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We studied icy CH4 and its mixtures with N2 (temperature 16-40 K), using near-IR transmittance spectroscopy (1.0-3.6 μm), and monitoring the film growth using interference patterns of two lasers. We measured peak position, full width at half-maximum, and strengths of the methane bands, and density and real refractive index of the icy films. Results confirm and extend but also partially contradict previous studies on similar mixtures. Experimental data can be applied to interpret observations of solar system (trans-Neptunian objects) and interstellar ices, where methane and nitrogen are believed to be present. We predict the optical depths of two methane NIR bands in the line of sight of some dense molecular clouds.

We present the results of UV irradiation with Lyα photons of carbon grains with a water ice cap at 11 K. Formation of CO and CO2 molecules takes place during irradiation. An estimation of the formation cross section of these molecules by Lyα photons has been obtained from the intensity increase of their infrared stretching bands as a function of the photon fluence. The fraction of carbon in the grains converted to CO and CO2 by UV photons is 0.06 and 0.05, respectively. The spectral profile of the CO stretching feature and that of the CO2 bending mode indicate a polar environment for these molecules. On the basis of the present laboratory results and those obtained in previous work on ion irradiation of similar samples, it has been possible to estimate the contribution of polar CO and CO2 produced on carbon grains by energetic processing to the observed column densities of these molecules for dense clouds whose visual extinction is known. A significant amount of polar CO and CO2 is produced through the mechanism we have studied. Furthermore, we have found that the laboratory profile of the bending band of CO2 produced on carbon grains is compatible with that observed toward the field star Elias 16.

We present the results of experiments aimed at studying the influence of the type of grain on the chemical composition of the ice mantles during energetic processing under simulated dense medium conditions. Formation of CO and CO2 molecules occurs when hydrogenated carbon grains with a water ice cap are irradiated with 30 keV He+ ions at low temperature. The fraction of carbon in the grains converted to CO and CO2 by ions is at least 0.03 and 0.02, respectively. An estimation of the formation cross section of these molecules by 30 keV He+ ions has been derived from the intensity increase of their infrared stretching bands as a function of the ion fluence. On the basis of the laboratory results, it has been possible to evaluate the contribution of CO and CO2 produced on carbon grain by cosmic rays to the observed column densities of these molecules for dense clouds whose visual extinction is known. The mechanism we have studied does not dominate other CO2 formation processes; however, its contribution is in addition to other processes occurring on ice mantles. The spectral profile and the contribution to the observed column densities make solid CO formed by cosmic-ray irradiation of ice-layered carbon grains a good candidate for the red component of the interstellar CO stretching feature, which is generally attributed to CO mixed in with water ice. As a consequence of the formation of CO and CO2 molecules on carbon grains, a slow chemical erosion of the particles takes place.

We have investigated the effects induced by ion bombardment of mixtures containing nitrogen-bearing compounds at low temperatures. The results show the formation of a band at 2080 cm-1 in binary mixtures, NH3:CH4 and N2:CH4, which we attribute to HCN embedded in the organic residue formed by ion irradiation. In addition to this band, ternary mixtures containing an oxygen-bearing species (i.e., H2O) form a compound with a prominent absorption band at about 2165 cm-1 (4.62 microns). We ascribe this band to a nitrile compound containing O that is bonded to the organic residue. A detailed comparison of the laboratory results with astronomical data of the 4.62 microns absorption band in protostellar spectra shows good agreement in peak position and profile. Our experimental studies show that N2, which is a more likely interstellar ice component than NH3, can be the molecular progenitor of the carrier of the interstellar band. This is an alternative to the pathway by which UV photolysis of NH3-containing ices produces the 4.62 microns band and implies that ion bombardment may well play an important role in the evolution of interstellar ices. Here, we discuss the implications of our studies for the chemical route by which the carrier of the 4.62 microns band is formed in these laboratory experiments.

An absorption feature at 2040 cm~1 (4.90 km), previously observed only in W33A, has been detected toward two deeply embedded young stellar objects (YSOs), AFGL 989 and Mon R2 IRS 2. Upper limits are reported for several other YSOs and for one object located behind the Taurus Dark Cloud. We attribute this interstellar feature to solid carbonyl sul-de (OCS) embedded in icy grain mantles along the line of sight. As in the case of W33A, the best match of the newly observed features with laboratory spectra of astrophysically relevant mixtures is obtained for traces of OCS in a methanol-rich matrix. This, again, suggests the presence of independent grain mantle populations and, in particular, of a minor fraction of methanol-rich icy grain mantles in which OCS is embedded. From the strength of the absorp- tion feature we deduce OCS column densities and ratios toward the observed objects. Taking OCS/H 2 O into consideration sulfur chemistry and the origins of solid OCS, we conclude that a major fraction of the elemental sulfur is presently unaccounted for in dense molecular clouds. Subject headings: infrared: ISM: lines and bands E ISM: clouds E ISM: molecules E line: identi-cation E molecular processes

We present ground-based observations of the 2041 cm(exp -1) (4.9 mm) absorption feature toward the deeply embedded protostar W33A. We attribute this interstellar feature to solid carbonyl sulphide (OCS) embedded in icy grain mantles along the line of sight. We have made an extensive laboratory study of the infrared characteristics of OCS in various ice mixtures. Studies of the 2041 cm(exp -1) band of solid OCS and solid mixtures containing OCS show that its shape and peak position are sensitive to the molecular environment of the OCS molecule. Mie scattering calculations show that the peak position and profile of the OCS band depend on the shape and size of the absorbing grain when the OCS concentration is larger than 0.05. For lower OCS concentrations, laboratory-measured bulk spectra agree very well with the absorption spectra of small grains. We have compared the observed feature with laboratory and theoretical spectra of astrophysically relevant mixtures. The best agreement is obtained for mixtures with traces of OCS in a methanol-rich matrix. This would suggest the presence of independent grain components and in particular of a minor fraction of methanol-rich icy grain mantles in which OCS is embedded. From the strength of the absorption feature we deduce a OCS/H2O ratio, along the line of sight, of 4 x 10(exp -4).