Probing model interstellar grain surfaces with small molecules

Temperature-Programmed Desorption (TPD) and Reflection-Absorption Infrared Spectroscopy (RAIRS) have been used to explore the interaction of oxygen (O₂), nitrogen (N₂), carbon monoxide (CO) and water (H2O) with an amorphous silica film as a demonstration of the detailed characterisation of the silicate surfaces that might be present in the interstellar medium. The simple diatomic adsorbates are found to wet the silica surface and exhibit first order desorption kinetics in the regime up to monolayer coverage. Beyond that, they exhibit zero order kinetics as might be expected for sublimation of bulk solids. Water, in contrast, does not to wet the silica surface and exhibits zero order desorption kinetics at all coverages consistent with the formation of an islanded structure. Kinetic parameters for use in astrophysical modelling were obtained by inversion of the experimental data at sub-monolayer coverages and by comparison with models in the multilayer regime. Spectroscopic studies in the sub-monolayer regime show that the C-O stretching mode is at around 2137 cm^-1, a position consistent with a linear surface-CO interaction, and is inhomogenously broadened as resulting from the heterogeneity of the surface. These studies also reveal, for the first time, direct evidence for the thermal activation of diffusion, and hence de-wetting, of H₂O on the silica surface. Astrophysical implications of these findings could account for a part of the missing oxygen budget in dense interstellar clouds, and suggest that studies of the sub-monolayer adsorption of these simple molecules might be a useful probe of surface chemistry on more complex silicate materials.