Probing model interstellar grain surfaces with small molecules

Abstract

Temperature-programmed desorption and reflection-absorption infrared spectroscopy have been used to explore the interaction of oxygen (O 2 ), nitrogen (N 2 ), carbon monoxide (CO) and water (H 2 O) with an amorphous silica film as a demonstration of the detailed characterization 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 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 (5.43 μm), 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 H 2 O 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.

Peeling the astronomical onion

“…The latter feature was left unidentified. It has been suggested that this unassigned feature could be CO in a CH 3 OH matrix, 51 but an assignment of CO directly adsorbed to the silicate surface 6 is also compatible with the results of the investigation and model suggested herein. On carbonaceous surfaces, the situation is less clear.…”

“…As explained in the introduction, TPD experiments have shown that H 2 O clusters at all exposures on various surfaces. [6][7][8][9][10][11] A key consequence of these observations is that the established Onion Model of COM growth, i.e. a layered polar and apolar structure subject to energetic and thermal processing, should be revised.…”

“…At low CO exposures, equivalent to a coverage of 0.1 ML or less, the binding energy of CO on the aSiO 2 surface is greater by 1 -2 kJ mol -1 than on the c-ASW surface. 6,24,49 Pontoppidan et al 50 The first two features can be assigned to multi-layers of CO. The latter feature was left unidentified.…”

“…2 More recently, H 2 O ice has been directly detected on comet 67P/Churyumov-Gerasimenko by the Rosetta mission 5 confirming the icy grain origins of these primitive bodies. Temperature programmed desorption (TPD) experiments on H 2 O have consistently shown zero order desorption kinetics on amorphous silica (aSiO 2 ), 6 sapphire, 7 graphene 8 and both polycrystalline and single crystal metals (e.g. Au 9 and Ru(0001)/Al(001) 10 ).…”

Sensitivity Analysis of Grain Surface Chemistry to Binding Energies of Ice Species

“…The final results depend on the nature of the substrate from which the species desorbs. As an example, we would like to mention the reported experimental values of E bind for water, which may range from 4800 K, as reported by Dulieu et al (2013) from studies of formation of water on amorphous silicate surfaces, to 5930±240 K, as reported by Collings et al (2015), based on experiments of desorption from amorphous silica. Whether experiments are performed in the monolayer or multilayer regime and whether the deposited ice is pure, mixed, or layered also has an effect.…”