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Detailed laboratory studies and progress in surface science technique, have allowed in recent years the first experimental confirmation of surface reaction schemes, as introduced by Tielens, Hagen and Charnley [1,2]. In this paper, we review few heterogeneous processes which give routes to form elementary molecules considered as precursors for explaining the variety and richness of molecular species in the interstellar medium. Adsorption, diffusion and reaction processes are discussed. With emphasis on the experimental approaches, but also supported by theoretical developments, progresses in the understanding of the “catalytic role” of a dust grain surface in various physical conditions are described. Recent advances made on few important species (H2, H2O, CH3OH) are used to illustrate basic properties and raise open questions.

Context. Ultraviolet photodesorption of molecules from icy interstellar grains can explain observations of cold gas in regions where thermal desorption is negligible. This non-thermal desorption mechanism should be especially important where UV fluxes are high. Aims. N2 and O2 are expected to play key roles in astrochemical reaction networks, both in the solid state and in the gas phase. Measurements of the wavelength-dependent photodesorption rates of these two infrared-inactive molecules provide astronomical and physical-chemical insights into the conditions required for their photodesorption. Methods. Tunable radiation from the DESIRS beamline at the SOLEIL synchrotron in the astrophysically relevant 7 to 13.6 eV range is used to irradiate pure N2 and O2 thin ice films. Photodesorption of molecules is monitored through quadrupole mass spectrometry. Absolute rates are calculated by using the well-calibrated CO photodesorption rates. Strategic N2 and O2 isotopolog mixtures are used to investigate the importance of dissociation upon irradiation. Results. N2 photodesorption mainly occurs through excitation of the b1Πu state and subsequent desorption of surface molecules. The observed vibronic structure in the N2 photodesorption spectrum, together with the absence of N3 formation, supports that the photodesorption mechanism of N2 is similar to CO, i.e., an indirect DIET (Desorption Induced by Electronic Transition) process without dissociation of the desorbing molecule. In contrast, O2 photodesorption in the 7-13.6 eV range occurs through dissociation and presents no vibrational structure. Conclusions. Photodesorption rates of N2 and O2 integrated over the far-UV field from various star-forming environments are lower than for CO. Rates vary between 10-3 and 10-2 photodesorbed molecules per incoming photon. [ABSTRACT FROM AUTHOR]

Over the last four years we have illustrated the potential of a novel wavelength-dependent approach in determining molecular processes at work in the photodesorption of interstellar ice analogs. This method, utilizing the unique beam characteristics of the vacuum UV beamline DESIRS at the French synchrotron facility SOLEIL, has revealed an efficient indirect desorption mechanism that scales with the electronic excitations in molecular solids. This process, known as DIET – desorption induced by electronic transition – occurs efficiently in ices composed of very volatile species (CO, N2), for which photochemical processes can be neglected. In the present study, we investigate the photodesorption energy dependence of pure and pre-irradiated CO2 ices at 10–40 K and between 7 and 14 eV. The photodesorption from pure CO2 is limited to photon energies above 10.5 eV and is clearly initiated by CO2 excitation and by the contribution of dissociative and recombination channels. The photodesorption from “pre-irradiated” ices is shown to present an efficient additional desorption pathway below 10 eV, dominating the desorption depending on the UV-processing history of the ice film. This effect is identified as an indirect DIET process mediated by photoproduced CO, observed for the first time in the case of less volatile species. The results presented here pinpoint the importance of the interconnection between photodesorption and photochemical processes in interstellar ices driven by UV photons having different energies., Astronomy

Context: Ultraviolet photodesorption of molecules from icy interstellar grains can explain observations of cold gas in regions where thermal desorption is negligible. This non-thermal desorption mechanism should be especially important where UV fluxes are high. Aims: \(N_2\) and \(O_2\) are expected to play key roles in astrochemical reaction networks, both in the solid state and in the gas phase. Measurements of the wavelength-dependent photodesorption rates of these two infrared-inactive molecules provide astronomical and physical-chemical insights into the conditions required for their photodesorption. Methods: Tunable radiation from the DESIRS beamline at the SOLEIL synchrotron in the astrophysically relevant 7 to 13.6 eV range is used to irradiate pure \(N_2\) and \(O_2\) thin ice films. Photodesorption of molecules is monitored through quadrupole mass spectrometry. Absolute rates are calculated by using the well-calibrated CO photodesorption rates. Strategic \(N_2\) and \(O_2\) isotopolog mixtures are used to investigate the importance of dissociation upon irradiation. Results: \(N_2\) photodesorption mainly occurs through excitation of the \(b^1\sqcap_u\) state and subsequent desorption of surface molecules. The observed vibronic structure in the \(N_2\) photodesorption spectrum, together with the absence of \(N_3\) formation, supports that the photodesorption mechanism of \(N_2\) is similar to CO, i.e., an indirect DIET (Desorption Induced by Electronic Transition) process without dissociation of the desorbing molecule. In contrast, \(O_2\) photodesorption in the 7−13.6 eV range occurs through dissociation and presents no vibrational structure. Conclusions: Photodesorption rates of \(N_2\) and \(O_2\) integrated over the far-UV field from various star-forming environments are lower than for CO. Rates vary between \(10^{-3}\) and \(10^{-2}\) photodesorbed molecules per incoming photon., Astronomy

The gas-phase reaction between the ethyl cation (C 2 H 5 ) and ethyne (C + ) and ethyne (C 2 ) is re-investigated by measuring absolute reactive cross sections (CSs) and branching ratios (BRs) as a function of collision energy, in the thermal and hyperthermal energy range, via tandem-guided ion beam mass spectrometry under single collision conditions. Dissociative photoionization of C 2 ) is re-investigated by measuring absolute reactive cross sections (CSs) and branching ratios (BRs) as a function of collision energy, in the thermal and hyperthermal energy range, via tandem-guided ion beam mass spectrometry under single collision conditions. Dissociative photoionization of C 2 Br using tuneable VUV radiation in the range 10.5-14.0 eV is employed to generate C 5 Br using tuneable VUV radiation in the range 10.5-14.0 eV is employed to generate C 2 , which has also allowed us to explore the impact of increasing (vibrational) excitation on the reactivity. Reactivity experiments are complemented by theoretical calculations, at the G4 level of theory, of the relative energies and structures of the most relevant stationary points on the reactive potential energy hypersurface (PES) and by mass-analyzed ion kinetic energy (MIKE) spectrometry experiments to probe the metastable decomposition from the [C 5 + , which has also allowed us to explore the impact of increasing (vibrational) excitation on the reactivity. Reactivity experiments are complemented by theoretical calculations, at the G4 level of theory, of the relative energies and structures of the most relevant stationary points on the reactive potential energy hypersurface (PES) and by mass-analyzed ion kinetic energy (MIKE) spectrometry experiments to probe the metastable decomposition from the [C 4 H 7 C + PES and elucidate the underlying reaction mechanisms. Two main product channels have been identified at a centre-of-mass collision energy of ∼0.1 eV: (a) C 3 H 3 C + +CH 4 , with BR = 0.76±0.05 and (b) , with BR = 0.22±0.02. A third channel giving C 4 H 5 in association with C + +H 2 is shown to emerge at both high internal excitation of C 2 H 3 + in association with C 2 H 4 , from which barrierless and exothermic pathways exist for the production of both cyclic c-C 2 H 5 + and high collision energies. From CS measurements, energy-dependent total rate constants in the range 4.3×10-11-5.2×10-10 cm3·molecule-1·s-1 have been obtained. Theoretical calculations indicate that both channels stem from a common covalently bound intermediate, CH 3 isomers of the main product channel. For the minor C 2 CHCH + product, two isomers are energetically accessible: the three-member cyclic isomer c-C 3 H 3 + and linear H 2 CCCH + isomers of the main product channel. For the minor C 4 H 5 + product, two isomers are energetically accessible: the three-member cyclic isomer c-C 3 H 2 (CH 3 ) + and the higher energy linear structure CH 2 CHCCH 2 + , but their formation requires multiple isomerization steps and passages via transition states lying only 0.11 eV below the reagents' energy, thus explaining the smaller BR. Results have implications for the modeling of hydrocarbon chemistry in the interstellar medium and the atmospheres of planets and satellites as well as in laboratory plasmas (e.g., plasma-enhanced chemical vapor deposition of carbon nanotubes and diamond-like carbon films).

Most polycyclic aromatic hydrocarbons (PAHs) can isomerize with internal energies near to or below the dissociation threshold. The C 10 H + 8 group of ions, made up of the naphthalene (Naph + ) radical cations, is a prototypical example. C + ) radical cations, is a prototypical example. C 8 isomers are important species in the growth kinetics and formation of complex organic molecules, and more generally fragments from larger PAHs, yet information about C + 6 structures is scarce and contradictory. Here, ion-molecule reactions were carried out and the tunable photoionization chemical monitoring technique was used to probe the C 8 H + isomers formed upon C 6 structures is scarce and contradictory. Here, ion-molecule reactions were carried out and the tunable photoionization chemical monitoring technique was used to probe the C 8 H + 6 isomers formed upon C 2 and kinetics calculations. To facilitate the interpretation of these data, chemical reactivity starting from phenylacetylene (PA) was studied. It was found that most of the C 2 -loss from Naph + ions formed from C + . The experimental findings were interpreted with the support of ab initio and kinetics calculations. To facilitate the interpretation of these data, chemical reactivity starting from phenylacetylene (PA) was studied. It was found that most of the C 8 H + 6 ions formed from C 10 H 8 , in a timescale of 40 μs, are PA + in the vicinity of the dissociation threshold. No evidence of the pentalene radical cation (PE + ) was observed and explanations to reconcile previous results are presented., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Acetamide, a small organic compound containing a peptide bond, was observed in the interstellar medium, but reaction pathways leading to the formation of this prebiotic molecule remain uncertain. We investigated the possible formation of a peptide-like bond from the reaction between acetic acid (CH 3 -COOH) and methylamine (CH 3 -NH 2 ) that were identified in the interstellar medium. From an experimental point of view, a quadrupole/octopole/quadrupole mass spectrometer was used in combination with synchrotron radiation as a tunable source of VUV photons for monitoring the reactivity of selected ions. Acetic acid was photoionized, and the reactivity of CH 3 COOH +• as well as COOH + (produced from either acetic acid or formic acid) ions with neutral CH 3 NH 2 was further studied. With no surprise, charge transfer, proton transfer, and concomitant dissociation processes were found to largely dominate the reactivity. However, a C(O)-N bond formation process between the two reactants was also evidenced, with a weak cross section reaction. From a theoretical point of view, results concerning reactivity and barrier heights were obtained using density functional theory, with the LC-ωPBE range-separated functional in combination with the 6-311++G(d,p) Pople basis set and are in perfect agreement with the experimental data.

The reactivity of CO 2 + with CD 4 has been experimentally investigated for its relevance in the chemistry of plasmas used for the conversion of CO 2 in carbon-neutral fuels. Non-equilibrium plasmas are currently explored for their capability to activate very stable molecules (such as methane and carbon dioxide) and initiate a series of reactions involving highly reactive species (e.g., radicals and ions) eventually leading to the desired products. Energy, in the form of kinetic or internal excitation of reagents, influences chemical reactions. However, putting the same amount of energy in a different form may affect the reactivity differently. In this paper, we investigate the reaction of CO 2 + with methane by changing either the kinetic energy of CO 2 + or its vibrational excitation. The experiments were performed by a guided ion beam apparatus coupled to synchrotron radiation in the VUV energy range to produce vibrationally excited ions. We find that the reactivity depends on the reagent collision energy, but not so much on the vibrational excitation of CO 2 + . Concerning the product branching ratios ( CD 4 + / CD 3 + /DOCO + ) there is substantial disagreement among the values reported in the literature. We find that the dominant channel is the production of CD 4 + , followed by DOCO + and CD 3 + , as a minor endothermic channel.

The reaction of C3N(-) with acetylene was studied using three different experimental setups, a triple quadrupole mass spectrometer (Trento), a tandem quadrupole mass spectrometer (Prague), and the "CERISES" guided ion beam apparatus at Orsay. The process is of astrophysical interest because it can function as a chain elongation mechanism to produce larger anions that have been detected in Titan's ionosphere by the Cassini Plasma Spectrometer. Three major products of primary processes, C2H(-), CN(-), and C5N(-), have been identified, whereby the production of the cyanide anion is probably partly due to collisional induced dissociation. The formations of all these products show considerable reaction thresholds and also display comparatively small cross sections. Also, no strong signals of anionic products for collision energies lower than 1 eV have been observed. Ab initio calculations have been performed to identify possible pathways leading to the observed products of the title reaction and to elucidate the thermodynamics of these processes. Although the productions of CN(-) and C5N(-) are exoergic, all reaction pathways have considerable barriers. Overall, the results of these computations are in agreement with the observed reaction thresholds. Due to the existence of considerable reaction energy barriers and the small observed cross sections, the title reaction is not very likely to play a major role in the buildup of large anions in cold environments like the interstellar medium or planetary and satellite ionospheres.

An experimental study has been carried out on the reactions of state selected O(+)((4)S, (2)D, (2)P) ions with methane with the aims of characterizing the effects of both the parent ion internal energy and collision energy on the reaction dynamics and determining the fate of oxygen species in complex media, in particular the Titan ionosphere. Absolute cross sections and product velocity distributions have been determined for the reactions of (16)O(+) or (18)O(+) ions with CH4 or CD4 from thermal to 5 eV collision energies by using the guided ion beam (GIB) technique. Dissociative photoionization of O2 with vacuum ultraviolet (VUV) synchrotron radiation delivered by the DESIRS beamline at the SOLEIL storage ring and the threshold photoion photoelectron coincidence (TPEPICO) technique are used for the preparation of purely state-selected O(+)((4)S, (2)D, (2)P) ions. A complete inversion of the product branching ratio between CH4(+) and CH3(+) ions in favor of the latter is observed for excitation of O(+) ions from the (4)S ground state to either the (2)D or the (2)P metastable state. CH4(+) and CH3(+) ions, which are by far the major products for the reaction of ground state and excited states, are strongly backward scattered in the center of mass frame relative to O(+) parent ions. For the reaction of O(+)((4)S), CH3(+) production also rises with increasing collision energy but with much less efficiency than with O(+) excitation. We found that a mechanism of dissociative charge transfer, mediated by an initial charge transfer step, can account very well for all the observations, indicating that CH3(+) production is associated with the formation of H and O atoms (CH3(+) + H + O) rather than with OH formation by an hydride transfer process (CH3(+) + OH). Therefore, as the CH4(+) production by charge transfer is also associated with O atoms, the fate of oxygen species in these reactions is essentially the O production, except for the reaction of O(+)((4)S), which also produces appreciable amounts of H2O(+) ions but only at very low collision energy. The production of O atoms and the nature of the states in which they are formed are discussed for the reactions of O(+) ions with CH4 and N2.