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13 pags., 5 figs., 5 tabs., The recent detection of urea in the interstellar medium raises questions about its stability in different astronomical environments. In this work, we have studied the stability of urea ices and urea/water ice mixtures under vacuum-ultraviolet (VUV; 6.3–10.9 eV) irradiation and high-energy (5 keV) electron bombardment at 30, 100, and 200 K. The evolution of the ices was monitored with infrared spectroscopy. CO2, HNCO, and OCN− were identified as reaction products in the 30 K samples. At the higher temperatures CO2 and HNCO were hardly found in the processed ices. The measurements provided destruction cross-sections and allowed the derivation of radiation yields, G100, and half-life doses for urea. G100 values were found to be low (≈3.6–0.3 molecules/100 eV) both for VUV photons and high-energy electrons with electrons being slightly more efficient for the destruction of the molecule. These low G100 values are likely due to favourable mechanisms of energy dissipation or urea recombination. The stability of urea under irradiation increases with temperature which suggests that higher mobility improves the repair mechanisms. Estimates based on these laboratory data indicate that urea should be stable (≈108–109 yr) against irradiation in cold dense clouds and hot cores. It would not survive long (≈103–104 yr) on the bare surface of a Kuiper belt object, but would be well protected (≈109 yr) against radiation below a 30 μm ice layer. The high resistance of the molecule to radiation damage makes it a good candidate for prebiotic chemistry., BM, IT, and VJH acknowledge support from the Ministerio de Economia y Competitividad (MINECO) and the Ministerio de Ciencia e Innovacion´ (MICINN) of Spain under grants FIS2016-77726-C3-1-P and PID2020-113084GB-100 and from the European Union under grant ERC-2013-Syg-210656-NANOCOSMOS. GMMC and HC are grateful for funding from MICINN under grant PID2020-118974GB-C21, and PhD fellowship FPU-17/03172. IJ-S acknowledges financial support from grant No PID2019-105552RBC41 by the Spanish Ministry of Science and Innovation/State Agency of Research MCIN/AEI/10.13039/501100011033. GMMC, HC, and IJ-S also acknowledge support from MDM-2017–0737 Unidad de Excelencia ‘Maria de Maeztu’ – CAB (CSIC-INTA). The i-LINK program (grant LINKA 20353) from CSIC is also acknowledged.

9 pags., 6 figs., Evolved stars are foundries of chemical complexity, gas and dust that provide the building blocks of planets and life, and dust nucleation first occurs in their photosphere. The circumstellar regions enveloping these stars, despite their importance, remain hidden to many observations, and dust formation processes are therefore still poorly understood. Laboratory astrophysics provides complementary routes to unveil these chemical processes, but most experiments rely on combustion or plasma decomposition of molecular precursors under physical conditions far removed from those in space. To reproduce and characterize the bottom-up dust formation process, we have built an ultra-high vacuum machine combining atomic gas aggregation with advanced in situ characterization techniques. We show that carbonaceous dust analogues that formed from low-pressure gas-phase condensation of carbon atoms in a hydrogen atmosphere, in a ratio of carbon to molecular hydrogen similar to that reported for evolved stars, lead to the formation of amorphous carbon nanograins and aliphatic carbon clusters. Aromatic species and fullerenes do not form effectively under these conditions, raising implications for a revision of the chemical mechanisms taking place in circumstellar envelopes., We thank the European Research Council for funding support under Synergy Grant ERC-2013-SyG, G.A. 610256 (NANOCOSMOS). We also acknowledge partial support from the Spanish Research Agency (AEI) through grants MAT2017-85089-c2-1R, FIS2016-77578-R and FIS2016-77726-C3-1-P. Support from the FotoArt-CM Project (P2018/NMT-4367) through the Program of R&D activities between research groups in Technologies 2013, cofinanced by European Structural Funds, is also recognized.

16 pags., 9 figs., 9 tabs. -- Unified Astronomy Thesaurus concepts: Pre-biotic astrochemistry (2079); Astrochemistry (75); Ice destruction (2091); Molecular physics (2058); Molecule destruction (2075); Spectral line identification (2073); Laboratory astrophysics (2004); Theoretical models (2107); Experimental techniques (2078), 2-aminooxazole (2AO), a N-heterocyclic molecule, has been proposed as an intermediate in prebiotic syntheses. It has been demonstrated that it can be synthesized from small molecules such as cyanamide and glycoaldehyde, which are present in interstellar space. The aim of this work is to provide infrared (IR) spectra, in the solid phase for conditions typical of astrophysical environments and to estimate its stability toward UV photons and cosmic rays. IR (4000-600 cm-1) absorption spectra at 20 K, 180 K, and 300 K, IR band strengths, and room-temperature UV (120-250 nm) absorption spectra are given for the first time for this species. Destruction cross sections of ≈9.5 10-18 cm2 and ≈2 10-16 cm2 were found in the irradiation at 20 K of pure 2AO and 2AO:H2O ices with UV (6.3-10.9 eV) photons or 5 keV electrons, respectively. These data were used to estimate half-life times for the molecule in different environments. It is estimated that 2AO could survive UV radiation and cosmic rays in the ice mantles of dense clouds beyond cloud collapse. In contrast, it would be very unstable on the surface of cold solar system bodies like Kuiper Belt objects, but the molecule could still survive within dust grain agglomerates or cometesimals., B.M., V.T., I.T., and V.J.H. are grateful to the Ministerio de Economia y Competitividad (MINECO) of Spain under grant FIS2016-77726-C3-1-P. G.M.M.C., H.C., and C.G.D. acknowledge MINECO support under grant AYA2017–85322-R (AEI/ FEDER, UE), Ph.D. fellowship FPU-17/03172, and MDM2017–0737 Unidad de Excelencia “María de Maeztu”—CAB (CSIC-INTA). I.J.S. acknowledges partial support from the Spanish FEDER (ESP2017-86582-C4-1-R) and the State Research Agency (PID2019-105552RB-C41).

IOP Publishing [Society Publisher] Silicon is present in interstellar dust grains, meteorites and asteroids, and to date 13 silicon-bearing molecules have been detected in the gas phase toward late-type stars or molecular clouds, including silane and silane derivatives. In this work, we have experimentally studied the interaction between atomic silicon and hydrogen under physical conditions mimicking those in the atmosphere of evolved stars. We have found that the chemistry of Si, H, and H2 efficiently produces silane (SiH4), disilane (Si2H6) and amorphous hydrogenated silicon (a-Si:H) grains. Silane has been definitely detected toward the carbon-rich star IRC +10216, while disilane has not been detected in space yet. Thus, based on our results, we propose that gas-phase reactions of atomic Si with H and H2 are a plausible source of silane in C-rich asymptotic giant branch stars, although its contribution to the total SiH4 abundance may be low in comparison with the suggested formation route by catalytic reactions on the surface of dust grains. In addition, the produced a-Si:H dust analogs decompose into SiH4 and Si2H6 at temperatures above 500 K, suggesting an additional mechanism of formation of these species in envelopes around evolved stars. We have also found that the exposure of these dust analogs to water vapor leads to the incorporation of oxygen into Si–O–Si and Si–OH groups at the expense of SiH moieties, which implies that if this kind of grain is present in the interstellar medium, it will probably be processed into silicates through the interaction with water ices covering the surface of dust grains. We thank the European Research Council for funding support under Synergy grant ERC-2013-SyG, G.A. 610256 (NANOCOSMOS). Also, we acknowledge partial support from the Spanish MINECO through grants MAT2017-85089-c2-1R, FIS2016-77726-C3-1-P, FIS2016-77578-R, AYA2016-75066-C2-1-P and RyC-2014-16277. Support from the FotoArt-CM Project (P2018/NMT 4367) through the Program of R&D activities between research groups in Technologies 2013, cofinanced by European Structural Funds, is also acknowledged. G.T.C. acknowledges funding from the Comunidad Autonoma de Madrid (PEJD-2018-PRE/IND-9029). Peerreview

11 pags., 6 figs. -- This article is part of the themed collection: Festschrift for Peter Toennies - New Horizons in the Dynamics of Molecules: from Gases to Surfaces, The excitation functions (reaction cross-section as a function of collision energy) of the F + HD(v= 0, 1;j= 0, 1) benchmark system have been calculated in the 0.01-6 meV collision energy interval using a time-independent hyperspherical quantum dynamics methodology. Special attention has been paid to orbiting resonances, which bring about detailed information on the three-atom interaction during the reactive encounter. The location of the resonances depends on the rovibrational state of the reactants HD(v,j), but is the same for the two product channels HF + D and DF + H, as expected for these resonances that are linked to the van der Waals well at the entrance. The resonance intensities depend both on the entrance and on the exit channels. The peak intensities for the HF + D channel are systematically larger than those for DF + H. Vibrational excitation leads to an increase of the peak intensity by more than an order of magnitude, but rotational excitation has a less drastic effect. It deceases the resonance intensity of the F + HD(v= 1) reaction, but increases somewhat that of F + HD(v= 0). Polarization of the rotational angular momentum with respect to the initial velocity reveals intrinsic directional preferences in the F + HD(v= 0, 1;j= 1) reactions that are manifested in the resonance patterns. The helicities (Ω= 0,Ω= ±1) possible forj= 1 contribute to the resonances, but that fromΩ± 1 is, in general, dominant and in some cases exclusive. It corresponds to a preferential alignment of the HD internuclear axis perpendicular to the initial direction of approach and, thus, to side-on collisions. This work also shows that external preparation of the reactants, following the intrinsic preferences, would allow the enhancement or reduction of specific resonance features, and would be of great help for their eventual experimental detection., This work has been funded by the MINECO and by the the Ministry of Science and innovation of Spain under grants FIS2016-77726-C3-1P and PGC2018-096444-B-I00 respectively. VJH acknowledges also the funding from the EU project ERC2013-Syg 610256 (Nanocosmos). The research was conducted within the Unidad Asociada between the Department of Physical Chemistry of the UCM and the CSIC of Spain.

8 pags., 9 figs., 4 tabs., Urea is a molecule of great interest in chemistry and biology. In particular, it is considered a key building block in prebiotic chemistry on Earth. The hypothesis of its possible exogenous origin has been reinforced after the recent detection of this molecule in the interstellar medium, where it is believed to form in the ice mantles of dust grains. In this work the infrared spectra of urea ices and urea:HO ice mixtures have been studied both experimentally and theoretically. Urea ices were generated by vapour deposition at temperatures between 10 K and 270 K. It was found that an amorphous phase is formed at temperatures below 200 K. A theoretical modelling of crystalline urea and of a tentative amorphous urea solid phase, as well as of amorphous urea:HO ice mixtures, has been performed. The corresponding infrared spectra were simulated with density functional theory. The main spectral features observed in the various solid samples are interpreted with the help of the theoretical results. Infrared band strengths are also provided for amorphous and crystalline urea. The infrared spectroscopic information given in this work is expected to be useful for the detection and quantification of urea in astrophysical ices., This work was funded by the Ministerio de Economia y Competitividad (MINECO) and the Ministerio de Ciencia e Innovacion (MCI) of Spain under grants FIS2016-77726-C3-1-P and PID2020-113084GB-I00, respectively, and by the European Union under grant ERC-2013-Syg-210656-NANOCOSMOS. The computation time provided by the Centro Tecnico de Informatica, cluster Trueno from CSIC and Centro de Supercomputacio´n de Galicia CESGA is deeply acknowledged.

Interstellar carbonaceous dust is mainly formed in the innermost regions of circumstellar envelopes around carbon-rich asymptotic giant branch (AGB) stars. In these highly chemically stratified regions, atomic and diatomic carbon, along with acetylene are the most abundant species after H(2) and CO. In a previous study, we addressed the chemistry of carbon (C and C(2)) with H(2) showing that acetylene and aliphatic species form efficiently in the dust formation region of carbon-rich AGBs whereas aromatics do not. Still, acetylene is known to be a key ingredient in the formation of linear polyacetylenic chains, benzene and polycyclic aromatic hydrocarbons (PAHs), as shown by previous experiments. However, these experiments have not considered the chemistry of carbon (C and C(2)) with C(2)H(2). In this work, by employing a sufficient amount of acetylene, we investigate its gas-phase interaction with atomic and diatomic carbon. We show that the chemistry involved produces linear polyacetylenic chains, benzene and other PAHs, which are observed with high abundances in the early evolutionary phase of planetary nebulae. More importantly, we have found a non-negligible amount of pure and hydrogenated carbon clusters as well as aromatics with aliphatic substitutions, both being a direct consequence of the addition of atomic carbon. The incorporation of alkyl substituents into aromatics can be rationalized by a mechanism involving hydrogen abstraction followed by methyl addition. All the species detected in gas phase are incorporated into the nanometric sized dust analogues, which consist of a complex mixture of sp, sp(2) and sp(3) hydrocarbons with amorphous morphology.

Time-independent, fully converged, quantum dynamical calculations have been performed for the F + HD (v = 0, j = 0) and F + HD (v = 1, j = 0) reactions on an accurate potential energy surface down to collision energies of 0.01 meV. The two isotopic exit channels, HF + D and DF + H, have been investigated. The calculations reproduce satisfactorily the Feshbach resonance structures for collision energies between 10 and 40 meV, previously reported in the literature for the HF + D channel. Contrary to the results of a former literature work, vibrational excitation of HD is found to enhance reactivity in all cases down to the lowest collision energy investigated. Shape-type orbiting resonances are found for collision energies lower than 2 meV. The resonances appear as peaks in the reaction cross sections that are associated to specific values of the total angular momentum, J. In contrast with the Feshbach resonances at higher energies, the orbiting resonance structure, which is caused by the van der Waals well of the entrance channel, is identical for the HF + D and DF + H exit channels. The orbiting resonance peaks for F + HD (v = 0) are very small, but those for F + HD (v = 1) could be observed, in principle, with a combination of Raman pumping and merged beams methods., This work has been funded by the meiyu of Spain under grant PGC2018-09644-B-100 and by the MINECO of Spain under grants , CTQ-2015-65033-P and FIS2016-77726-C3-1P. VJH acknowledges also funding from the EU project ERC-2013-Syg 610256. The research was conducted within the Unidad Asociada between the Department of Physical Chemistry of the UCM and the CSIC of Spain

2 págs., 1 fig. -- Proceedings IAU Symposium 2019, The desorption of volatile molecules from dust grains in cold dense clouds is crucial for the chemical inventory in the various stages of cloud collapse. In this work we investigate the desorption of N2, CO, CH4 and CO2 from surfaces of hydrogenated amorphous carbon (HAC), which, according to IR observations, is one of the main components of interstellar dust., This work was funded by the EU, ERC-2013-Syg-210656-NANOCOSMOS and by the MINECO of Spain, Grant FIS 2016-77726-C3-1-P.

8 pags., 5 figs., 5 tabs., 1 app., Context. Dinitriles with a saturated hydrocarbon skeleton and a −C≡N group at each end can have large electric dipole moments. Their formation can be related to highly reactive radicals such as CH2CN, C2N, or CN. Thus, these saturated dinitriles are potential candidates to be observed in the interstellar medium (ISM). Aims. Our goal is the investigation of the rotational spectrum of one of the simplest dinitriles N≡C−CH2−CH2−C≡N, succinonitrile, whose actual rotational parameters are not precise enough to allow its detection in the ISM. In addition, the rotational spectra for its vibrational excited states will be analysed. Methods. The rotational spectra of succinonitrile was measured in the frequency range 72−116.5 GHz using a new broadband millimeter-wave spectrometer based on radio astronomy receivers with Fast Fourier Transform backends. The identification of the vibrational excited states of succinonitrile was supported by high-level ab initio calculations on the harmonic and anharmonic force fields. Results. A total of 459 rotational transitions with maximum values of J and Ka quantum numbers 70 and 14, respectively, were measured for the ground vibrational state of succinonitrile. The analysis allowed us to accurately determine the rotational, quartic, and sextic centrifugal distortion constants. Up to eleven vibrational excited states, resulting from the four lowest frequency vibrational modes ν13, ν12, ν24, and ν23 were identified. In addition to the four fundamental modes, we observed overtones together with some combination states. The rotational parameters for the ground state were employed to unsuccessfully search for succinonitrile in the cold and warm molecular clouds Orion KL, Sgr B2(N), B1-b, and TMC-1, using the spectral surveys captured by IRAM 30 m at 3 mm and the Yebes 40 m at 1.3 cm and 7 mm., We thank the European Research Council for funding support under Synergy Grant ERC-2013-SyG, G.A. 610256 (NANOCOSMOS). CB would like to thank the Ministerio de Ciencia, Innovación y Universidades for a Juan de la Cierva postdoctoral fellowship (FJCI-2016-27983). IT, VJH, and JLD acknowledge additional partial support from the Spanish State Research Agency (AEI) through grant FIS2016-77726-C3-1-P. This work was also based on observations carried out with the IRAM 30-meter telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).

13 pags., 7 figs., 1 tab.-- Published as part of The Journal of Physical Chemistry virtual special issue “F. Javier Aoiz Festschrift”., The ionic polymerization of acetylene in cold plasmas of C2H2/He and C2H2/Ar has been experimentally studied and modeled in radio frequency (rf) discharges with conditions selected to avoid particle formation. Steady-state distributions of positive and negative ions were measured with mass spectrometry. All the measured distributions are dominated by ions with an even number of carbon atoms, reflecting the characteristic polyyne structures typical for the polymerization of acetylene. The distributions show a monotonic decrease in intensity from ions with two carbon atoms until the highest number of atoms detected. For cations, the distributions extend until 12 carbon atoms. The anion distributions extend further, and negative ions with 20 C atoms are observed in the C2H2/Ar plasma. From the measured mass spectra it is not possible to decide on the possible presence of aromatic species in ions with more than six carbon atoms. A simple model assuming a homogeneous discharge was used to describe the plasma kinetics and could account for the measured ion distributions with reasonable values of charge density and electron temperature. The results of this work stress the important role of the vinylidene anion and indicate that Ar and He do not have much influence on the carbon chemistry., Funding from EU project ERC-2013-Syg-610256 NANOCOSMOS and from the MINECO of Spain under grant FIS2016-77726-C3-1P is gratefully acknowledged.

74th ISMS 2019, University of Illinois, at Urbana-Champaign, June 17 - 21, 2019

25th International Conference on High Resolution Molecular Spectroscopy. 3 al 7 de septiembre en el Bizkaia Aretoa (Bilbao, Spain)(2018). --.http://www.hrms-bilbao2018.com/

13 pags., 12 figs., 2 tabs., Interstellar carbonaceous dust is mainly formed in the innermost regions of circumstellar envelopes around carbon-rich asymptotic giant branch stars (AGBs). In these highly chemically stratified regions, atomic and diatomic carbon, along with acetylene, are the most abundant species after H and CO. In a previous study, we addressed the chemistry of carbon (C and C) with H showing that acetylene and aliphatic species form efficiently in the dust formation region of carbon-rich AGBs whereas aromatics do not. Still, acetylene is known to be a key ingredient in the formation of linear polyacetylenic chains, benzene, and polycyclic aromatic hydrocarbons (PAHs), as shown by previous experiments. However, these experiments have not considered the chemistry of carbon (C and C) with CH. In this work, by employing a sufficient amount of acetylene, we investigate its gas-phase interaction with atomic and diatomic carbon. We show that the chemistry involved produces linear polyacetylenic chains, benzene, and other PAHs, which are observed with high abundances in the early evolutionary phase of planetary nebulae. More importantly, we have found a nonnegligible amount of pure and hydrogenated carbon clusters as well as aromatics with aliphatic substitutions, both being a direct consequence of the addition of atomic carbon. The incorporation of alkyl substituents into aromatics can be rationalized by a mechanism involving hydrogen abstraction followed by methyl addition. All the species detected in the gas phase are incorporated into nanometric-sized dust analogs, which consist of a complex mixture of sp, sp, and sp hydrocarbons with amorphous morphology., We thank the European Research Council for funding support under Synergy grant ERC-2013-SyG, G.A. 610256 (NANOCOSMOS). Also, partial support from the Spanish Research Agency (AEI) through grants MAT2017-85089-c2- 1R, FIS2016-77578-R, FIS2016-77726-C3-1-P, AYA2016- 75066-C2-1-P, and RyC-2014-16277 is acknowledged. Support from the FotoArt-CM Project (P2018/NMT 4367) through the Program of R&D activities between research groups in Technologies 2013, co-financed by European Structural Funds, is also acknowledged. G.TC. acknowledges funding from the Comunidad Autónoma de Madrid (PEJD-2018-PRE/ IND-9029)

Champaign-Urbana, Illinois, United States, 17 - 21 June 2019

12 pags., 8 figs., 3 tabs., The interaction of volatile species with carbonaceous interstellar dust analogues is of relevance in the chemistry and physics of dense clouds in the interstellar medium. Two deposits of hydrogenated amorphous carbon (HAC), with different morphologies and aromatic versus aliphatic ratio in their structure, have been grown to model interstellar dust. The interaction of N2, CO, CH4, and CO2 with these two surfaces has been investigated using thermal programmed desorption (TPD). Desorption energy distributions were obtained by analysing TPD spectra for one monolayer coverage with the Polanyi–Wigner equation. The desorption energies found in this work for N2, CO, and CH4 are larger by 10–20 per cent than those reported in the literature for siliceous or amorphous solid water surfaces. Moreover, the experiments suggest that the interaction of the volatiles with the aromatic substructure of HAC is stronger than that with the aliphatic part. Desorption of CO2 from the HAC surfaces follows zero-order kinetics, reflecting the predominance of CO2–CO2 interactions. A model simulation of the heating of cold cloud cores shows that the volatiles considered in this work would desorb sequentially from carbonaceous dust surfaces with desorption times ranging from hundreds to tens of thousands of years, depending on the molecule and on the mass of the core, This work was funded by the Ministerio de Economia y Competitividad (MINECO) of Spain under grant FIS2016-77726-C3-1-P and by the European Union under grant ERC-2013-Syg-210656- NANOCOSMOS

8 pags. 6 figs., One of the most relevant features of the O(P) + H reaction is that it occurs on two different potential energy surfaces (PESs) of symmetries A′ and A′′ that correlate reactants and products. The respective saddle points, which correspond to a collinear arrangement, are the same for both PESs, whilst the barrier height rises more abruptly on the A′ PES than on the A′′ PES. Accordingly, the reactivity on the A′′ PES should be always higher than on the A′ PES. In this work, we present accurate quantum-scattering calculations showing that this is not always the case for rotationless reactants, where dynamical factors near the reaction threshold cause the A′ PES to dominate at energies around the barrier. Further calculation of cross sections and Λ-doublet populations has allowed us to establish how the reaction mechanism changes from the deep tunneling regime to hyperthermal energies., MINECO/FEDER-CTQ2015-65033-P, and PGC2018-096444-B-I00 and FIS2016-77726-C3-1-P) is also acknowledged. P.G.J. and A. Z. acknowledge funding by Fundación Salamanca City of Culture and Knowledge (programme for attracting scientific talent to Salamanca).

13 pags., 8 figs. -- Open Access funded by Creative Commons Atribution Licence 4.0, The increasing demand for nanostructured materials is mainly motivated by their key role in a wide variety of technologically relevant fields such as biomedicine, green sustainable energy or catalysis. We have succeeded to scale-up a type of gas aggregation source, called a multiple ion cluster source, for the generation of complex, ultra-pure nanoparticles made of different materials. The high production rates achieved (tens of g/day) for this kind of gas aggregation sources, and the inherent ability to control the structure of the nanoparticles in a controlled environment, make this equipment appealing for industrial purposes, a highly coveted aspect since the introduction of this type of sources. Furthermore, our innovative UHV experimental station also includes in-flight manipulation and processing capabilities by annealing, acceleration, or interaction with background gases along with in-situ characterization of the clusters and nanoparticles fabricated. As an example to demonstrate some of the capabilities of this new equipment, herein we present the fabrication of copper nanoparticles and their processing, including the controlled oxidation (from Cu to CuO through CuO, and their mixtures) at different stages in the machine., This work was supported by the European Union [grant number ERC-2013-SyG 610256 NANOCOSMOS]; the Spanish MINECO [grant numbers MAT2017-85089-C2-1-R, MAT2014-54231-C4-1-P, MAT2014-54231- C4-4-P, MAT2014-59772-C2-2-P, FIS2016-77578-R, FIS2013-48087-C2-1P, FIS2016-77726-C3-1P and CSIC13-4E-1775].

10 pags., 9 figs., 2 tabs., Interstellar (IS) dust analogs, based on amorphous hydrogenated carbon (a-C:H) were generated by plasma deposition in radio frequency discharges of CH4 + He mixtures. The a-C:H samples were characterized by means of secondary electron microscopy, infrared (IR) spectroscopy and UV-visible reflectivity. DFT calculations of structure and IR spectra were also carried out. From the experimental data, atomic compositions were estimated. Both IR and reflectivity measurements led to similar high proportions (≈50%) of H atoms, but there was a significant discrepancy in the sp2/sp3 hybridization ratios of C atoms (sp2/sp3 = 1.5 from IR and 0.25 from reflectivity). Energetic processing of the samples with 5 keV electrons led to a decay of IR aliphatic bands and to a growth of aromatic bands, which is consistent with a dehydrogenation and graphitization of the samples. The decay of the CH aliphatic stretching band at 3.4 μm upon electron irradiation is relatively slow. Estimates based on the absorbed energy and on models of cosmic ray (CR) flux indicate that CR bombardment is not enough to justify the observed disappearance of this band in dense IS clouds., This work was funded by the Spanish MINECO under grants FIS2013-48087-C2-1P and FIS2016-77726-C3-1P. European Union funding under the project ERC-2013-Syg 610256 (NANOCOSMOS) is also acknowledged.

10 págs.; 10 figs.; 1 tab.; Special Issue: Dynamics of Molecular Collisions XXV: Fifty Years of Chemical Reaction Dynamics, We have analyzed the influence of the rotational excitation on the H + D(v = 0, j) reaction through quantum mechanical (QM) and quasiclassical trajectories (QCT) calculations at a wide range of total energies. The agreement between both types of calculations is excellent. We have found that the rotational excitation largely increases the reactivity at large values of the total energy. Such an increase cannot be attributed to a stereodynamical effect but to the existence of recrossing trajectories that become reactive as the target molecule gets rotationally excited. At low total energies, however, recrossing is not significant and the reactivity evolution is dominated by changes in the collision energy; the reactivity decreases with the collision energy as it shrinks the acceptance cone. When state-to-state results are considered, rotational excitation leads to cold products rovibrational distributions, so that most of the energy is released as recoil energy., The authors acknowledge funding by the Spanish Ministry of Science and Innovation (grant Consolider Ingenio 2010 CSD2009-00038). J.A., F.J.A. and P.G.J. acknowledge also funding by the Spanish Ministry of Economy and Competitiveness (grant CTQ2012-37404-C02), and V.J.H. acknowledges additional funding by the Spanish Ministry of Science and Innovation (FIS2013-48087-C2-1P) and by the European Research Council (ERC-2013-Syg-610256).

I. Tanarro et al. -- 16 pags., 18 figs., app., We present a proof of concept on the coupling of radio astronomical receivers and spectrometers with chemical reactors and the performances of the resulting setup for spectroscopy and chemical simulations in laboratory astrophysics. Several experiments including cold plasma generation and UV photochemistry were performed in a 40 cm long gas cell placed in the beam path of the Aries 40 m radio telescope receivers operating in the 41–49 GHz frequency range interfaced with fast Fourier transform spectrometers providing 2 GHz bandwidth and 38 kHz resolution. The impedance matching of the cell windows has been studied using di erent materials. The choice of the material and its thickness was critical to obtain a sensitivity identical to that of standard radio astronomical observations. Spectroscopic signals arising from very low partial pressures of CH3OH, CH3CH2OH, HCOOH, OCS, CS, SO2 (, The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC-SyG-2013 Grant Agreement No. 610256 NANOCOSMOS and from spanish MINECO CSD2009-00038 (ASTROMOL) under the Consolider-Ingenio Program. We also thank spanish MINECO for funding under grants AYA2012-32032, AYA2016-75066-C2-1-P, FIS2013-48087-C2-1-P, FIS2016-77726-C3-1-P, FIS2016-77578-R, MAT2014- 54231-C4-1-P.

International Symposium on Molecular Spectroscopy. 73rd Meeting - June 18-22, 2018 - Champaign-Urbana, Illinois (USA). .--http://isms.illinois.edu / http://hdl.handle.net/2142/100503

Carbonaceous grains make a large part of cosmic dust and contain a significant fraction of the carbon available in the interstellar medium (ISM). Observational evidence for the presence of carbonaceous grains comes mostly from IR absorption spectra. The most intense IR absorption is the 3.4 μm band which is ubiquitous in the diffuse ISM and is also present in other galaxies. It is attributed to CH stretching vibrations of CH2 and CH3 groups; weaker absorptions at 6.85 and 7.27 μm are due to aliphatic bendings, and a broad, weak absorption between 6 and 6.4 μm is assigned to C=C stretching. These observations, together with laboratory work on dust analogs, point to amorphous hydrogenated carbon (abbreviated a-C:H or HAC), as the most likely carrier of these IR bands, and thus as the predominant component of carbonaceous grains; however, the H/C ratio and the proportion of sp2 and sp3 carbon atoms are presently debated. Emission IR bands at 3.3, 6.2, 7.7, 8.6 and 11.2 μm are also seen at many places in the ISM. They have been associated with stretching and bending vibrations of aromatic material and are often referred to as aromatic IR bands (AIB). They are mostly attributed to polycyclic aromatic hydrocarbons (PAHs). Refinements to the original PAH hypothesis including the presence of heteroatoms, aliphatic chains or non-planar defective π domains have been introduced to account for the presence of aliphatic emissions and for the observed band shapes. It is conjectured that the PAHs or PAHs-related molecules responsible for the IR emissions are related with the a-C:H structures responsible for dust absorption either as formation precursors or as destruction products. IR spectroscopy provides also clues on grain evolution. It is generally assumed that carbonaceous grains are formed as nanoparticles in the outflows of carbon-rich AGB stars and evolve from this point. When they reach the diffuse ISM, the grains are subjected to H atom bombardment, which leads to the formation of aliphatic structures, and to UV photon and cosmic ray irradiation that destroy aliphatic structures and lead to graphitization. A core/mantel structure is often assumed for the grains in the diffuse ISM, but models differ on whether the aliphatic chains responsible for the 3.4 μm band are located in the core or in the mantle and present day observations cannot decide on that point. In dense clouds the 3.4 μm band disappears. It is believed that the aliphatic carriers of this band are destroyed somewhere in the transition from the diffuse to the dense ISM, since in the interior of dense clouds UV and cosmic ray fluxes are too weak.

Methyl isocyanate (CH3NCO) was recently found in hot cores and suggested to exist on comet 67P/CG. The incorporation of this molecule into astrochemical networks requires data on its formation and destruction. In this work, ices of pure CH3NCO and of CH3NCO(4%–5%)/H2O mixtures deposited at 20 K were irradiated with a UV D2 lamp (120–400 nm) and bombarded by 5 keV electrons to mimic the secondary electrons produced by cosmic rays (CRs). The destruction of CH3NCO was studied using IR spectroscopy. After processing, the νa–NCO band of CH3NCO disappeared and IR bands corresponding to CO, CO2, OCN−, and HCN/CN− appeared instead. The products of photon and electron processing were very similar. Destruction cross sections and half-life doses were derived from the measurements. Water ice provides a good shield against UV irradiation (half-life dose of ∼64 eV molecule−1 for CH3NCO in water ice), but is not so good against high-energy electrons (half-life dose ∼18 eV molecule−1). It was also found that CH3NCO does not react with H2O over the temperature range 20–200 K. These results indicate that hypothetical CH3NCO in the ices of dense clouds should be stable against UV photons and relatively stable against CRs over the lifetime of a cloud (∼107 yr), and could sublime in the hot core phase. On the surface of a Kuiper Belt object (the original location of comet 67P/CG) the molecule would be swiftly destroyed, by both photons and CRs, but embedded below just 10 μm of water ice, the molecule could survive for ∼109 yr., This work has been funded by the Spanish MINECO under grants FIS2013-48087-C2-1P, FIS2016-C3-1P, and AYA2016- 75066-C2-1-P. J.-C.G. and J.C. also thank the ANR-13-BS05- 0008 IMOLABS and J.-C.G. thanks the Program PCMI (INSU-CNRS) and the Centre National d’Etudes Spatiales (CNES) for funding support. European Union funding under grant ERC-2013-Syg 610256 (NANOCOSMOS) is also acknowledged.

11 págs.; 5 figs.; 6 tabs., A theoretical model of hydrogenated amorphous carbon (HAC) is developed and applied to study the interaction of glycine with HAC surfaces at astronomical temperatures. Two models with different H content are tried for the HAC surface. The theory is applied at the Density Functional Theory (DFT) level, including a semiempirical dispersion correlation potential, d-DFT or Grimme DFT-D2. The level of theory is tested on glycine adsorption on a Si(001) surface. Crystalline glycine is also studied in its two stable phases, a and b, and the metastable g phase. For the adsorption on Si or HAC surfaces, molecular glycine is introduced in the neutral and zwitterionic forms, and the most stable configurations are searched. All theoretical predictions are checked against experimental observations. HAC films are prepared by plasma enhanced vapor deposition at room temperature. Glycine is deposited at 20 K into a high vacuum, cold temperature chamber, to simulate astronomical conditions. Adsorption takes place through the acidic group COO and when several glycine molecules are present, they form H-bond chains among them. Comparison between experiments and predictions suggests that a possible way to improve the theoretical model would require the introduction of aliphatic chains or a polycyclic aromatic core. The lack of previous models to study the interaction of amino-acids with HAC surfaces provides a motivation for this work. © the Owner Societies 2015, This research has been carried out with funding from Spanish MINECO, Projects FIS2013-48087-C2-1-P and CDS2009-00038, and EU project ERC-2013-Syg 610256. O. G. acknowledges Ramón y Cajal Program and project CGL2013-48415-C2-1-R. Some calculations were performed at CESGA and TRUENO.

9 pags., 5 figs., 6 tabs., Methyl isocyanate has been recently detected in comet 67P/Churyumov–Gerasimenko (67P/CG) and in the interstellar medium. New physicochemical studies on this species are now necessary as tools for subsequent studies in astrophysics. In this work, infrared spectra of solid CH3NCO have been obtained at temperatures of relevance for astronomical environments. The spectra are dominated by a strong, characteristic multiplet feature at 2350–2250 cm−1, which can be attributed to the asymmetric stretching of the NCO group. A phase transition from amorphous to crystalline methyl isocyanate is observed at ∼90 K. The band strengths for the absorptions of CH3NCO in ice at 20 K have been measured. Deuterated methyl isocyanate is used to help with the spectral assignment. No X-ray structure has been reported for crystalline CH3NCO. Here we advance a tentative theoretical structure, based on density functional theory (DFT) calculations, derived taking the crystal of isocyanic acid as a starting point. A harmonic theoretical spectrum is then calculated for the proposed structure and compared with the experimental data. A mixed ice of H2O and CH3NCO was formed by simultaneous deposition of water and methyl isocyanate at 20 K. The absence of new spectral features indicates that methyl isocyanate and water do not react appreciably at 20 K, but form a stable mixture. The high CH3NCO/H2O ratio reported for comet 67P/CG, and the characteristic structure of the 2350–2250 cm−1 band, makes it a very good candidate for future astronomical searches., This work was funded by the European Union under grant ERC-2013-Syg 610256 (NANOCOSMOS) and by the Spanish MINECO CSD2009-00038 (ASTROMOL) within the Consolider–Ingenio Program. MINECO funding from grants FIS2013-48087-C2-1P, FIS2016-C3-1P, AYA2012- 32032 and AYA2016-75066-C2-1-P is also acknowledged. G. M. acknowledges MINECO PhD grant BES-2014-0693. JCG and JC also thank the ANR-13-BS05-0008 IMOLABS and JCG thanks the Program PCMI (INSU-CNRS) and the Centre National d’Etudes Spatiales (CNES) for funding support

9 págs.; 8 figs.; 3 tabs., 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. 2016 The Authors; Published by Oxford University Press on behalf of the Royal Astronomical Society, This work has been funded by the Ministerio de Ciencia y Competitividad (MINECO) of Spain under grants FIS2013-48087-C2-1P, FIS2013-48087-C2-2P and AYA2015-71975-REDT ‘Polvo Cosmico’ by the Ministerio de Ciencia e Innovacion of Spain under grant CDS2009-00038 and by the European Research Council project ERC-2013-Syg 610256 ‘Nanocosmos’. GM acknowledges MINECO PhD grant BES-2014-069355.

The angular momentum polarization of the products of the reaction F + H2(v = 0, j = 0) → HF(v′) + H is calculated via the QCT methodology at a collision energy of 0.119 eV. The HF rotational angular momentum distribution is found to display both alignment and orientation, the latter along the y-axis, perpendicular to the k-k′ scattering plane, which depend sensitively on the product vibrational level. The origin of polarization behaviour is traced back to different dynamical mechanisms leading to production of HF(v′ = 0), and to a lesser extent HF(v′ =1), compared with higher product vibrational states, with the former originating primarily from repulsive insertion type trajectories, and the latter primarily from repulsive abstraction type trajectories. 1997 Elsevier Science B.V., Financial support of the Spanish Ministry of Education, through the DGICYT grant PB9509 1%CO-2, the British Council and Spanish and German Ministries for the award of British-Spanish and German-Spanish “Acciones Integradas” grants.

8 pags., 6 figs., The dynamics of the D + MuH(v = 1) reaction has been investigated using time-independent quantum mechanical calculations. The total reaction cross sections and rate coefficients have been calculated for the two exit channels of the reaction leading, respectively, to DMu + H and DH + Mu. Over the 100-1000 K temperature range investigated the rate coefficients for the DMu + H channel are of the order of 10 cm s and those for the DH + Mu channel vary between 1 × 10 and 8 × 10 cm s. These results point to a virtually barrierless reaction for the DMu + H channel and to the presence of a comparatively small barrier for the DH + Mu channel and are consistent with the profiles of their respective collinear vibrationally adiabatic potentials (VAPs). The effective barrier in the VAP of the DH + Mu channel is located in the reactant valley and, consequently, translation is found to be more efficient than vibration for the promotion of the reaction over a large energy interval in the post threshold region. Below this barrier, the DH + Mu channel can be accessible through an indirect mechanism implying crossing from the DMu + H pathway. The most salient feature found in the present study is revealed in the total reaction cross section for the DMu + H channel, which shows a sharp resonance caused by the presence of a deep well in the vibrationally adiabatic potential. This well has a dynamical origin, reminiscent of that found recently in the vibrationally bonded BrMuBr complex [Fleming, et al., Angew. Chem., Int. Ed., 2014, 53, 1], and is due to the stabilizing effect of the light Mu atom oscillating between the heavier H and D isotopes and to the bond softening associated with vibrational excitation of MuH., This work has been funded by the Spanish Ministry of Science and Innovation under grants CSD2009-00038 and by the MINECO of Spain under grant FIS2013-48087-C2-1P, CTQ2012-37404, CTQ-2015-65033-P. VJH acknowledges also funding from the EU project ERC-2013-Syg 610256. The research was conducted within the Unidad Asociada Química Física Molecular between the UCM and the CSIC of Spain.

The 6.85 {mu}m band observed in the spectra of young stellar objects has been analyzed recently and the most usually accepted assignment to the nu{sub 4} bending mode of NH{sub 4} {sup +} is still under debate. We present here a laboratory study of frozen solutions of NH{sub 4} {sup +}Cl{sup -} in water in an astrophysical range of concentrations and temperatures. The samples are prepared by hyper-quenching of liquid droplets on a cold substrate. The nu{sub 4} band of NH{sub 4} {sup +}, which is very strong in the pure crystal and in the liquid solution at ambient temperature, becomes almost blurred in IR spectra of the frozen solution. The effect of the chlorine anion is expected to be of little relevance in this study. The experimental results are supported by theoretical calculations, which predict a broad range of weak nu{sub 4} features for amorphous samples containing different ammonium environments. The present results indicate that the ammonium ion surrounded by water molecules only cannot suffice to explain this spectral feature. This paper contributes with new evidence to the discussion on the assignment of the 6.85 {mu}m band.

The interaction of carbon dioxide and amorphous water ice at 95 K is studied using transmission infrared spectroscopy. Samples are prepared in two ways: co-deposition of the gases admitted simultaneously or sequential deposition, in which amorphous water ice (ASW) is grown first and CO 2 vapor is added subsequently. In either case, a fraction of the CO 2 molecules is found to interact with water in a way that gives rise to shifts and splittings in the infrared bands with respect to those of a pure CO 2 solid. In co-deposition experiments, a larger amount of carbon dioxide is trapped within the amorphous water than in sequential deposition samples, where a substantial proportion of molecules appears to be trapped in macropores of the ASW. The specific surface area of sequential samples is evaluated and compared to previous literature results. When the sequential samples are heated to 140 K, beyond the onset temperature at which water ice undergoes a phase transition, the CO 2 molecules at the pores relocate inside the bulk in a structure similar to that found in co-deposited samples, as deduced by changes in the shape of the CO 2 infrared bands.

9 págs; 5 figs.; 6 tabs., The effects of cosmic rays on the carriers of the interstellar 3.4 μm absorption band have been investigated in the laboratory. This band is attributed to stretching vibrations of CH3 and CH2 in carbonaceous dust. It is widely observed in the diffuse interstellar medium, but disappears in dense clouds. Destruction of CH3 and CH2 by cosmic rays could become relevant in dense clouds, shielded from the external ultraviolet field. For the simulations, cules, formed by the recombination of H atoms liberated through CH bond breaking, diffuse out of the sample. The CH bond destruction rates derived from the present experiments are in good accordance with those from previous ion irradiation experiments of HAC. The experimental simplicity of electron the 3.4 μm band carriers lie in the 108 yr range and cannot account for the disappearance of this band in dense clouds, which have characteristic lifetimes of 3×107 yr. The results invite a more detailed investigation of the mechanisms of CH bond formation and breaking in the intermediate region between diffuse and dense clouds., This work has been funded by the MINECO of Spain under grant FIS2013-48087-C2-1P, by the MICINN of Spain under grant CDS2009-00038, and by the European project ERC-2013-Syg 610256. G.M. acknowledges MINECO PhD grant BES-2014-069355.

The chloroniumyl cation, HCl+, has been recently identified in space from Herschel’s spectra. A joint analysis of extensive vis-UV spectroscopy emission data together with a few high-resolution and high-accuracy millimiter-wave data provided the necessary rest frequencies to support the astronomical identification. Nevertheless, the analysis did not include any infrared (IR) vibration-rotation data. Furthermore, with the end of the Herschel mission, infrared observations from the ground may be one of the few available means to further study this ion in space. In this work, we provide a set of accurate rovibrational transition wavenumbers as well as a new and improved global fit of vis-UV, IR and millimiter-wave spectroscopy laboratory data, that will aid in future studies of this molecule.

The recent discovery of ArH+ in the interstellar medium has awakened the interest in the chemistry of this ion. In this work, the ion-molecule kinetics of cold plasmas of Ar/H2 is investigated in glow discharges spanning the whole range of [H2]/([H2]+[Ar]) proportions for two pressures, 1.5 and 8 Pa. Ion concentrations are determined by mass spectrometry, and electron temperatures and densities, with Langmuir probes. A kinetic model is used for the interpretation of the results. The selection of experimental conditions evinces relevant changes with plasma pressure in the ion distributions dependence with the H2 fraction, particularly for the major ions: Ar+, ArH+ and H3+. At 1.5 Pa, ArH+ prevails for a wide interval of H2 fractions: 0.3 0.1. The analysis of the data with the kinetic model allows the identification of the sources and sinks of the major ions over the whole range of experimental conditions sampled. Two key factors turn out to be responsible for the different ion distributions observed: the electron temperature, which determines the rate of Ar+ formation and thus of ArH+, and the equilibrium ArH+ + H2 ⇄ H3+ + Ar, which can be strongly dependent of the degree of vibrational excitation of H3+. The results are discussed and compared with previously published data on other Ar/H2 plasmas.

Oral presentation given at the 70th International Symposium on Molecular Spectroscopy, held on June 22-26th, 2015 at The University of Illinois at Urbana-Champaign (United States)., HCl^+ is a key intermediate in the interstellar chemistry of chlorine. It has been recently identified in space from Herschel's spectra and it has also been detected in the laboratory through its optical emission, infrared and mm-wave spectra. Now that Hershchel is decomissioned, further astrophysical studies on this radical ion will likely rely on ground-based observations in the mid-infrared. We have used a difference frequency laser spectrometer coupled to a hollow cathode discharge to measure the absorption spectrum of H35Cl^+ and H37Cl^+ in the v=0-1 band of the ^2Π state with Dopppler limited resolution. The accuracy of the individual measurements (˜ 10 MHz (3σ)) relies on a solid state wavemeter referenced to an iodine-stabilized Ar^+ laser. The new data are being fit using the CALPGM software from JPL, and the current status will be presented.

Astrophysical Ices in the Lab, Workshop, Instituto de Estructura de la Materia, IEM-CSIC, Madrid (Spain), 7-8 March 2016 ; http://www.iem.csic.es/fismol/AILab/program.htm

26 pags. ; 2 tabs. ; 6 figs., The chemistry of low pressure H2 + O2 discharges with different mixture ratios has been studied in a hollow cathode DC reactor. Neutral and positive ion distributions have been measured by mass spectrometry, and Langmuir probes have been used to provide charge densities and electron temperatures. A simple zero order kinetic model including neutral species and positive and negative ions, which takes into account gas-phase and heterogeneous chemistry, has been used to reproduce the global composition of the plasmas over the whole range of mixtures experimentally studied, and allows for the identification of the main physicochemical mechanisms that may explain the experimental results. To our knowledge, no combined experimental and modelling studies of the heavy species kinetics of low pressure H2 + O2 plasmas including ions has been reported before. As expected, apart from the precursors, H2O is detected in considerable amounts. The model also predicts appreciable concentrations of H and O atoms and the OH radical. The relevance of the metastable species O(1D) and O2(a1Δg) is analysed. Concerning the charged species, positive ion distributions are dominated by H3O+ for a wide range of intermediate mixtures, while H3 + and O2 + are the major ions for the higher and lower H2/O2 ratios, respectively. The mixed ions OH+, H2O+ and HO2 + are also observed in small amounts. Negative ions are shown to have a limited relevance in the global chemistry; their main contribution is the reduction of the electron density available for electron impact processes. © 2015 IOP Publishing, This work has been funded by the MCINN of Spain under grants FIS2010-16455, FIS2013- 48087-C2-1-P and the Consolider Astromol, CDS2009-00038. We thank also the European Research Council for additional support under ERC-2013-Syg 610256-NANOCOSMOS. MJR has received funding from the FPI program of the MCINN, and EC from the JdC program of the same ministry.

Theoretical absorption spectra of HCl trihydrate, HCl hexahydrate, and HNO 3 trihydrate in the mid-IR have been calculated using the S iesta suite of programs, a DFT method especially designed for periodic systems of large size. Infrared intensities are obtained from the macroscopic polarization changes per normal mode. The theoretical results can account globally for the observed experimental features. In all spectra, the largest absorption intensities are due to stretching vibrations of hydrated oxonium ions. Oxonium stretching absorptions appear as broad bands in the experimental spectra, with band widths growing with increasing number of water molecules coordinated to the H 3 O + ion.

This paper focuses on recent progress in the understanding of the H + H2 reaction and its isotopic variants. The detailed agreement between theory and experiment attained during the last years is emphasized and major experimental and theoretical advances are highlighted. The excellent description of most experimental findings, from state-resolved cross-sections to thermal rate constants, provided by the available quantum mechanical (QM) treatments, as well as the good overall behaviour of classical mechanics are underlined. Debated issues on short-lived complexes and delayed scattering, resonances and interferences, geometric-phase (GP) effects, or product rotational distributions are discussed. Finally, some prospects for future research on this prototypic system are presented.

The low-temperature rotational relaxation of CO in self-collisions and in collisions with the rare-gas atoms Ne and He has been investigated in supersonic expansions with a combination of resonance-enhanced multiphoton ionization (REMPI) spectroscopy and time-of-flight techniques. For the REMPI detection of CO, a novel 2 + 1′ scheme has been employed through the A1 state of CO. From the measured data, average cross sections for rotational relaxation have been derived as a function of temperature in the range 5-100 K. For CO-Ne and CO-He, the relaxation cross sections grow, respectively, from values of ∼20 and 7 Å2 at 100 K to values of ∼65-70 and ∼20 A°2 in the 5-20 K temperature range. The cross section for the relaxation of CO-CO grows from a value close to 40 Å2 at 100 K to a maximum of 60 Å2 at 20 K and then decreases again to 40 Å2 at 5 K. These results are qualitatively similar to those obtained previously with the same technique for N2-N 2, N2-Ne, and N2-He collisions, although in the low-temperature range (T < 20 K) the CO relaxation cross sections are significantly larger than those for N2. Some discrepancies have been found between the present relaxation cross sections for CO-CO and CO-He and the values derived from electron-induced fluorescence experiments. © 2005 American Chemical Society., Financial support through the Ramón y Cajal program. Financial support from the EU Research Training Network “Reaction Dynamics”, Grant HPRNCT-1999-00007.Financial support from the Spanish Ministry of Education through the State Secretary of Education and University. Support from the Plan Andaluz de Investigación (Group FQM-205). Funding by the MEC of Spain under Grants FIS2004-00456 and BQU2002-04627-C02-02

9 pages, 7 figures, 1 table.-- Printed version published Jul 21, 2005., The neutral and charged species present in a direct current (dc) hollow cathode, gas flow, air reactor are experimentally studied by quadrupole mass spectrometry. The degree of ionization of the plasma and the electron mean temperature with decreasing air pressure, for constant discharge current, are measured with a double Langmuir probe. The chemical composition of the plasma changes appreciably over the 3x10(-3) to 5x10(-2) mbar range investigated: at the lowest pressures studied, O2 dissociation is up to 60% and the concentration of NO is half that of N2; concerning ions, NO+ and N2+ are dominant for the whole pressure range. A kinetic model of the plasma including electrons, neutrals, and positive ions is developed to account for the experimental observations; it is consistent with energy balance and predicts that heterogeneous processes are the main source of NO and that the contribution of ions to the global chemistry of neutrals is of minor significance even for the lowest pressures., The SEUID of Spain (Projects FIS2004-00456, FTN2003-08228-C03-03) and C.A.M. (07N/0058/2002) are gratefully acknowledged for financial support.

The crystal of hydrogen chloride trihydrate has been studied from theoretical and experimental points of view. This species may be relevant in atmospheric processes. A DFT study of the crystal using the program siesta has been performed. The geometrical structure has been refined and the vibrational modes of the crystal have been calculated. The crystal has been formed in the laboratory by exposure of amorphous water ice to excess pressure of HCl, followed by annealing and further cooling to 87 K. Reflection–absorption infrared spectra were recorded at this temperature. The experimental spectrum is assigned by comparison with the theoretical prediction, with the help of the atomic Cartesian displacements per normal mode. The results are compared with previous investigations on the HCl trihydrate and also with theoretical works on the Zundel ion H5O2+, which is present in the atomic arrangement of the crystal of HCl·3(H2O).

Progress in the understanding of the H + H2 reaction and its isotopic variants is reviewed with special attention to the achievements of the last decade. The detailed agreement between theory and experiment attained during this period is emphasized and major experimental and theoretical advances are highlighted. The excellent description of most experimental findings, from state-resolved cross sections to thermal rate constants, provided by the available quantum mechanical treatments, as well as the good overall behaviour of classical mechanics are underlined. Debated issues on short-lived complexes and delayed scattering, resonances and interferences, or geometric phase effects are extensively discussed. Finally, the state-of-the-art is summarized and prospects for future research on this prototypic system are presented.

A combined theoretical and experimental study of crystalline hydrogen chloride hexahydrate HCl AE 6(H2O) has been performed. This species is stable in the range of atmospheric pressure and temperature, and some apparent discrepancies existed between previous crystallographic and vibrational studies. This work presents theoretical calculations based on the SIESTA method that allow a refinement of the geometrical structure and a prediction of the vibrational modes of the crystal. The experimental part consists in new reflection–absorption infrared spectra, that are analysed and assigned with the help of the theoretical results, and compared to previous assignments of infrared transmission spectra. This research supports the equivalence of the samples prepared by different methods in previous experimental investigations. � 2004 Elsevier B.V. All rights reserved.

The transients associated with the ignition and extinction of the cold plasma produced in a low-frequency, square wave modulated, hollow cathode discharge of air are characterized by time resolved mass spectrometry and emission spectroscopy. The time evolution of the concentrations of neutral products measured in the discharge (NO, N2O, N and O) is compared with the predictions of a simple kinetic model previously developed to characterize low-pressure plasmas of N2O, NO and NO2 and a good agreement is found.© 2004 IOP Publishing Ltd, SEUID of Spain (Projects REN2000-1557, FTN2003-08228-C03-03) and CAM (07N/0058/2002)

Complete suppression of amorphous C:H film deposition has been reported for N2/CH4 ratios ∼1, in low-pressure technical plasmas. This finding has been recently used by the authors for the proposal of a possible technique for the inhibition of re-deposited T-containing carbon films at the divertor region of fusion devices. Although several works aiming to the understanding of the underlying physicochemical processes have been published, the complexity of the system is far from being properly described by the proposed models. In the present work, experiments in DC glow discharges at low pressure of H2/CH4/N2 admixtures (90:0–5:0–5) are described. Mass spectrometry of neutral species as well as plasma mass spectrometry for ion detection have been used as the main diagnostics. Several plasma conditions (plasma current, gas composition, etc.) as well as isotopic exchange (H/D) have been investigated. Also, the effect of progressive film growing on the metal walls of the reactor in the composition of gas-phase species has been investigated. It is concluded that wall carbonisation is required to trigger the inhibition process. Ethylene and acetylene are found as the main reaction products. © 2004 Elsevier Ltd, supported by Dirección General de Investigación Científica y Técnica (DGICYT) under Project FTN2000-0915-C03-01, by CAM (Project 07N/0058/2002) and by M.E.C.D. of Spain (Project SB2000-0051)

The transients associated with the ignition and the extinction of the cold plasma produced in a low frequency, square-wave modulated, hollow cathode discharge of nitrogen dioxide are characterized by time resolved emission spectroscopy, mass spectrometry and electrical probes. The temporal evolution of the concentrations of neutral species created or destroyed in the NO 2 discharges are compared with the predictions of a simple kinetic model previously developed for discharges of other nitrogen oxides (N 2O and NO). The physical conditions of pressure, gas flow rate, modulation frequency and electrical current in the NO2 plasma were selected in order to highlight the time-dependent behaviour of some of the stable species formed in the discharge, especially the nitrogen oxide products, whose concentrations show transient maxima. The usefulness of the analysis of the transient results is emphasized as a means to evaluate the relevance of the different elementary processes and as a key to estimate the values of some of the rate constants critical to the modelling.© 2004 IOP Publishing Ltd, Support of the SEUID of Spain (Projects REN2000-1557, FTN2000-0915-C03-03, PB98-0762-C03-02) and CAM (07N/0058/2002)