HDO infrared detection sensitivity and D/H isotopic exchange in amorphous and crystalline ICE

6 p. : gráf.The sensitivity of the OD stretching band as a probe to detect HDO in astrophysical ice is discussed based on IR laboratory spectra of HDO molecules embedded in H2O ice. This band is extremely broad and tends to disappear into the absorption continuum of H2O for low-temperature amorphous samples. Detectable HDO/H2O ratios with this technique may range from a few percent for amorphous samples to a few per thousand in crystalline ice. These relatively high upper limits and the appreciable dependence of the band shape on temperature, which would complicate the interpretation of data from many lines of sight, decisively limit the usefulness of the technique for HDO detection in astronomical observations. The process of isotopic H/D exchange in mixed ice of H2O/D2O is also studied through the evolution of the OD band in IR spectra. Isotopic exchange starts at ∼120 K and is greatly accelerated at 150 K, as crystallization proceeds in the ice. Annealed amorphous samples prove to be more favorable for isotope exchange than samples directly formed in crystalline phase. The annealing process seems to favor a polycrystalline ice morphology with a higher defect activity. These morphology differences can be of relevance for deuterium fractionation in astronomical . © 2011 The American Astronomical Society.Funded by the MCINN of Spain under grants FIS2007-61686 and FIS2010-16455; “Ramon y Cajal” programPeer reviewe

An infrared study of solid glycine in environments of astrophysical relevance

9 p.: gráf.The conversion from neutral to zwitterionic glycine is studied using infrared spectroscopy from the point of view of the interactions of this molecule with polar (water) and non-polar (CO2, CH4)surroundings. Such environments could be found on astronomical or astrophysical matter.The samples are prepared by vapour-deposition on a cold substrate (25 K), and then heated up to sublimation temperatures of the co-deposited species. At 25 K, the neutral species is favoured over the zwitterionic form in non-polar environments, whereas for pure glycine, or in glycine/water mixtures, the dominant species is the latter. The conversion is easily followed by the weakening of two infrared bands in the mid-IR region, associated to the neutral structure. Theoretical calculations are performed on crystalline glycine and on molecular glycine, both isolated and surrounded by water. Spectra predicted from these calculations are in reasonable agreement with the experimental spectra, and provide a basis to the assignments. Different spectral features are suggested as probes for the presence of glycine in astrophysical media, depending on its form (neutral or zwitterionic), their temperature and composition. © the Owner Societies 2011Funding from the Spanish Ministry of Education, Projects FIS2007-61686 and FIS2010-16455.Financial support from Ministerio de Ciencia e Innovación, ‘‘Ramón y Cajal’’ program and CSIC, JAE-Doc ProgramPeer reviewe

SOLID INDENE PURE AND IN WATER ICE: INFRARED SPECTRA AND DESTRUCTION CROSS SECTIONS

In 2021, the first pure PAH molecule, indene, was finally detected in the cold pre-stellar core TMC-1ᵃ, with an estimated gas-phase abundance of 1 - 1.6 10−⁹ with respect to H₂. The observed high relative abundance of indene in cold molecular clouds raises the question about the cycling of this molecule between the gas and the ice mantles of dust grains, and further modeling and laboratory data are required to understand these processes. The present work is focused on the IR spectroscopy of solid phases of indene at low temperatures that, to our knowledge, have not been reported previously. Using the same experimental setup described in our previous works on urea, IR spectra of vapor deposited amorphous and crystalline indene and of indene mixtures with water ice have been recorded. Solid structures and vibrational spectra have been calculated using density functional theory and the results of the calculations have been used for the assignment of the measured IR spectra. Experimental and theoretical band strengths have also been determined. The IR spectra provided are expected to guide the possible detection of this species in the solid phase with the JWST. Our results suggest that some weak absorptions tentatively attributed to mixtures of large PAHs in the IR spectra of interstellar ices should have a large contribution of indene and other small aromatic hydrocarbons. Additionally, experiments on energetic processing of indene ices with 5 keV and VUV photons have been performed, to mimic the effect that Cosmic Rays and the secondary UV field, respectively, will have on this species if present on the surface of dust grains in dense clouds. Indene radiolysis and VUV photolysis destruction cross sections have been derived

High energy electron irradiation of interstellar carbonaceous dust analogs: Cosmic ray effects on the carriers of the 3.4 μm absorption band

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.Peer reviewe

Experimental and theoretical determination of rotational-translational state-to-state rate constants for N2:He collisions at low temperature (3<T<20 K)

10 pages, 7 figures.-- PACS nr.: 34.50.Fa.We present an experimental determination of state-to-state rotational–translational (RT) rate constants of N2:He collisions in the vibrational ground state as a function of temperature in the range 3<T<20 K. Raman spectroscopy in supersonic expansions of N2/He mixtures is used to determine the primary data that, together with the N2:N2 state-to-state RT rates previously determined [Ramos et al., Phys. Rev. A 66, 022702 (2002)], are needed to solve the master equation according to a procedure that does not impose any particular scaling law. We also report first principle calculations of the N2:He state-to-state RT rate constants performed using the full three-dimensional potential energy surface of Reid et al. [J. Chem. Phys. 107, 2329 (1997)], in the 3<T<300 K temperature range. The coupled-channel method, and the coupled-states approximation, were applied in the low (0–610 cm–1) and in the high (610–1500 cm–1) energy limits, respectively. A good agreement between theoretical and experimental results is found in the temperature range where comparison is possible.Thanks are due to the Spanish MCYT for financial support of the experimental part of this work (Research Project No. BFM2001-2276).Peer reviewe

Structure and infrared spectra of hydrocarbon interstellar dust analogs

9 págs.; 6 figs.; 5 tabs.A theoretical study of the structure and mid infrared (IR) spectra of interstellar hydrocarbon dust analogs is presented, based on DFT calculations of amorphous solids. The basic molecular structures for these solids are taken from two competing literature models. The first model considers small aromatic units linked by aliphatic chains. The second one assumes a polyaromatic core with hydrogen and methyl substituents at the edges. The calculated spectra are in reasonably good agreement with those of aliphatic-rich and graphitic-rich samples of hydrogenated amorphous carbon (HAC) generated in our laboratory. The theoretical analysis allows the assignment of the main vibrations in the HAC spectra and shows that there is a large degree of mode mixing. The calculated spectra show a marked dependence on the density of the model solids, which evinces the strong influence of the environment on the strengths of the vibrational modes. The present results indicate that the current procedure of estimating the hydrogen and graphitic content of HAC samples through the decomposition of IR features into vibrational modes of individual functional groups is problematic owing to the mentioned mode mixing and to the difficulty of assigning reliable and unique band strengths to the various molecular vibrations. Current band strengths from the literature might overestimate polyaromatic structures. Comparison with astronomical observations suggests that the average structure of carbonaceous dust in the diffuse interstellar medium lies probably in between those of the two models considered, though closer to the more aliphatic structure. ©the Owner Societies 2017This work has been funded by the MINECO of Spain under grant FIS2013-48087-C2-1P, and AYA2015-71975-REDT, by the MICINN of Spain under grant CDS2009-00038, and by the European project ERC-2013-SyG, Grant Agreement 610256 ‘‘NANOCOSMOS’’. G. M acknowledges MINECO PhD grant BES-2014-069355.Peer reviewe

Plasma generation and processing of interstellar carbonaceous dust analogs

ICPIG (2017) ; Presentación de 32 diapositivas ; Estoril, Lisbon, 9 to 14 July 2017 ; Invited Lecture to XXXIII ICPIG ; http://icpig2017.tecnico.ulisboa.pt/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 m, are small and cannot explain the disappearance of this band in dense interstellar clouds.N

Experimental and numerical investigation of an axisymmetric supersonic jet

21 pages, 10 figures, 2 tables.A comprehensive study of a steady axisymmetric supersonic jet of CO2, including experiment, theory, and numerical calculation, is presented. The experimental part, based on high-sensitivity Raman spectroscopy mapping, provides absolute density and rotational temperature maps covering the significant regions of the jet: the zone of silence, barrel shock, Mach disk, and subsonic region beyond the Mach disk. The interpretation is based on the quasi-gasdynamic (QGD) system of equations, and its generalization (QGDR) considering the translational–rotational breakdown of thermal equilibrium. QGD and QGDR systems of equations are solved numerically in terms of a finite-difference algorithm with the steady state attained as the limit of a time-evolving process. Numerical results show a good global agreement with experiment, and provide information on those quantities not measured in the experiment, like velocity field, Mach numbers, and pressures. According to the calculation the subsonic part of the jet, downstream of the Mach disk, encloses a low-velocity recirculation vortex ring.This research was supported by the Spanish Dirección General de Investigación Científica y Enseñanza Superior (DGICYES), Research Projects PB94{1526 and PB97{1203, and by the Fund for Fundamental Investigations of the Russian Academy of Sciences N 98-01-00155.Peer reviewe