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Methane (CH4) is one of the major components of the icy mantle of cosmic dust prevalent in cold, dense regions of interstellar media, playing an important role in the synthesis of complex organic molecules and prebiotic molecules. Solid CH4 is considered to be formed via the successive hydrogenation of C atoms accreting onto dust: C + 4H → CH4. However, most astrochemical models assume this reaction on the ice mantles of dust to be barrierless and efficient, without considering the states of adsorption. Recently, we found that C atoms exist in either the physisorbed or chemisorbed state on compact amorphous solid water, which is analogous to an interstellar ice mantle. These distinct adsorption states considerably affect the hydrogenation reactivity of the C atom. Herein, we elucidate the reactivities of physisorbed and chemisorbed C atoms with H atoms via sequential deposition and codeposition processes. The results indicate that only physisorbed C atoms can produce CH4 on ice. Combining this finding with a previous estimate for the fraction of physisorbed C atoms on ice, we determined the upper limit for the conversion of C atoms into CH4 to be 30%.

We report the first detection in the interstellar medium of $N$-cyanomethanimine (H$_2$CNCN), the stable dimer of HCN of highest energy, and the most complex organic molecule identified in space containing the prebiotically relevant NCN backbone. We have identified a plethora of $a$-type rotational transitions with 3 $\leq J_\text{up} \leq$ 11 and $K_\text{a} \leq$ 2 that belong to this species towards the Galactic Center G+0.693-0.027 molecular cloud, the only interstellar source showing the three cyanomethanimine isomers (including the $Z$- and $E$- isomers of $C$-cyanomethanimine, HNCHCN). We have derived a total column density for H$_2$CNCN of (2.9$\, \pm \,$0.1)$\times$10$^{12}$ cm$^{-2}$, which translates into a total molecular abundance with respect to H$_2$ of (2.1$\, \pm \,$0.3)$\times$10$^{-11}$. We have also revisited the previous detection of $E$- and $Z$-HNCHCN, and found a total $C/N$-cyanomethanimine abundance ratio of 31.8$\, \pm \,$1.8 and a $Z/E$-HNCHCN ratio of 4.5$\, \pm \,$0.2. While the latter can be explained on the basis of thermodynamic equilibrium, chemical kinetics are more likely responsible for the observed $C/N$-cyanomethanimine abundance ratio, where the gas-phase reaction between methanimine (CH$_2$NH) and the cyanogen radical (CN) arises as the primary formation route., Comment: 22 pages, 8 figures, 3 tables, 2 appendix - Accepted for publication in The Astrophysical Journal

Many interstellar complex organic molecules (COMs) are believed to be produced on the surfaces of icy grains at low temperatures. Atomic carbon is considered responsible for the skeletal evolution processes, such as C–C bond formation, via insertion or addition reactions. Before reactions, C atoms must diffuse on the surface to encounter reaction partners; therefore, information on their diffusion process is critically important for evaluating the role of C atoms in the formation of COMs. In situ detection of C atoms on ice was achieved by a combination of photostimulated desorption and resonance-enhanced multiphoton ionization methods. We found that C atoms weakly bound to the ice surface diffused above approximately 30 K and produced C2 molecules. The activation energy for C-atom surface diffusion was experimentally determined to be 88 meV (1,020 K), indicating that the diffusive reaction of C atoms is activated at approximately 22 K on interstellar ice. The facile diffusion of C atoms at temperatures above 22 K on interstellar ice opens a previously overlooked chemical regime where the increase in complexity of COMs is driven by C atoms. Carbon addition chemistry can be an alternative source of chemical complexity in translucent clouds and protoplanetary disks with crucial implications in our current understanding on the origin and evolution of organic chemistry in our Universe.

Many interstellar complex organic molecules (COMs) are believed to be produced on the surfaces of icy grains at low temperatures. Atomic carbon is considered responsible for the skeletal evolution processes, such as C-C bond formation, via insertion or addition reactions. Before reactions, C atoms must diffuse on the surface to encounter reaction partners; therefore, information on their diffusion process is critically important for evaluating the role of C atoms in the formation of COMs. In situ detection of C atoms on ice was achieved by a combination of photostimulated desorption and resonance enhanced multiphoton ionization methods. We found that C atoms weakly bound to the ice surface diffused approximately above 30 K and produced C2 molecules. The activation energy for C-atom surface diffusion was experimentally determined to be 88 meV (1,020 K), indicating that the diffusive reaction of C atoms is activated at approximately 22 K on interstellar ice. The facile diffusion of C at T > 22 K atoms on interstellar ice opens a previously overlooked chemical regime where the increase in complexity of COMs as driven by C atoms. Carbon addition chemistry can be an alternative source of chemical complexity in translucent clouds and protoplanetary disks with crucial implications in our current understanding on the origin and evolution of organic chemistry in our Universe., Comment: 33 pages (main + SI), 14 figures, 1 table

CO2 is one of the dominant components of the interstellar ice. Recent observations show CO2 exists more abundantly in polar (H2O-dominated) ice than in apolar (H2O-poor) ice. CO2 ice formation is primarily attributed to the reaction between CO and OH, which has a barrier. Highly accurate quantum chemical calculations were employed to analyze the stationary points of the potential energy surfaces of the title reaction in the gas phase on a H2O and CO clusters. Microcanonical transition state theory was used as a diagnostic tool for the efficiency of the reaction under ISM conditions. We simulate the kinetics of ice chemistry, considering different scenarios involving non-thermal processes and energy dissipation. The CO + OH reaction proceeds through the remarkably stable intermediate HOCO radical. On the H2O cluster, the formation of this intermediate is efficient, but the subsequent reaction leading to CO2 formation is not. Conversely, HOCO formation on the CO cluster is inefficient without external energy input. Thus, CO2 ice cannot be formed by the title reaction alone either on the H2O cluster or CO cluster. In the polar ice, CO2 ice formation is possible via CO + OH -> HOCO, followed by HOCO + H ->CO2 + H2, as demonstrated by abundant experimental literature. In apolar ice, CO2 formation is less efficient because HOCO formation requires external energy. Our finding is consistent with the JWST observations. Further experimental work is encouraged using low-temperature OH radicals., Comment: Accepted for publication in Astronomy and Astrophysics

Energy redistribution after a chemical reaction is one of the few mechanisms to explain the diffusion and desorption of molecules which require more energy than the thermal energy available in quiescent molecular clouds (10 K). This energy distribution can be important in phosphorous hydrides, elusive yet fundamental molecules for interstellar prebiotic chemistry. We studied the reaction dynamics of the \ce{P + H -> PH} reaction on amorphous solid water, a reaction of astrophysical interest, using \emph{ab-initio} molecular dynamics with atomic forces evaluated by a neural network interatomic potential. We found that the exact nature of the initial phosphorous binding sites is less relevant for the energy dissipation process because the nascent PH molecule rapidly migrates to sites with higher binding energy after the reaction. Non-thermal diffusion and desorption-after-reaction were observed and occurred early in the dynamics, essentially decoupled from the dissipation of the chemical reaction energy. From an extensive sampling of reactions on sites, we constrained the average dissipated reaction energy within the simulation time (50 ps) to be between 50 and 70 %. Most importantly, the fraction of translational energy acquired by the formed molecule was found to be mostly between 1 and 5 %. Including these values, specifically for the test cases of 2% and 5% of translational energy conversion, in astrochemical models, reveals very low gas-phase abundances of PH$_{x}$ molecules and reflects that considering binding energy distributions is paramount for correctly merging microscopic and macroscopic modelling of non-thermal surface astrochemical processes. Finally, we found that PD molecules dissipate more of the reaction energy. This effect can be relevant for the deuterium fractionation and preferential distillation of molecules in the interstellar medium., Comment: Accepted for publication in Astronomy and Astrophysics

Hydroxylamine, NH2OH, is one of the already detected interstellar molecules with the highest prebiotic potential. Yet, the abundance of this molecule found by astronomical observations is rather low for a relatively simple molecule, ∼10-10 relative to H2. This seemingly low abundance can be rationalized by destruction routes operating on interstellar dust grains. In this work, we tested the viability of this hypothesis under several prisms, finding that the origin of a lower abundance of NH2OH can be explained by two chemical processes, one operating at low temperature (10 K) and the other at intermediate temperature (20 K). At low temperatures, enabling the hydrogen abstraction reaction HNO + H → NO + H2, even in small amounts, partially inhibits the formation of NH2OH through successive hydrogenation of NO, and reduces its abundance on the grains. We found that enabling a 15-30 per cent of binding sites for this reaction results in reductions of NH2OH abundance of approximately one to two orders of magnitude. At warmer temperatures (20 K, in our study), the reaction NH2OH + H → HNOH + H2, which was found to be fast (k ∼106 s-1) in this work, followed by further abstractions by adsorbates that are immobile at 10 K (O, N) are the main route of NH2OH destruction. Our results shed light on the abundance of hydroxylamine in space and pave the way to constraining the subsequent chemistry experienced by this molecule and its derivatives in the interstellar prebiotic chemistry canvas.

The abundance ratios between isomers of a COM observed in the ISM provides valuable information about the chemistry and physics of the gas and eventually, the history of molecular clouds. In this context, the origin of an abundance of c-HCOOH acid of only 6% the isomer c-HCOOH abundance in cold cores, remains unknown. Herein, we explain the presence of c-HCOOH in dark molecular clouds through the destruction and back formation of c-HCOOH and t-HCOOH in a cyclic process that involves HCOOH and highly abundant molecules such as HCO+ and NH3. We use high-level ab initio methods to compute the potential energy profiles for the cyclic destruction/formation routes of c-HCOOH and t-HCOOH. Global rate constants and branching ratios were calculated based on the transition state theory and the master equation formalism under the typical conditions of the ISM. The destruction of HCOOH by reaction with HCO+ in the gas phase leads to three isomers of the cation HC(OH)2+. The most abundant cation can react in a second step with other abundant molecules of the ISM like NH3 to form back c-HCOOH and t-HCOOH. This mechanism explains the formation of c-HCOOH in dark molecular clouds. Considering this mechanism, the fraction of c-HCOOH with respect t-HCOOH is 25.7%. To explain the 6% reported by the observations we propose that further destruction mechanisms of the cations of HCOOH should be taken into account. The sequential acid-base (SAB) mechanism proposed in this work involves fast processes with very abundant molecules in the ISM. Thus, HCOOH very likely suffers our proposed transformations in the conditions of dark molecular clouds. This is a new approach in the framework of the isomerism of organic molecules in the ISM which has the potential to try to explain the ratio between isomers of organic molecules detected in the ISM.

The isomerism of molecules in the interstellar medium and the mechanisms behind it are essential questions in the chemistry of organic molecules in space. In particular, for the simple formic and thioformic acids, the low temperatures found in molecular clouds indicate that cis-trans isomerization in the gas-phase must be impeded. Reactions happening on top of interstellar dust grains may explain the isomer interconversion at low temperatures. We studied the isomerization processes of formic and thioformic acid susceptible to happen on the surface of interstellar dust grains and initiated by H abstraction reactions. Similarly, deuterium enrichment of the acids can occur by the same mechanism. Our objective is to shed light on both topics to increase our understanding of key precursors of organic molecules in space., Comment: Accepted for publication in Astronomy and Astrophysics

The isomerism of molecules in the interstellar medium and the mechanisms behind it are essential questions in the chemistry of organic molecules in space. In particular, for the simple formic and thioformic acids, the low temperatures found in molecular clouds indicate that cis-trans isomerization in the gas-phase must be impeded. Reactions happening on top of interstellar dust grains may explain the isomer interconversion at low temperatures. We studied the isomerization processes of formic and thioformic acid susceptible to happen on the surface of interstellar dust grains and initiated by H abstraction reactions. Similarly, deuterium enrichment of the acids can occur by the same mechanism. Our objective is to shed light on both topics to increase our understanding of key precursors of organic molecules in space., Comment: Accepted for publication in Astronomy and Astrophysics

The diffusion of atoms and radicals on interstellar dust grains is a fundamental ingredient for predicting accurate molecular abundances in astronomical environments. Quantitative values of diffusivity and diffusion barriers usually rely heavily on empirical rules. In this paper, we compute the diffusion coefficients of adsorbed nitrogen atoms by combining machine-learned interatomic potentials, metadynamics, and kinetic Monte Carlo simulations. With this approach, we obtain a diffusion coefficient of nitrogen atoms on the surface of amorphous solid water of merely $(3.5 \pm 1.1)10^{-34}$cm$^2$s$^{-1}$ at 10 K for a bare ice surface. Thus, we find that nitrogen, as a paradigmatic case for light and weakly bound adsorbates, is unable to diffuse on bare amorphous solid water at 10 K. Surface coverage has a strong effect on the diffusion coefficient by modulating its value over 9--12 orders of magnitude at 10 K and enables diffusion for specific conditions. In addition, we have found that atom tunneling has a negligible effect. Average diffusion barriers of the potential energy surface (2.56 kJ mol$^{-1}$) differ strongly from the effective diffusion barrier obtained from the diffusion coefficient for a bare surface (6.06 kJ mol$^{-1}$) and are, thus, inappropriate for diffusion modeling. Our findings suggest that the thermal diffusion of N on water ice is a process that is highly dependent on the physical conditions of the ice., Comment: Accepted in MNRAS

We report new computational and experimental evidence of an efficient and astrochemically relevant formation route to formaldehyde (H$_2$CO). This simplest carbonylic compound is central to the formation of complex organics in cold interstellar clouds, and is generally regarded to be formed by the hydrogenation of solid-state carbon monoxide. We demonstrate H$_2$CO formation via the reaction of carbon atoms with amorphous solid water. Crucial to our proposed mechanism is a concerted proton transfer catalyzed by the water hydrogen bonding network. Consequently, the reactions $^3$C + H$_2$O -> $^3$HCOH and $^1$HCOH -> $^1$H$_2$CO can take place with low or without barriers, contrary to the high-barrier traditional internal hydrogen migration. These low barriers or absence thereof explain the very small kinetic isotope effect in our experiments when comparing the formation of H$_2$CO to D$_2$CO. Our results reconcile the disagreement found in the literature on the reaction route: C + H$_2$O -> H$_2$CO., Comment: Accepted for publication in JPCL

With the presence of evermore complex S-bearing molecules being detected lately, studies on their chemical formation routes need to keep up the pace to rationalize observations, suggest new candidates for detection, and provide input for chemical evolution models. In this paper, we theoretically characterize the hydrogenation channels of OCS on top of amorphous solid water as an interstellar dust grain analog in molecular clouds. Our results show that the significant reaction outcome is trans-HC(O)SH, a recently detected prebiotic molecule toward G+0.693. The reaction is diastereoselective, explaining the seemingly absence of the cis isomer in the astronomical observations. We found that the reaction proceeds through a highly localized radical intermediate (cis-OCSH), which could be essential in the formation of other sulfur-bearing complex organic molecules due to its slow isomerization dynamics on top of amorphous solid water., Comment: Accepted in ApJ

The recent wave of detections of interstellar aromatic molecules has sparked interest in the chemical behavior of aromatic molecules under astrophysical conditions. In most cases, these detections have been made through chemically related molecules, called proxies, that implicitly indicate the presence of a parent molecule. In this study, we present the results of the theoretical evaluation of the hydrogenation reactions of different aromatic molecules (benzene, pyridine, pyrrole, furan, thiophene, silabenzene, and phosphorine). The viability of these reactions allows us to evaluate the resilience of these molecules to the most important reducing agent in the interstellar medium, the hydrogen atom (H). All significant reactions are exothermic and most of them present activation barriers, which are, in several cases, overcome by quantum tunneling. Instanton reaction rate constants are provided between 50 K and 500 K. For the most efficiently formed radicals, a second hydrogenation step has been studied. We propose that hydrogenated derivatives of furan, pyrrole, and specially 2,3-dihydropyrrole, 2,5-dihydropyrrole, 2,3-dihydrofuran, and 2,5-dihydrofuran are promising candidates for future interstellar detections.

The diffusion of atoms and radicals on interstellar dust grains is a fundamental ingredient for predicting accurate molecular abundances in astronomical environments. Quantitative values of diffusivity and diffusion barriers usually rely heavily on empirical rules. In this paper, we compute the diffusion coefficients of adsorbed nitrogen atoms by combining machine-learned interatomic potentials, metadynamics, and kinetic Monte Carlo simulations. With this approach, we obtain a diffusion coefficient of nitrogen atoms on the surface of amorphous solid water of merely $(3.5 \pm 1.1)10^{-34}$cm$^2$s$^{-1}$ at 10 K for a bare ice surface. Thus, we find that nitrogen, as a paradigmatic case for light and weakly bound adsorbates, is unable to diffuse on bare amorphous solid water at 10 K. Surface coverage has a strong effect on the diffusion coefficient by modulating its value over 9--12 orders of magnitude at 10 K and enables diffusion for specific conditions. In addition, we have found that atom tunneling has a negligible effect. Average diffusion barriers of the potential energy surface (2.56 kJ mol$^{-1}$) differ strongly from the effective diffusion barrier obtained from the diffusion coefficient for a bare surface (6.06 kJ mol$^{-1}$) and are, thus, inappropriate for diffusion modeling. Our findings suggest that the thermal diffusion of N on water ice is a process that is highly dependent on the physical conditions of the ice., Comment: Accepted in MNRAS

We report new computational and experimental evidence of an efficient and astrochemically relevant formation route to formaldehyde (H$_2$CO). This simplest carbonylic compound is central to the formation of complex organics in cold interstellar clouds, and is generally regarded to be formed by the hydrogenation of solid-state carbon monoxide. We demonstrate H$_2$CO formation via the reaction of carbon atoms with amorphous solid water. Crucial to our proposed mechanism is a concerted proton transfer catalyzed by the water hydrogen bonding network. Consequently, the reactions $^3$C + H$_2$O -> $^3$HCOH and $^1$HCOH -> $^1$H$_2$CO can take place with low or without barriers, contrary to the high-barrier traditional internal hydrogen migration. These low barriers or absence thereof explain the very small kinetic isotope effect in our experiments when comparing the formation of H$_2$CO to D$_2$CO. Our results reconcile the disagreement found in the literature on the reaction route: C + H$_2$O -> H$_2$CO., Comment: Accepted for publication in JPCL

With the presence of evermore complex S-bearing molecules being detected lately, studies on their chemical formation routes need to keep up the pace to rationalize observations, suggest new candidates for detection, and provide input for chemical evolution models. In this paper, we theoretically characterize the hydrogenation channels of OCS on top of amorphous solid water as an interstellar dust grain analog in molecular clouds. Our results show that the significant reaction outcome is trans-HC(O)SH, a recently detected prebiotic molecule toward G+0.693. The reaction is diastereoselective, explaining the seemingly absence of the cis isomer in the astronomical observations. We found that the reaction proceeds through a highly localized radical intermediate (cis-OCSH), which could be essential in the formation of other sulfur-bearing complex organic molecules due to its slow isomerization dynamics on top of amorphous solid water., Comment: Accepted in ApJ

The recent wave of detections of interstellar aromatic molecules has sparked interest in the chemical behavior of aromatic molecules under astrophysical conditions. In most cases, these detections have been made through chemically related molecules, called proxies, that implicitly indicate the presence of a parent molecule. In this study, we present the results of the theoretical evaluation of the hydrogenation reactions of different aromatic molecules (benzene, pyridine, pyrrole, furan, thiophene, silabenzene, and phosphorine). The viability of these reactions allows us to evaluate the resilience of these molecules to the most important reducing agent in the interstellar medium, the hydrogen atom (H). All significant reactions are exothermic and most of them present activation barriers, which are, in several cases, overcome by quantum tunneling. Instanton reaction rate constants are provided between 50 K and 500 K. For the most efficiently formed radicals, a second hydrogenation step has been studied. We propose that hydrogenated derivatives of furan, pyrrole, and specially 2,3-dihydropyrrole, 2,5-dihydropyrrole, 2,3-dihydrofuran, and 2,5-dihydrofuran are promising candidates for future interstellar detections.

Context. The diffusion of volatile species on amorphous solid water ice affects the chemistry on dust grains in the interstellar medium as well as the trapping of gases enriching planetary atmospheres or present in cometary material. Aims. The aim of the work is to provide diffusion coefficients of CH$_4$ on amorphous solid water (ASW), and to understand how they are affected by the ASW structure. Methods. Ice mixtures of H$_2$O and CH$_4$ were grown in different conditions and the sublimation of CH$_4$ was monitored via infrared spectroscopy or via the mass loss of a cryogenic quartz crystal microbalance. Diffusion coefficients were obtained from the experimental data assuming the systems obey Fick's law of diffusion. Monte Carlo simulations modeled the different amorphous solid water ice structures investigated and were used to reproduce and interpret the experimental results. Results. Diffusion coefficients of methane on amorphous solid water have been measured to be between 10$^{-12}$ and 10$^{-13}$ cm$^2$ s$^{-1}$ for temperatures ranging between 42 K and 60 K. We showed that diffusion can differ by one order of magnitude depending on the morphology of amorphous solid water. The porosity within water ice, and the network created by pore coalescence, enhance the diffusion of species within the pores.The diffusion rates derived experimentally cannot be used in our Monte Carlo simulations to reproduce the measurements. Conclusions. We conclude that Fick's law can be used to describe diffusion at the macroscopic scale, while Monte Carlo simulations describe the microscopic scale where trapping of species in the ices (and their movement) is considered., Comment: 17 pages, 15 figures

In this work, we present the results of our investigation into the chemistry of Z- and E-cyanomethanimine (HNCHCN), both of which are possible precursors to the nucleobase adenine. Ab initio quantum chemical calculations for a number of reactions with atomic hydrogen were carried out. We find that the reaction H + Z/E-HNCHCN leading both to H-addition as well as H$_2$-abstraction proceed via similar short-range barriers with bimolecular rate coefficients on the order of $\sim10^{-17}$ cm$^{3}$ s$^{-1}$. These results were then incorporated into astrochemical models and used in simulations of the giant molecular cloud G+0.693. The calculated abundances obtained from these models were compared with previous observational data and found to be in good agreement, with a predicted [Z/E] ratio of $\sim3$ - somewhat smaller than the previously derived value of $6.1\pm2.4$. We find that the [Z/E] ratio in our simulations is due mostly to ion-molecule destruction rates driven by the different permanent dipoles of the two conformers. Based on these results, we propose a general rule-of-thumb for estimating the abundances of isomers in interstellar environments., Comment: The Astrophysical Journal, accepted: 17 pages, 9 figures, 7 tables

Context. The diffusion of volatile species on amorphous solid water ice affects the chemistry on dust grains in the interstellar medium as well as the trapping of gases enriching planetary atmospheres or present in cometary material. Aims. The aim of the work is to provide diffusion coefficients of CH4 on amorphous solid water (ASW) and to understand how they are affected by the ASW structure. Methods. Ice mixtures of H2O and CH4 were grown in different conditions and the sublimation of CH4 was monitored via infrared spectroscopy or via the mass loss of a cryogenic quartz crystal microbalance. Diffusion coefficients were obtained from the experimental data assuming the systems obey Fick's law of diffusion. Monte Carlo simulations were used to model the different amorphous solid water ice structures investigated and were used to reproduce and interpret the experimental results. Results. Diffusion coefficients of methane on amorphous solid water have been measured to be between 10-12 and 10-13 cm2 s-1 for temperatures ranging between 42 K and 60 K. We show that diffusion can differ by one order of magnitude depending on the morphology of amorphous solid water. The porosity within water ice and the network created by pore coalescence enhance the diffusion of species within the pores. The diffusion rates derived experimentally cannot be used in our Monte Carlo simulations to reproduce the measurements. Conclusions. We conclude that Fick's laws can be used to describe diffusion at the macroscopic scale, while Monte Carlo simulations describe the microscopic scale where trapping of species in the ices (and their movement) is considered., Astrodynamics & Space Missions

Dynamics of adsorption and desorption of (4S)-N on amorphous solid water are analyzed using molecular dynamics simulations. The underlying potential energy surface was provided by machine-learned interatomic potentials. Binding energies confirm the latest available theoretical and experimental results. The nitrogen sticking coefficient is close to unity at dust temperatures of 10 K but decreases at higher temperatures. We estimate a desorption time scale of 1 {\mu}s at 28 K. The estimated time scale allows chemical processes mediated by diffusion to happen before desorption, even at higher temperatures. We found that the energy dissipation process after a sticking event happens on the picosecond timescale at dust temperatures of 10 K, even for high energies of the incoming adsorbate. Our approach allows the simulation of large systems for reasonable time scales at an affordable computational cost and ab-initio accuracy. Moreover, it is generally applicable for the study of adsorption dynamics of interstellar radicals on dust surfaces.

In this work, we present the results of our investigation into the chemistry of Z- and E-cyanomethanimine (HNCHCN), both of which are possible precursors to the nucleobase adenine. Ab initio quantum chemical calculations for a number of reactions with atomic hydrogen were carried out. We find that the reaction H + Z/E-HNCHCN leading both to H-addition as well as H$_2$-abstraction proceed via similar short-range barriers with bimolecular rate coefficients on the order of $\sim10^{-17}$ cm$^{3}$ s$^{-1}$. These results were then incorporated into astrochemical models and used in simulations of the giant molecular cloud G+0.693. The calculated abundances obtained from these models were compared with previous observational data and found to be in good agreement, with a predicted [Z/E] ratio of $\sim3$ - somewhat smaller than the previously derived value of $6.1\pm2.4$. We find that the [Z/E] ratio in our simulations is due mostly to ion-molecule destruction rates driven by the different permanent dipoles of the two conformers. Based on these results, we propose a general rule-of-thumb for estimating the abundances of isomers in interstellar environments., Comment: The Astrophysical Journal, accepted: 17 pages, 9 figures, 7 tables

Context. The diffusion of volatile species on amorphous solid water ice affects the chemistry on dust grains in the interstellar medium as well as the trapping of gases enriching planetary atmospheres or present in cometary material. Aims. The aim of the work is to provide diffusion coefficients of CH4 on amorphous solid water (ASW) and to understand how they are affected by the ASW structure. Methods. Ice mixtures of H2O and CH4 were grown in different conditions and the sublimation of CH4 was monitored via infrared spectroscopy or via the mass loss of a cryogenic quartz crystal microbalance. Diffusion coefficients were obtained from the experimental data assuming the systems obey Fick's law of diffusion. Monte Carlo simulations were used to model the different amorphous solid water ice structures investigated and were used to reproduce and interpret the experimental results. Results. Diffusion coefficients of methane on amorphous solid water have been measured to be between 10-12 and 10-13 cm2 s-1 for temperatures ranging between 42 K and 60 K. We show that diffusion can differ by one order of magnitude depending on the morphology of amorphous solid water. The porosity within water ice and the network created by pore coalescence enhance the diffusion of species within the pores. The diffusion rates derived experimentally cannot be used in our Monte Carlo simulations to reproduce the measurements. Conclusions. We conclude that Fick's laws can be used to describe diffusion at the macroscopic scale, while Monte Carlo simulations describe the microscopic scale where trapping of species in the ices (and their movement) is considered., Astrodynamics & Space Missions

Context. The diffusion of volatile species on amorphous solid water ice affects the chemistry on dust grains in the interstellar medium as well as the trapping of gases enriching planetary atmospheres or present in cometary material. Aims. The aim of the work is to provide diffusion coefficients of CH4 on amorphous solid water (ASW) and to understand how they are affected by the ASW structure. Methods. Ice mixtures of H2O and CH4 were grown in different conditions and the sublimation of CH4 was monitored via infrared spectroscopy or via the mass loss of a cryogenic quartz crystal microbalance. Diffusion coefficients were obtained from the experimental data assuming the systems obey Fick's law of diffusion. Monte Carlo simulations were used to model the different amorphous solid water ice structures investigated and were used to reproduce and interpret the experimental results. Results. Diffusion coefficients of methane on amorphous solid water have been measured to be between 10-12 and 10-13 cm2 s-1 for temperatures ranging between 42 K and 60 K. We show that diffusion can differ by one order of magnitude depending on the morphology of amorphous solid water. The porosity within water ice and the network created by pore coalescence enhance the diffusion of species within the pores. The diffusion rates derived experimentally cannot be used in our Monte Carlo simulations to reproduce the measurements. Conclusions. We conclude that Fick's laws can be used to describe diffusion at the macroscopic scale, while Monte Carlo simulations describe the microscopic scale where trapping of species in the ices (and their movement) is considered.

Water is one of the most abundant molecules in the form of solid ice phase in the different regions of the interstellar medium (ISM). This large abundance cannot be properly explained by using only traditional low-temperature gas-phase reactions. Thus, surface chemical reactions are believed to be major synthetic channels for the formation of interstellar water ice. Among the different proposals, hydrogenation of atomic O (i.e. 2H + O ¿ H2O) is a chemically `simple¿ and plausible reaction toward water formation occurring on the surfaces of interstellar grains. Here, novel theoretical results concerning the formation of water adopting this mechanism on the crystalline (010) Mg2SiO4 surface (a unequivocally identified interstellar silicate) are presented. The investigated reaction aims to simulate the formation of the first water ice layer covering the silicate core of dust grains. Adsorption of the atomic O as a first step of the reaction has been computed, results indicating that a peroxo (O2¿2¿) group is formed. The following steps involve the adsorption, diffusion, and reaction of two successive H atoms with the adsorbed O atom. Results indicate that H diffusion on the surface has barriers of 4¿6 kcal mol¿1, while actual formation of OH and H2O present energy barriers of 22¿23 kcal mol¿1. Kinetic study results show that tunneling is crucial for the occurrence of the reactions and that formation of OH and H2O are the bottlenecks of the overall process. Several astrophysical implications derived from the theoretical results are provided as concluding remarks.

A method to predict the near-infrared spectra of amorphous solids by means of ab initio molecular dynamics is presented. These solids can simulate molecular ices. To test the method, mixtures of methane, water and nitrogen are generated as amorphous samples of various concentrations. The full theoretical treatment includes as a first step, the optimization of their geometrical structure for a range of densities, after which, the most stable systems are taken as initial structures for molecular dynamics, performed at 200 K in trajectories of 4 ps duration with a 0.2 fs time step. All the dynamics are carried out using the first principles method, solving the quantum problem for the electrons using density-functional theory (DFT), and integrating the DFT forces, following the Born–Oppenheimer dynamics. After the dynamics, near-IR spectra are predicted by the Fourier transform of the macroscopic polarization autocorrelation function. The calculated spectra are compared with the experimental spectra of ice mixtures of CH4 and H2O recorded in our laboratory, and with some spectra recorded by the New Horizons mission on Pluto.

[EN] Contact. The increasing capabilities of space missions like the James Webb Space Telescope or ground-based observatories like the European Extremely Large Telescope demand high quality laboratory data of species in astrophysical conditions for the interpretation of their findings. Aims. We provide new physical and spectroscopic data of solid methanol that will help to identify this species in astronomical environments. Methods. Ices were grown by vapour deposition in high vacuum chambers. Densities were measured via a cryogenic quartz crystal microbalance and laser interferometry. Absorbance infrared spectra of methanol ices of different thickness were recorded to obtain optical constants using an iterative minimization procedure. Infrared band strengths were determined from infrared spectra and ice densities. Results. Solid methanol densities measured at eight temperatures vary between 0.64 g cm(-3) at 20 K and 0.84 g cm(-3 )at 130 K. The visible refractive index at 633 nm grows from 1.26 to 1.35 in that temperature range. New infrared optical constants and band strengths are given from 650 to 5000 cm(-1) (15.4-2.0 mu m) at the same eight temperatures. The study was made on ices directly grown at the indicated temperatures, and amorphous and crystalline phases have been recognized. Our optical constants differ from those previously reported in the literature for an ice grown at 10 K and subsequently warmed. The disagreement is due to different ice morphologies. The new infrared band strengths agree with previous literature data when the correct densities are considered.

[EN] Contact. The increasing capabilities of space missions like the James Webb Space Telescope or ground-based observatories like the European Extremely Large Telescope demand high quality laboratory data of species in astrophysical conditions for the interpretation of their findings. Aims. We provide new physical and spectroscopic data of solid methanol that will help to identify this species in astronomical environments. Methods. Ices were grown by vapour deposition in high vacuum chambers. Densities were measured via a cryogenic quartz crystal microbalance and laser interferometry. Absorbance infrared spectra of methanol ices of different thickness were recorded to obtain optical constants using an iterative minimization procedure. Infrared band strengths were determined from infrared spectra and ice densities. Results. Solid methanol densities measured at eight temperatures vary between 0.64 g cm(-3) at 20 K and 0.84 g cm(-3 )at 130 K. The visible refractive index at 633 nm grows from 1.26 to 1.35 in that temperature range. New infrared optical constants and band strengths are given from 650 to 5000 cm(-1) (15.4-2.0 mu m) at the same eight temperatures. The study was made on ices directly grown at the indicated temperatures, and amorphous and crystalline phases have been recognized. Our optical constants differ from those previously reported in the literature for an ice grown at 10 K and subsequently warmed. The disagreement is due to different ice morphologies. The new infrared band strengths agree with previous literature data when the correct densities are considered.

For the past 30 years, Polycyclic Aromatic Hydrocarbons (PAHs) have been able to reproduce the Unidentified Infrared Emission (UIE) emission bands, a set of spectroscopic features found between 11.2 and 6.2 µm and at 3.3 µm in the mid IR, corresponding to aromatic C=C stretchings and bendings and C-H bendings, respectively [1,2]. The so-called PAH hypothesis is able to account for the general appearance of the bands explaining them as a combination of PAHs molecules in different oxidation states [1]. One of the main advantages of PAH molecules is that they can undergo transient heating to effective temperatures of 1000K upon absorption of a single UV photon, relaxing afterwards. With the years, refinements of the classical theory have been developed and successfully applied to the fitting of astronomical sources [3,4]. Several criticisms arise from the classical PAH hypothesis, being the most important one that no single PAH molecule has been positively identified in the Interstellar Medium (ISM) [5]. Among the years, several alternatives have been postulated like considering small solid particles as candidates for carriers of such band and assuming that isolated aromatic domains within these particles may suffer transient heating by UV photons [6,7]. The present work is devoted to the simulation of the UV absorption and IR emission spectra of a series of randomly packed hydrogenated amorphous carbon (HAC) chemical analogues [8] using ab initio methods. We have employed CASTEP [9] for the calculation of the vibrational properties and the optical constants in the ultraviolet and visible range. The calculation of the emission curves has been done implementing a canonical photocooling emission model from the literature [4 and references therein]. We show that the IR photo-cooling of our randomly packed amorphous analogs is consistent with the spectral characteristics of the interstellar UIE bands, offering a reconciling scenario between the PAH hypothesis and the mixed

This work describes the first experimental results on the combined diagnostics of a capacitively coupled RF discharge and an inductively coupled RF discharge built in our laboratory, and the compared behaviour relative to the production of carbonaceous dust and deposits of astrophysical interest in low pressure He+C2H2 plasmas.

Methyl isocyanate has been recently detected in the interstellar medium [1-2], whereas a reported detection on comet 67P/ Churyumov-Gerasimenko (67P/C-G) could not be confirmed [3]. Although this molecule was usually neglected in astrochemical networks, this new evidence fosters new astrophysical studies on this species to be carried out. We present here our work on the IR spectra and on the energetic processing of ices of this molecule.

Infrared band strengths are affected by ice density. Up to now no density values were available for temperatures covering from amorphous to crystalline ices. This presentation will show new experiments where density was measured at different temperatures from 20 K to 130 K. Ices were grown by vapour background deposition in two high vacuum chambers in the same conditions. Densities were measured via a cryogenic quartz crystal microbalance (mass deposited per surface unit) and laser interferometry (thickness). Absorbance infrared spectra of methanol ices of different thickness were recorded to obtain optical constants using an iterative minimization procedure. Infrared band strengths were determined from infrared spectra and ice densities.Solid methanol densities measured at eight temperatures vary between 0.64 g cm3at 20 K and 0.84 g cm3at 130 K. The visible refractive index at 633 nm grows from 1.26 to 1.35 in that temperature range. New infrared optical constants and band strengths aregiven from 650 to 5000 cm-1(15.4¿2.0 ¿m) at the same eight temperatures. The study was made on ices directly grown at the indicated temperatures, and amorphous and crystalline phases have been recognized. Our optical constants differ from those previously reported in the literature for an ice grown at 10 K and subsequently warmed. The disagreement is due to different ice morphologies. The new infrared band strengths agree with previous literature data when the correct densities are considered.

Many observations in the solar system are carried out in the near-IR region, 1.25-2.5 ¿m, or 8000-4000 cm-1, where ices of simple molecules and mixtures of them present relatively strong absorption or reflexion spectra, corresponding to their overtone or combination bands. However, whereas theoretical methods to predict and analyze the fundamental vibrationsof molecules and solids, appearing usually in the mid-IR, are well-established and straightforward to apply, there are no similar procedures available to deal with the vibrational modes leading to higher excited states. We propose the use of ab initio Molecular Dynamics to predict the spectra of solid samples containing mixtures of molecules such as those found in different bodies of the Solar system. In particular, we are focused on the exciting recent data obtained by the New Horizons mission on Pluto and Charon [1], where spectra of ices with strikingly different content of methane, water, nitrogen and other gases were recorded.We have carried out an initial study of these systems using the CASTEP program [2], as contained in the Materials Studio software [3], which uses a first-principles description of the electrons based on density functional theory with plane waves as basis sets. We are currently working on similar systems with a different theoretical approach, using numerical atomic orbitals as basis functions, implemented in the Siesta code[4].We will present the current state of results at the meeting.

Ices mantles on dust grains in dense clouds in the interstellar medium are formed mainly by H2O, CO and CO2. Methanol is the fourth most abundant species in those mantles with an abundance of 6% with respect to water [1]. Solid methanol has also been detected on the surface of minor bodies of the Solar System like some TNOs and Centarus [2]. The information about the composition and structure of astrophysical ices, and in particular confirmation of the presence of solid methanol, is based on the comparison of infrared absorption spectra retrieved by spatial missions or ground based observatories with laboratory data [3]. The goal of this work is to provide new spectroscopic data of solid methanol in the near and mid IR spectral range, that could help to identify and quantify this species in astrophysical ices. Densities and visible refractive index for methanol ices at eight temperatures, from 20 K to 130 K, have been measured. The infrared band strengths and optical constants of those ices from 650 cm tto 5000 cm (15.4 ¿m to 2.0 ¿m) were also determined, following the procedure described previously [4]. Comparison with other literature data, when available, will be commented., The increasing capabilities of space missions like the James Webb Space Telescope or ground-based observatories like the European Extremely Large Telescope demand high quality laboratory data of species in astrophysical conditions for the interpretation of their findings.

The increasing capabilities of space missions, like the James Webb Space Telescope, or groundbased observatories, like the European Extremely Large Telescope, demand high quality laboratory data of species in astrophysical conditions for the interpretation of their #12;ndings. We have measured new physical and spectroscopic data of solid methanol that will help to identify this species in astronomical environments. Methanol ices were grown by vapor deposition in high vacuum chambers. On one hand, ice densities were measured via a cryogenic Quartz Crystal Microbalance and laser interferometry. On the other hand, absorbance infrared spectra of ice layers of dierent thickness where recorded to obtain optical constants using an iterative minimization procedure. Finally, infrared band strengths were determined from infrared spectra and ice densities. Some discrepancies are found between our optical constants and those previously reported in the literature, for an ice grown at 10 K and subsequently warmed. The disagreement is explained as due to dierent ice morphologies. The new infrared band strengths agree with previous literature data when the correct densities are considered.

[EN] Physical and spectroscopic properties of ices of C2H4 and CH4:C2H4 mixtures with 3:1, 1:1 and 1:3 ratios have been investigated at 30 K. Two laboratories are involved in this work. In one of them, the density and refractive index of the samples have been measured by using a cryogenic quartz microbalance and laser interferometric techniques. In the other one, IR spectra have been recorded in the near- and mid-infrared regions, and band shifts with respect to the pure species, band strengths of the main bands, and the optical constants in both regions have been determined. Previous data on ethylene and the mixtures studied here were scarce. For methane, both the wavenumbers and band strengths have been found to follow a regular pattern of decrease with increasing dilution, but no pattern has been detected for ethylene vibrations. The method employed for the preparation of the samples, by vapour deposition under vacuum, is thought to be adequate to mimic the structure of astrophysical ices. Possible astrophysical implications, especially by means of the optical constants reported here, have been discussed.

[EN] We report the density and refractive index, at 633 nm, of ethane and ethylene ices at temperatures from 13 to 65 K, measured by double laser interferometry and a cryogenic quartz crystal microbalance in a high vacuum chamber. Both quantities rise with the temperature of deposition from 13 K up to a plateau, 40 K for ethane and 22 K for ethylene. An amorphous structure is suggested for temperatures below 40 K for ethane and below 20 K for ethylene. Above these temperatures, 40 K for ethane and 20 K for ethylene, a crystalline structure is proposed. Ethylene results deviate from linear growth between 25 and 35 K, where a metastable structure is reported in the literature. Density values increase from about 0.40 to 0.60 g cm(-3) for ethane and from about 0.45 to almost 0.65 g cm(-3) for ethylene. The real part of the refractive index changes from about 1.27 to 1.45 for ethane and from about 1.30 to almost 1.48 for ethylene. Results are relevant especially to the outer Solar System, where the presence of these molecules is reported, and for experiments involving them. (C) 2017 Elsevier Inc. All rights reserved.

[EN] Physical and spectroscopic properties of ices of C2H4 and CH4:C2H4 mixtures with 3:1, 1:1 and 1:3 ratios have been investigated at 30 K. Two laboratories are involved in this work. In one of them, the density and refractive index of the samples have been measured by using a cryogenic quartz microbalance and laser interferometric techniques. In the other one, IR spectra have been recorded in the near- and mid-infrared regions, and band shifts with respect to the pure species, band strengths of the main bands, and the optical constants in both regions have been determined. Previous data on ethylene and the mixtures studied here were scarce. For methane, both the wavenumbers and band strengths have been found to follow a regular pattern of decrease with increasing dilution, but no pattern has been detected for ethylene vibrations. The method employed for the preparation of the samples, by vapour deposition under vacuum, is thought to be adequate to mimic the structure of astrophysical ices. Possible astrophysical implications, especially by means of the optical constants reported here, have been discussed.

[EN] We report the density and refractive index, at 633 nm, of ethane and ethylene ices at temperatures from 13 to 65 K, measured by double laser interferometry and a cryogenic quartz crystal microbalance in a high vacuum chamber. Both quantities rise with the temperature of deposition from 13 K up to a plateau, 40 K for ethane and 22 K for ethylene. An amorphous structure is suggested for temperatures below 40 K for ethane and below 20 K for ethylene. Above these temperatures, 40 K for ethane and 20 K for ethylene, a crystalline structure is proposed. Ethylene results deviate from linear growth between 25 and 35 K, where a metastable structure is reported in the literature. Density values increase from about 0.40 to 0.60 g cm(-3) for ethane and from about 0.45 to almost 0.65 g cm(-3) for ethylene. The real part of the refractive index changes from about 1.27 to 1.45 for ethane and from about 1.30 to almost 1.48 for ethylene. Results are relevant especially to the outer Solar System, where the presence of these molecules is reported, and for experiments involving them. (C) 2017 Elsevier Inc. All rights reserved.

[EN] Physical and spectroscopic properties of ices of C2H4 and CH4:C2H4 mixtures with 3:1, 1:1 and 1:3 ratios have been investigated at 30 K. Two laboratories are involved in this work. In one of them, the density and refractive index of the samples have been measured by using a cryogenic quartz microbalance and laser interferometric techniques. In the other one, IR spectra have been recorded in the near- and mid-infrared regions, and band shifts with respect to the pure species, band strengths of the main bands, and the optical constants in both regions have been determined. Previous data on ethylene and the mixtures studied here were scarce. For methane, both the wavenumbers and band strengths have been found to follow a regular pattern of decrease with increasing dilution, but no pattern has been detected for ethylene vibrations. The method employed for the preparation of the samples, by vapour deposition under vacuum, is thought to be adequate to mimic the structure of astrophysical ices. Possible astrophysical implications, especially by means of the optical constants reported here, have been discussed.

Carbonaceous compounds are found in very diverse astronomical media [1], a significant amount consisting in small dust grains. This dust originates characteristic IR absorption bands revealing the presence of aliphatic and aromatic functional groups in variable proportions [2, 3]. Notably, the characteristic 3.4 ¿m (2940 cm-1) band is observed in the diffuse interstellar media, but not in dense clouds, and is mostly attributed to CH stretching vibrations of aliphatic groups (CH2, CH3). Among the various materials investigated as possible carriers of these bands, plasma generated hydrogenated amorphous carbon (a-C:H) led to the best agreement with observations [2-4], although the exact composition and structure are still not clear and under intensive investigation. In this work we present a study of the structure and spectroscopic features of carbonaceous dust analogues generated in He + CH4 inductively coupled RF discharges. Spectra obtained from density functional theory (DFT) calculations of different hydrocarbon structures are compared to the IR spectra of the samples to identify the structures that better fit the experimental results. The effects of cosmic rays (CRs) on the 3.4 ¿m IR band of a-C:H are also simulated by bombarding the samples with a beam of 5 keV electrons, which impart energy doses comparable to those of MeV protons [5].

Solar system objects like outer planets, their satellites, comets and asteroids contain a carbonaceous structure often covered by ices of different composition. The carbonaceous content is assumed to be formed by hydrogenated amorphous carbon (HAC). The most abundant ices are made of H2O, CH4, CH3OH, NH3 and others. Our goal in this work is to make theoretical models using quantum chemical tools, like ab initio and DFT methods, for mixtures of ices with various compositions and predict their spectra, aiming to match the space observations. Were the theoretical models that are constructed to study these systems are shown. Systems that have been recently studied by our laboratory group also. Related to methane-water ice, the so-called breathing mode of methane due its tetrahedral structure, the C atom and the charge distribution remain undisturbed during the vibration, and no net dipole moment is generated. Thus, as is well known, this vibrational mode is inactive in the infrared spectrum. However, in low-temperature methane/water mixtures, like those generated to model astrophysical ices, a small but distinguishable feature has been observed in IR spectra, at the expected frequency of this vibration, Gálvez et al [1]. We will present in this communication our results for a number of quantum chemical models for methane/water mixtures with an explanation of this phenomenon [2] Fig.1. Two different models for the arrangements of a-C:H particles, Fig. 2, have been studied. Calculations with varying density values allow proposing structures with low density, in the range 0.75 to 1 g cm-3 as the most likely constituents of the experimental a-C:H samples named RC, (rings and chains ) model and SG (substituted graphite) model[3].

Different samples of plasma deposited amorphous hydrogenated carbon (HAC or a-C:H) are used as analogs of carbonaceous dust in the diffuse interstellar (IS) medium. Comparison of measured and theoretically calculated spectra suggests that IS dust grains are likely made of small aromatic islands linked by aliphatic chains. Irradiation of the HAC deposits with 5 keV electrons shows that the effects of cosmic rays on the aliphatic dust component, characterized by an absorption band at 3.4 um, are small and cannot explain the disappearance of this band in dense interstellar clouds.

Astronomical observations have revealed the presence of a ubiquitous absroption band at 3.4 ¿m inthe diffuse interstellar (IS) medium. It is generally assumed that this band corresponds to some sort of amorphous hydrogenated carbon (a¿C:H), whose structure has been explained by two alternative models: small aromatic islands linked by aliphatic chains [1] or large polyaromatic structures with small aliphatic substituents at the edges [2]. In addition, it has been found that the 3.4 ¿m absorption band related to the aliphatic component of IS carbonaceous dust disappears inside dense molecular clouds. In this environment, cosmic rays (CR) could provide a realistic destruction mechanism. In order to clarify these issues, IS carbonaceous dust analogs produced in cold hydrocarbon plasmas and processed by highly energetic electrons has been analyzed by IR absorption, and the spectral signatures have been related with chemical structures described theoretically. Thin films and dust grains of a¿C:H with a variable proportion of aliphatic and aromatic structures are produced by plasma deposition in RF discharges using mixtures of hydrocarbons and He as plasma precursors. Optical spectroscopy, mass spectrometry and Langmuir probes are used for plasma diagnostics. The a¿C:H deposits are analyzed with IR absorption spectroscopy and surface analysis techniques (SEM, AFM,...). The effects of cosmic rays are investigated by irradiating the samples with 5 keV electrons. Models of a¿C:H solids of variable density, based on the mentioned competing structures [1,2], are constructed using Montecarlo/Molecular Mechanics and their electronic energies and IR spectra are computed at Density Functional Theory (DFT) level. Comparison between theoretical and experimental IR spectra [3] shows that the structure of carbonaceous dust in the diffuse IS medium is most probably intermediate between those of the two literature models but closer to that with small aromatic units [1]. Electron bombardm

The prebiotic molecule methyl isocyanate (CH3NCO) was recently detected at the surface of comet 67P/CG [1] and shortly afterwards in the gas-phase of the interstellar medium [2,3]. Its estimated abundance with respect to water was found to be very high (1.3%) in the comet. The crystal structure of CH3NCO is unknown. In order to make a prediction, we first carried out theoretical calculations at Density Functional Theory (DFT) level on the known structure of isocyanic acid, HNCO. We achieved a correct reproduction of the crystalline structure of this molecule. Then we replaced the H atom by a CH3 group, optimized the structure and predicted the infrared spectrum of solid CH3NCO. We have recorded in our lab experimental spectra of this species and of CD3NCO at temperatures of relevance for astronomical environments. Comparison between theoretical and experimental spectra leads us to propose a tentative structure for the CH3NCO crystal.

Carbonaceous dust in the diffuse interstellar (IS) medium is largely made of some sort of amorphous hydrogenated carbon (a-C:H), whose structure has been explained by two alternative models: small aromatic islands linked by aliphatic chains [1] or large polyaromatic structures with small aliphatic substituents at the edges [2]. In addition, IR spectra show that the 3.4 ¿m absorption band related to the aliphatic component of IS carbonaceous dust disappears inside dense molecular clouds. In this environment, cosmic rays (CR) could provide a realistic destruction mechanism. In order to clarify these questions, IS carbonaceous dust analogs produced in cold hydrocarbon plasmas and processed by highly energetic electrons has been analyzed by IR absorption, and the spectral signatures have been related with chemical structures described theoretically. Dust grains and thin films of a-C:H with a variable proportion of aliphatic and aromatic structures are generated by plasma deposition in RF discharges. Mixtures of hydrocarbons and He are used as plasma precursors. Optical spectroscopy, mass spectrometry and Langmuir probes are used for plasma diagnostic. a-C:H deposits are analyzed with IR absorption spectroscopy and surface analysis techniques (SEM, AFM,...). The effects of cosmic rays are investigated by irradiating the samples with 5 keV electrons. Models of a-C:H solids of variable density, based on the mentioned competing structures [1,2], are constructed using Montecarlo/Molecular Mechanics and their electronic energies and IR spectra are computed at Density Functional Theory (DFT) level. The comparison of measured and calculated IR spectra [3] reveals that the structure of carbonaceous dust in the diffuse IS medium is intermediate between those of the two literature models but closer to that with small aromatic units [1]. The experimental destruction rates suggest that the effects of cosmic rays are not enough to explain the disappearance of the 3.4 ¿m band inside d