyads was correctly described with TvibI=894 +/- 47K, while the population distribution within a given polyad (namely, the dyad or the pentad) was modeled correctly by TvibII=54 +/- 4K, testifying to a more rapid vibrational relaxation between the vibrational energy levels constituting a polyad.

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International audience; The role of water vapor (H2O) and its hydrogen-bonded complexes in the gas-phase reactivity of organic compounds with hydroxyl (OH) radicals has been the subject of many recent studies. Contradictory effects have been reported at temperatures between 200 and 400 K. For the OH + acetaldehyde reaction, a slight catalytic effect of H2O was previously reported at temperatures between 60 and 118 K. In this work, we used Laval nozzle expansions to reinvestigate the impact of H2O on the OH-reactivity with acetaldehyde between 21.7 and 135.0 K. The results of this comprehensive study demonstrate that water, instead, slows down the reaction by factors of ∼3 (21.7 K) and ∼2 (36.2-89.5 K), and almost no effect of added H2O was observed at 135.0 K. © 2021 Author(s).

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The low temperature reaction between CN and benzene (C$_6$H$_6$) is of significant interest in the astrochemical community due to the recent detection of benzonitrile, the first aromatic molecule identified in the interstellar medium (ISM) using radio astronomy. Benzonitrile is suggested to be a low temperature proxy for benzene, one of the simplest aromatic molecules, which may be a precursor to polycyclic aromatic hydrocarbons (PAHs). In order to assess the robustness of benzonitrile as a proxy for benzene, low temperature kinetics measurements are required to confirm whether the reaction remains rapid at the low gas temperatures found in cold dense clouds. Here, we study the C$_6$H$_6$ + CN reaction in the temperature range 15--295 K, using the well-established CRESU technique (a French acronym standing for Reaction Kinetics in Uniform Supersonic Flow) combined with Pulsed Laser Photolysis-Laser-Induced Fluorescence (PLP-LIF). We obtain rate coefficients, $k(T)$, in the range (3.6--5.4) $\times$ 10$^{-10}$ cm$^3$ s$^{-1}$ with no obvious temperature dependence between 15--295 K, confirming that the CN + C$_6$H$_6$ reaction remains rapid at temperatures relevant to the cold ISM.

International audience; Product channel-specific reaction kinetics at low temperature presents a significant challenge for both experiment and theory but provides essential inputs for astrochemical models of cold interstellar environments. Reaction kinetics studies using chirped pulse Fourier transform microwave (CP-FTMW) spectroscopy provide a potential solution but require the use of a buffer gas to thermalize the reactants and products; however, collisions with the buffer gas reduce the length of the detected signal, the free induction decay, through pressure broadening. The effect of this on time domain signals is largely unexplored using CP-FTMW spectrometers. A new E-band (60–90 GHz) spectrometer has been constructed using previously unavailable equipment in order to maximize its performance for detecting molecules in collisional environments. The design of this spectrometer is described in detail. The pressure broadening of OCS in He was used to test the spectrometer under collisional conditions similar to those that are routinely used to perform reaction kinetics measurements. The corresponding pressure broadening coefficients for transitions in this frequency range were determined at room temperature and the performance of the spectrometer was assessed in relation to recently reported chirped pulse in uniform flow experiments. © 2020

International audience; CN is known for its fast reactions with hydrocarbons at low temperatures, but relatively few studies have focused on the reactions between CN and aromatic molecules. The recent detection of benzonitrile in the interstellar medium, believed to be produced by the reaction of CN and benzene, has ignited interest in studying these reactions. Her; we report rate constants of the CN + toluene (C7H8) reaction between 15 and 294 K using a CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme; reaction kinetics in uniform supersonic flow) apparatus coupled with the pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) technique. We also present the stationary points on the potential energy surface of this reaction to study the available reaction pathways. We find the rate constant does not change over this temperature range, with an average value of (4.1 +/- 0.2) x 10(-10) cm(3) s(-1), which is notably faster than the only previous measurement at 105 K. While the reason for this disagreement is unknown, we discuss the possibility that it is related to enhanced multiphoton effects in the previous work.

The gas phase reactivity of the CN radical in the presence of dimethylamine (CH3)2NH and trimethylamine (CH3)3N has been investigated experimentally using a uniform supersonic expansion CRESU react...

International audience; We review progress, since publication of the book "Collisional effects on molecular spectra Laboratory experiments and models, consequences for applications" (Elsevier, Amsterdam, 2008), on measuring, modeling and predicting the influence of pressure (ie of intermolecular collisions) on the spectra of gas molecules. We first introduce recently developed experimental techniques of high accuracy and sensitivity. We then complement the aforementioned book by presenting the theoretical approaches, results and data proposed (mostly) in the last decade on the topics of isolated line shapes, line-broadening and-shifting, line-mixing, the far wings and associated continua, and collision-induced absorption. Examples of recently demonstrated consequences of the progress in the description of spectral shapes for some practical applications (metrology, probing of gas media, climate predictions) are then given. Remaining issues and directions for future research are finally discussed.

International audience; The reaction between cyano radicals (which are ubiquitous in interstellar clouds) and methylamine (a molecule detected in various interstellar sources) has been investigated in a synergistic experimental and theoretical study. The reaction has been found to be very fast in the entire range of temperatures investigated (23-297 K) by using a CRESU apparatus coupled to pulsed laser photolysis-laser induced fluorescence. The global experimental rate coefficient is given by In addition, dedicated electronic structure calculations of the underlying potential energy surface have been performed, together with capture theory and RRKM calculations. The experimental data have been interpreted in the light of the theoretical calculations and the product branching ratio has been established. According to the present study, in the range of temperatures investigated the title reaction is an efficient interstellar route of formation of cyanamide, NH2CN, another interstellar species. The second most important channel is the one leading to methyl cyanamide, CH3NHCN (an isomer of aminoacetonitrile), via a CN/H exchange mechanism with a yield of 12% of the global reaction in the entire range of temperatures explored. For a possible inclusion in future astrochemical models we suggest, by referring to the usual expression the following values α = 3.68 × 10-12 cm3 molec-1 s-1, β = -1.80, γ = 7.79 K for the channel leading to NH2CN + CH3; α = 5.05 × 10-13 cm3 molec-1 s-1, β = -1.82, γ = 7.93 K for the channel leading to CH3NHCN + H. © 2018 the Owner Societies.

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International audience; The interstellar medium is host to a rich chemistry that produces a number of complex organic molecules (COMs) i.e. organic molecules with six or more atoms. A large number of COMs have been observed in astrophysical regions, even in very cold (

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International audience; The reactivity of methanol (CH3OH) toward the hydroxyl (OH) radical was investigated in the temperature range 11.7-177.5 K using the CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique. In the present study, the temperature dependence of the rate coefficient for the OH + CH3OH reaction, k(T), has been revisited and additional experimental and computational data are reported. New kinetic measurements were performed to fill the existing gaps (andlt;22 K, 22-42 K and 88-123 K), reporting k(T andlt; 20 K) for the first time. The lowest temperature ever achieved by a pulsed CRESU has been obtained in this work (11.7 K). k(T) abruptly increases by almost 2 orders of magnitude from 177.5 K to around 100 K. At T andlt; 100 K, this increase is less pronounced, reaching the capture limit at temperatures below 22 K. The pressure dependence of k(T) has been investigated for selected temperatures and gas densities (1.5 × 10 16 to 4.3 × 10 17 cm -3 ), combining our results with those previously reported. No dependence was observed within the experimental uncertainties below 110 K. The high- and low-pressure rate coefficients, k HPL (T) and k LPL (T), were also studied in detail using high-level quantum chemical and theoretical kinetic methodologies, closely reproducing the experimental data between 20 and 400 K. The results suggest that the experimental data are near the high pressure limit at the lowest temperatures, but that the reaction remains a fast and effective source of CH2OH and CH3O at the low pressures and temperatures prevalent in the interstellar medium. © 2019 the Owner Societies.

The last 50 years have been characterized by the fast development of astrochemistry as a science. To date, more than 150 gas-phase neutral-neutral reactions have been investigated at low temperatures relevant to planetary atmospheres and in cold regions of the interstellar medium. However, the rate constants and nature of the products for many potentially important gas-phase processes remain unknown. We performed kinetic studies of reactive and non-reactive removal processes between electronically excited atoms [C(1D), O(1D) and N(2D)] with several molecules in order to quantify their importance in the chemistry of planetary atmospheres. Furthermore, we also investigated the reaction between carbon atoms in their ground electronic state (3P) with water, providing new evidence of a quantum mechanical tunnelling mechanism at low temperatures, which could play an important role in the chemistry of interstellar clouds. Rate constants and branching ratios for these processes were determined over the 50 - 296 K temperature range using a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus coupled with pulsed laser photolysis (PLP) and vacuum ultraviolet laser induced fluorescence (VUV LIF).; Les 50 dernières années ont été caractérisées par le développement rapide de l’astrochimie. Plus de 150 réactions entre espèces neutres ont déjà été étudiées aux basses températures qui sont celles du le milieu interstellaire et des atmosphères planétaires. Néanmoins, les constantes de vitesse, et la nature des produits, restent inconnus pour de nombreuses réactions potentiellement importantes pour caractériser ces milieux. Nous avons effectué des études cinétiques pour des processus réactifs, et non réactifs, entre des atomes dans un état électronique excité [C(1D), O(1D) et N(2D)] et plusieurs molécules stables afin de quantifier leur importance dans la chimie des atmosphères planétaires. Nous avons aussi étudié la réaction entre les atomes de carbone dans leur état électronique fondamental (3P) et l’eau, confirmant l’importance, pour certaines réactions avec barrière, de l’effet tunnel pour la réactivité à basse température. Les constantes de vitesse et les rapports de branchement pour ces processus ont été déterminés dans la gamme de température entre 50 et 296 K en utilisant un appareil CRESU, les atomes étudiés ont été produits par photolyse à l’aide d’un laser pulsé (PLP) et détectés par fluorescence induite dans l’ultraviolet sous vide (VUV LIF).

The High Enthalpy Source (HES) is a novel high temperature source developed to measure infrared line-by-line integrated absorption cross sections of flowing gases up to 2000 K. The HES relies on a porous graphite furnace designed to uniformly heat a constant flow of gas. The flow compensates thermal dissociation by renewing continuously the gas sample and eliminating dissociation products. The flowing characteristics have been investigated using computational fluid dynamics simulation confirming good temperature uniformity. The HES has been coupled to a high-resolution Fourier transform spectrometer to record emission spectra of methane at temperatures ranging between 700 and 1400 K. A radiative model has been developed to extract absolute line intensities from the recorded spectra., SCOPUS: ar.j, info:eu-repo/semantics/published

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Electron attachment to phosphoryl chloride (phosphorus oxychloride) POCl 3 has been studied in the gas phase by mass spectrometry at several low temperatures (47.7, 74.5, 169.7 and 199.5 K) with the CRESU method. By measuring over this temperature range and data from [8] , we have demonstrated the inversion of the branching ratio between the exothermic non-dissociative exit channel POCl 3 − and the thermo-neutral dissociative exit channel POCl 2 − + Cl. A kinetic model in terms of statistical theory is used to fit the experimental data.

International audience; The gas phase reaction between the CN radical and acetonitrile CH3CN was investigated experimentally, at low temperatures, with the CRESU apparatus and a slow flow reactor to explore the temperature dependence of its rate coefficient from 354 K down to 23 K. Whereas a standard Arrhenius behavior was found at T > 200 K, indicating the presence of an activation barrier, a dramatic increase in the rate coefficient by a factor of 130 was observed when the temperature was decreased from 168 to 123 K. The reaction was found to be pressure independent at 297 K unlike the experiments carried out at 52 and 132 K. The work was complemented by ab initio transition state theory based master equation calculations using reaction pathways investigated with highly accurate thermochemical protocols. The role of collisional stabilization of a CN⋯CH3CN van der Waals complex and of tunneling induced H atom abstractions were also considered. The experimental pressure dependence at 52 and 132 K is well reproduced by the theoretical calculations provided that an anharmonic state density is considered for the van der Waals complex CH3CN⋯CN and its Lennard-Jones radius is adjusted. Furthermore, these calculations indicate that the experimental observations correspond to the fall-off regime and that tunneling remains small in the low-pressure regime. Hence, the studied reaction is essentially an association process at very low temperature. Implications for the chemistry of interstellar clouds and Titan are discussed.

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