Rate constants have been measured for the C(3P) + CH3CN reaction between 50 K and 296 K using a continuous-flow supersonic reactor. C(3P) atoms were created by the in-situ pulsed laser photolysis of CBr4 at 266 nm, while the kinetics of C(3P) atom loss were followed by direct vacuum ultra-violet laser induced fluorescence at 115.8 nm. Secondary measurements of product H(2S) atom formation were also made, allowing absolute H-atom yields to be obtained by comparison with those obtained for the C(3P) + C2H4 reference reaction. In parallel, quantum chemical calculations were performed to obtain the various complexes, adducts and transition states relevant to the title reaction over the triplet potential energy surface, allowing us to better understand the preferred reaction pathways. The reaction is seen to be very fast, with measured rate constants in the range (3-4) x 10-10 cm3 s-1 with little or no observed temperature dependence. As the C + CH3CN reaction is not considered in current astrochemical networks, we test its influence on interstellar methyl cyanide abundances using a gas-grain dense interstellar cloud model. Its inclusion leads to predicted CH3CN abundances that are significantly lower than the observed ones.
We explore the energetics of the titular reaction, which current astrochemical databases consider open at typical dense molecular (i.e., dark) cloud conditions. As is common for reactions involving the transfer of light particles, we assume that there are no intersystem crossings of the potential energy surfaces involved. In the absence of any such crossings, we find that this reaction is endoergic and will be suppressed at dark cloud temperatures. Updating accordingly a generic astrochemical model for dark clouds changes the predicted gas-phase abundances of 224 species by greater than a fact…
The reaction between atomic carbon in its ground electronic state, C(3P), and nitrous oxide, N2O, has been studied below room temperature due to its potential importance for astrochemistry, with both species considered to be present at high abundance levels in a range of interstellar environments. On the experimental side, we measured rate constants for this reaction over the 50-296 K range using a continuous supersonic flow reactor. C(3P) atoms were generated by the pulsed photolysis of carbon tetrabromide at 266 nm and were detected by pulsed laser induced fluorescence at 115.8 nm. Additiona…
We have incorporated our experimentally derived thermal rate coefficients for C + H$_3^+$ forming CH$^+$ and CH$_2^+$ into a commonly used astrochemical model. We find that the Arrhenius-Kooij equation typically used in chemical models does not accurately fit our data and use instead a more versatile fitting formula. At a temperature of 10 K and a density of 10$^4$ cm$^{-3}$, we find no significant differences in the predicted chemical abundances, but at higher temperatures of 50, 100, and 300 K we find up to factor of 2 changes. Additionally, we find that the relatively small error on our the…
Modeling the UV-photon irradiation of CS$_2$-bearing ices in the laboratory with the pyRate gas-grain astrochemical code. New insights into the missing sulfur problem
Max-Planck-Institut für Extraterrestrische Physik (MPE)（马克斯·普朗克地外物理研究所）；Centro de Astrobiología (CSIC-INTA)（天体生物学中心（西班牙国家科学技术研究委员会-国家航空航天技术研究所））
The emission line spectra of cyanoacetylene and methanol reveal chemical and physical heterogeneity on very small (< 0.1 pc) scales toward the peak in cyanopolyyne emission in the Taurus Molecular Cloud, TMC-1 (CP). We generate grids of homogeneous chemical models using a three-phase rate equation approach to obtain all time-dependent abundances spanning the physical conditions determined from molecular tracers of compact and extended regions of emission along this line of sight. Each time-dependent abundance is characterized by one of four features: a maximum/minimum, a monotonic increase/dec…
Positive ions play a fundamental role in the interstellar chemistry, especially in cold environments where chemistry is believed to be mainly ion-driven. However, in contrast with neutral species, most of the cations present in the astrochemical reaction networks are not fully characterized in the astrochemical literature. To fill up this gap, we have carried out new accurate quantum chemical calculations to identify the structures and energies of 262 cations with up to 14 atoms that are postulated to have a role in the interstellar chemistry. Optimised structures and rotational constants were…
This review paper summarizes the state-of-the-art in laboratory based interstellar ice chemistry. The focus is on atom addition reactions, illustrating how water, carbon dioxide and methanol can form in the solid state at astronomically relevant temperatures, and also the formation of more complex species such as hydroxylamine, an important prebiotic molecule, and glycolaldehyde, the smallest sugar, is discussed. These reactions are particularly relevant during the dark ages of star and planet formation, i.e., when the role of UV light is restricted. A quantitative characterization of such pro…
Astrochemical surface reactions are thought to be responsible for the formation of complex organic molecules, which are of potential importance for the origin of life. In a situation, when the chemical composition of dust surfaces is not precisely known, the fundamental knowledge concerning such reactions gains significance. We describe an experimental technique, which can be used to measure the energy released in reactions of a single pair of reactants. These data can be directly compared with the results of quantum chemical computations leading to unequivocal conclusions regarding the reacti…
Our current understanding of interstellar carbon fractionation hinges on the interpretation of astrochemical kinetic models. Yet, the various reactions included carry large uncertainties in their (estimated) rate coefficients, notably those involving C with C$_{2}$. In this work, we provide theoretical thermal rate coefficients as a function of the temperature for all possible gas-phase isotope-exchange reactions of C+C$_{2}(X^{1}Σ_{g}^{+},a^{3}Π_{u})$. For this, we employ the quasi-classical trajectory method, with the previously obtained potential energy surfaces of C$_{3}$ dictating the for…
Among all the interstellar complex organic molecules (iCOMs), acetaldehyde is one of the most widely detected species. The question of its formation route(s) is, therefore, of a major interest regarding astrochemical models. In this paper, we provide an extensive review of the gas-phase formation paths that were, or are, reported in the literature and the major astrochemical databases. Four different gas-phase formation routes stand out : (1) CH$_3$OCH$_3$ + H$^+$ / CH$_3$CHOH$^+$ + e$^-$, (2) C$_2$H$_5$ + O($^3$P), (3) CH$_3$OH + CH and (4) CH$_3$CH$_2$OH + OH / CH$_3$CHOH + O($^3$P). Paths (…
The origin of bistable solutions in the kinetic equations describing the chemistry of dense interstellar clouds is explained as being due to the autocatalysis and feedback of oxygen nuclei from the oxygen dimer (O2). We identify four autocatalytic processes that can operate in dense molecular clouds, driven respectively by reactions of H+, He+, C+, and S+ with O2. We show that these processes can produce the bistable solutions found in previous studies, as well as the dependence on various model parameters such as the helium ionization rate, the sulfur depletion and the {H}3+ electron recombin…
OH radicals play a key role as an intermediate in the water formation chemistry of the interstellar medium. For example the reaction of OH radicals with H$_2$ molecules is among the final steps in the astrochemical reaction network starting from O, O$_2$, and O$_3$. Experimentally it was shown that even at 10 K this reaction occurs on ice surfaces. As the reaction has a high activation energy only atom tunneling can explain such experimental findings. In this study we calculated reaction rate constants for the title reaction on a water-ice I$_h$ surface. To our knowledge, low-temperature rate …
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 i…
Reaction Mechanisms and Kinetics of CN and CCH with H2CS: Implications for Interstellar Sulfur Chemistry
De Vinci Higher Education, De Vinci Research Center（德文奇高等教育中心，德文奇研究中心）；Institut Supérieur des Arts Multimédia de la Manouba, Université de la Manouba（马努巴多媒体艺术学院，马努巴大学）；Faculté des Sciences de Tunis, Laboratoire de Physique de la Matière Condensée, Université Tunis El Manar（突尼斯理学院，凝聚态物质物理实验室，突尼斯埃尔曼纳尔大学）；Observatoire de Paris, Université PSL, CNRS-UMR 8112, LUX（巴黎天文台，巴黎文理研究大学，法国国家科学研究中心联合研究单位8112，LUX）；Department of Physics, College of Khurma University, Taif University（物理系，库尔玛学院，塔伊夫大学）
Grain surface chemistry and its treatment in gas-grain chemical models is an area of large uncertainty. Whilst laboratory experiments are making progress, there is still much that is unknown about grain surface chemistry. Further, the results and parameters produced by experiment are often not easily translated to the rate equation approach most commonly used in astrochemical modelling. It is possible that statistical methods can reduce the uncertainty in grain surface chemical networks. In this work, a simple model of grain surface chemistry in a molecular cloud is developed and a Bayesian in…
The ionization fraction plays a key role in the physics and chemistry of the neutral interstellar medium, from controlling the coupling of the gas to the magnetic field to allowing fast ion-neutral reactions that drive interstellar chemistry. Most estimations of the ionization fraction have relied on deuterated species such as DCO+, whose detection is limited to dense cores representing an extremely small fraction of the volume of the giant molecular clouds they are part of. As large field-of-view hyperspectral maps become available, new tracers may be found. We search for the best observable …
Quantum mechanical tunneling of atoms is increasingly found to play an important role in many chemical transformations. Experimentally, atom-tunneling can be indirectly detected by temperature-independent rate constants at low temperature or by enhanced kinetic isotope effects. On the contrary, using computational investigations the influence of tunneling on the reaction rates can directly be monitored. The tunnel effect, for example, changes reaction paths and branching ratios, enables chemical reactions in an astrochemical environment that would be impossible by thermal transition, and influ…
We propose a new model for treating solid-phase photoprocesses in interstellar ice analogues. In this approach, photoionization and photoexcitation are included in more detail, and the production of electronically-excited (suprathermal) species is explicitly considered. In addition, we have included non-thermal, non-diffusive chemistry to account for the low-temperature characteristic of cold cores. As an initial test of our method, we have simulated two previous experimental studies involving the UV irradiation of pure solid O$_2$. In contrast to previous solid-state astrochemical model calcu…
The feasibility of contemporary gas-grain astrochemical models depends on the availability of accurate kinetics data, in particular, for surface processes. We study the sensitivity of gas-grain chemical models to the energy barrier Ea of the important surface reaction between some of the most abundant species: C and H2 (surface C + surface H2 = surface CH2). We used the gas-grain code ALCHEMIC to model the time-dependent chemical evolution over a 2D grid of densities (nH: 10^{3} - 10^{12}cm^{-3}) and temperatures (T: 10 - 300 K), assuming UV-dark (Av = 20 mag) and partly UV-irradiated (Av = 3 …
We calculated reaction rate constants including atom tunneling of the reaction of dihydrogen with the hydroxy radical down to a temperature of 50 K. Instanton theory and canonical variational theory with microcanonical optimized multidimensional tunneling (CVT/$μ$OMT) were applied using a fitted potential energy surface [J. Chem. Phys. 138, 154301 (2013)]. All possible protium/deuterium isotopologues were considered. Atom tunneling increases at about 250 K (200 K for deuterium transfer). Even at 50 K the rate constants of all isotopologues remain in the interval $ 4 \cdot 10^{-20}$ to $4 \cdot…
The final step of the water formation network on interstellar grain surfaces starting from the H + O$_2$ route is the reaction between H and H$_2$O$_2$. This reaction is known to have a high activation energy and therefore at low temperatures it can only proceed via tunneling. To date, however, no rate constants are available at temperatures below 200 K. In this work, we use instanton theory to compute rate constants for the title reaction with and without isotopic substitutions down to temperatures of 50 K. The calculations are based on density functional theory, with additional benchmarks fo…
Solid state astrochemical reaction pathways have the potential to link the formation of small nitrogen-bearing species, like NH3 and HNCO, and prebiotic molecules, specifically amino acids. To date, the chemical origin of such small nitrogen containing species is still not well understood, despite the fact that ammonia is an abundant constituent of interstellar ices toward young stellar objects and quiescent molecular clouds. This is mainly because of the lack of dedicated laboratory studies. The aim of the present work is to experimentally investigate the formation routes of NH3 and HNCO thro…
The hydrogen abstraction reaction between H and H$_2$S, yielding HS and H$_2$ as products, has been studied within the framework of interstellar surface chemistry. High-temperature rate constants up to 2000 K are calculated in the gas phase and are in agreement with previously reported values. Subsequently low-temperature rate constants down to 55 K are presented for the first time that are of interest to astrochemistry, i.e., covering both bimolecular and unimolecular reaction mechanisms. For this, a so-called implicit surface model is used. Strictly speaking, this is a structural gas-phase m…
Deuterated molecules are important chemical tracers of prestellar and protostellar cores. Up to now, the titular reaction has been assumed to contribute to the generation of these deuterated molecules. We have measured the merged-beams rate coefficient for this reaction as function of the relative collision energy in the range of about 10 meV to 10 eV. By varying the internal temperature of the reacting H$_3^+$ molecules, we found indications for the existence of a reaction barrier. We have performed detailed theoretical calculations for the zero-point-corrected energy profile of the reaction …
We present a combined experimental and theoretical study focussing on the quantum tunneling of atoms in the reaction between CH4 and OH. The importance of this reaction pathway is derived by investigating isotope substituted analogs. Quantitative reaction rates needed for astrochemical models at low temperature are currently unavailable both in the solid state and in the gas phase. Here, we study tunneling effects upon hydrogen abstraction in CH4 + OH by focusing on two reactions: CH4 + OD -> CH3 + HDO and CD4 + OH -> CD3 + HDO. The experimental study shows that the solid-state reaction rate R…
