On the spectral features of dangling bonds in CH4/H2O amorphous ice mixtures

[EN] Dangling bond (DB) bands in IR spectra, above 3600 cm(-1), are a source of information on structural properties of amorphous water ice, and especially on ice mixtures of water and other frozen gases. We deal in this paper with the spectroscopic behavior of DB bands of CH4/H2O mixtures. We use ab initio methodology to predict theoretical results which are compared with experimental results. Our model mixtures are created by inserting a variable number of molecules of either species into a cell of appropriate size to reach an initial density of 1 g cm(-3), which can be modified by including an empty space at the top, to simulate pores. The cell is taken as a unit cell for a solid state calculation The structure of the mixture is optimized and the IR spectrum is calculated for the converged geometry. We find two different kinds of dangling bonds, in which the O-H stretching responsible for this mode is directed either to an empty space of a pore or towards a nearby CH4 molecule, with which some interaction takes place. The spectral characteristics of these two DB types are clearly different, and follow satisfactorily the pattern observed in experimental spectra. Estimated band strengths for these DB bands are given for the first time.Funds from the Spanish MINECO/FEDER FIS2016-77726-C3-1-P and C3-3-P projects are acknowledged. We are indebted to V. J. Herrero for technical assistance with the CASTEP calculations, performed at SGAI-CSIC.Maté, B.; Satorre, MÁ.; Escribano, R. (2021). On the spectral features of dangling bonds in CH4/H2O amorphous ice mixtures. Physical Chemistry Chemical Physics. 23(15):9532-9538. https://doi.org/10.1039/d1cp00291k95329538231

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

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

Solid L-α-alanine: Spectroscopic properties and theoretical calculations

A spectroscopic study of L-α-alanine in different environments is presented, with special emphasis on the neutral to zwitterion transformation of this amino acid. Spectra of room temperature solids as deposited from the vapour and in KBr pellets are compared and discussed revealing some discrepancies. An assignment is proposed based on theoretical calculations of the solid structure. Vapour deposits at 25K are prepared both of pure alanine and of mixtures with polar (H 2O) and non-polar (CO 2, CH 4) components. The spectra of the solids contain IR bands which can be individually assigned to the neutral and to the zwitterion, and whose relative intensity variation can be used to follow the neutral to zwitterion transformation. The assignment of the spectrum of the neutral species is proposed, again with help from theoretical calculations of the single neutral molecule. The neutral/zwitterion ratio in deposits at 25K varies between ~60% for pure alanine and H 2O mixtures and ~90% for non-polar matrices. This ratio drops when the solid is heated until the ionic species only remains at 200K. The conversion process depends on the environment surrounding the alanine molecules. © 2012 Elsevier Ltd.funding from the Spanish Ministry of Science and Innovation, ProjectFIS2010-16455. financial support from Ministerio de Ciencia e Innovación, “Ramón y Cajal” programme and CSIC, JAE-Doc Programme, Sabbatical ProjectPR2010-0012.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

Densities, infrared band strengths, and optical constants of solid methanol

[EN] Contact. The increasing capabilities of space missions like the James Webb Space Telescope or ground-based observatories like the European Extremely Large Telescope demand high quality laboratory data of species in astrophysical conditions for the interpretation of their findings. Aims. We provide new physical and spectroscopic data of solid methanol that will help to identify this species in astronomical environments. Methods. Ices were grown by vapour deposition in high vacuum chambers. Densities were measured via a cryogenic quartz crystal microbalance and laser interferometry. Absorbance infrared spectra of methanol ices of different thickness were recorded to obtain optical constants using an iterative minimization procedure. Infrared band strengths were determined from infrared spectra and ice densities. Results. Solid methanol densities measured at eight temperatures vary between 0.64 g cm(-3) at 20 K and 0.84 g cm(-3 )at 130 K. The visible refractive index at 633 nm grows from 1.26 to 1.35 in that temperature range. New infrared optical constants and band strengths are given from 650 to 5000 cm(-1) (15.4-2.0 mu m) at the same eight temperatures. The study was made on ices directly grown at the indicated temperatures, and amorphous and crystalline phases have been recognized. Our optical constants differ from those previously reported in the literature for an ice grown at 10 K and subsequently warmed. The disagreement is due to different ice morphologies. The new infrared band strengths agree with previous literature data when the correct densities are considered.Funds have been provided for this research by the Spanish MINECO, Project FIS2016-77726-C3-1-P and FIS2016-77726-C3-3-P. German Molpeceres acknowledges MINECO PhD grant BES-2014-069355. We are grateful to R. Escribano for helpful discussions. Our skillful technicians C. Santonja, M. A. Moreno, and J. Rodriguez are also gratefully acknowledged.Luna Molina, R.; Molpeceres, G.; Ortigoso, J.; Satorre, MÁ.; Domingo Beltran, M.; Maté, B. (2018). Densities, infrared band strengths, and optical constants of solid methanol. Astronomy and Astrophysics. 617:1-9. https://doi.org/10.1051/0004-6361/201833463S19617Boogert, A. C. A., Pontoppidan, K. M., Knez, C., Lahuis, F., Kessler‐Silacci, J., van Dishoeck, E. F., … Stapelfeldt, K. R. (2008). The c2dSpitzerSpectroscopic Survey of Ices around Low‐Mass Young Stellar Objects. I. H2O and the 5–8 μm Bands1,2. The Astrophysical Journal, 678(2), 985-1004. doi:10.1086/533425Boogert, A. C. A., Gerakines, P. A., & Whittet, D. C. B. (2015). Observations of the Icy Universe. Annual Review of Astronomy and Astrophysics, 53(1), 541-581. doi:10.1146/annurev-astro-082214-122348Bossa, J.-B., Maté, B., Fransen, C., Cazaux, S., Pilling, S., Rocha, W. R. M., … Linnartz, H. (2015). POROSITY AND BAND-STRENGTH MEASUREMENTS OF MULTI-PHASE COMPOSITE ICES. The Astrophysical Journal, 814(1), 47. doi:10.1088/0004-637x/814/1/47Bottinelli, S., Boogert, A. C. A., Bouwman, J., Beckwith, M., van Dishoeck, E. F., Öberg, K. I., … Lahuis, F. (2010). THE c2dSPITZERSPECTROSCOPIC SURVEY OF ICES AROUND LOW-MASS YOUNG STELLAR OBJECTS. IV. NH3AND CH3OH. The Astrophysical Journal, 718(2), 1100-1117. doi:10.1088/0004-637x/718/2/1100Bouilloud, M., Fray, N., Bénilan, Y., Cottin, H., Gazeau, M.-C., & Jolly, A. (2015). Bibliographic review and new measurements of the infrared band strengths of pure molecules at 25 K: H2O, CO2, CO, CH4, NH3, CH3OH, HCOOH and H2CO. Monthly Notices of the Royal Astronomical Society, 451(2), 2145-2160. doi:10.1093/mnras/stv1021Cazaux, S., Bossa, J.-B., Linnartz, H., & Tielens, A. G. G. M. (2014). Pore evolution in interstellar ice analogues. Astronomy & Astrophysics, 573, A16. doi:10.1051/0004-6361/201424466Dohnálek, Z., Kimmel, G. A., Ayotte, P., Smith, R. S., & Kay, B. D. (2003). The deposition angle-dependent density of amorphous solid water films. The Journal of Chemical Physics, 118(1), 364-372. doi:10.1063/1.1525805Drabek-Maunder E., Greaves J., Fraser H. J., Clements D. L., & Alconcel L. N. 2017, Int. J. Astrobiol., DOI: 10.1017/S1473550417000428Gálvez, O., Maté, B., Martín-Llorente, B., Herrero, V. J., & Escribano, R. (2009). Phases of Solid Methanol. The Journal of Physical Chemistry A, 113(14), 3321-3329. doi:10.1021/jp810239rGerakines, P. A., Bray, J. J., Davis, A., & Richey, C. R. (2005). The Strengths of Near‐Infrared Absorption Features Relevant to Interstellar and Planetary Ices. The Astrophysical Journal, 620(2), 1140-1150. doi:10.1086/427166Hodyss, R., Parkinson, C. D., Johnson, P. V., Stern, J. V., Goguen, J. D., Yung, Y. L., & Kanik, I. (2009). Methanol on Enceladus. Geophysical Research Letters, 36(17). doi:10.1029/2009gl039336Hudgins, D. M., Sandford, S. A., Allamandola, L. J., & Tielens, A. G. G. M. (1993). Mid- and far-infrared spectroscopy of ices - Optical constants and integrated absorbances. The Astrophysical Journal Supplement Series, 86, 713. doi:10.1086/191796Hudson, R. L., Ferrante, R. F., & Moore, M. H. (2014). Infrared spectra and optical constants of astronomical ices: I. Amorphous and crystalline acetylene. Icarus, 228, 276-287. doi:10.1016/j.icarus.2013.08.029Ioppolo, S., van Boheemen, Y., Cuppen, H. M., van Dishoeck, E. F., & Linnartz, H. (2011). Surface formation of CO2 ice at low temperatures. Monthly Notices of the Royal Astronomical Society, 413(3), 2281-2287. doi:10.1111/j.1365-2966.2011.18306.xIsokoski, K., Bossa, J.-B., Triemstra, T., & Linnartz, H. (2014). Porosity and thermal collapse measurements of H2O, CH3OH, CO2, and H2O:CO2 ices. Physical Chemistry Chemical Physics, 16(8), 3456. doi:10.1039/c3cp54481hMaté, B., Gálvez, Ó., Herrero, V. J., & Escribano, R. (2008). INFRARED SPECTRA AND THERMODYNAMIC PROPERTIES OF CO2/METHANOL ICES. 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Cold plasmas in laboratory astrochemistry: Ions and carbonaceous dust analogues

Colloquium SFB 956, Physikalische Institute Köln, 18.05.2015Peer Reviewe