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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. [ABSTRACT FROM AUTHOR]

We report an experimental determination of the k00→02 rate coefficient for inelastic H2:H2 collisions in the temperature range from 2 to 110 K based on Raman spectroscopy data in supersonic expansions of para-H2. For this purpose a more accurate method for inverting the master equation of rotational populations is presented. The procedure permits us to reduce the measured k00→02 rate coefficient to the corresponding σ00→02 cross section in the range of precollisional energy from 360 to 600 cm-1. Numerical calculations of σ00→02 carried out in the frame of the coupled channel method are also reported for different intermolecular potentials of H2. A good agreement is found between the experimental cross section and the numerical one derived from Diep and Johnson’s potential [J. Chem. Phys. 112, 4465 (2000)]. [ABSTRACT FROM AUTHOR]

We present an experimental determination of state-to-state rotational-translational (RT) rate constants of N[SUB2]:He collisions in the vibrational ground state as a function of temperature in the range 3

The Q branches of the Raman bands associated to the 2ν1:4ν02:ν1+2ν02 Fermi resonance of 12C 16O2 have been observed in the gas phase at 2543, 2671, and 2797 cm-1 and, in addition, at 2514 cm-1, one hot band from their first excited vibrational state is seen. From the analysis of their cross sections, jointly with those of the main Fermi diad, ν1:2ν02 and its hot bands, an improved description of the molecular polarizability surface has been achieved. The following derivatives of the mean polarizability have been obtained: ∂α/∂q1=12.43×10-42 CV-1 m2; ∂2α/∂q22σ=2.81×10-42 CV-1 m2 (σ=a,b); ∂2α/∂q21=0.45×10-42 CV-1 m2; ∂2α/∂q23=0.67 or 0.15×10-42 CV-1 m2; ∂3α/∂q1∂q22σ=-0.06×10-42 CV-1 m2, in terms of the dimensionless normal coordinates, and ∂α/∂S1=3.15×10-30 CV-1 m; ∂2α/∂S22σ=0.36×10-20 CV-1; ∂2α/∂S21=2.9×10-20 CV-1; ∂2α/∂S23=2.0 or 0.5×10-20 CV-1; ∂3α/∂S1∂S22σ=-1.7×10-10 CV-1 m-1, in terms of the symmetry coordinates. The thermal dependence of the mean polarizability is discussed in terms of these quantities. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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:H 2 O 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:H 2 O 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.