RESUMEN
Interstellar (IS) dust analogs, based on amorphous hydrogenated carbon (a-C:H) were generated by plasma deposition in RF discharges of CH4 + He mixtures. The a-C:H samples were characterized by means of secondary electron microscopy (SEM), 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.
RESUMEN
Methyl isocyanate (CH3NCO) was recently found in hot cores and suggested 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 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 20-200 K temperature range. 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, both by photons and CRs, but embedded below just 10 µm of water-ice, the molecule could survive for ~ 109 yr.
RESUMEN
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 antisymmetric 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 as a starting point the crystal of isocyanic acid. A harmonic theoretical spectrum is calculated then 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, make of it a very good candidate for future astronomical searches.