Chemical and morphological evolution of a silicate surface under low-energy ion irradiation

Abstract

Aims. Olivine surfaces have been subjected to low-energy ion irradiation with H + , He + and Ar + at energies within the keV range in order to simulate the effects of energetic gas-grain interactions within shocked regions of the interstellar medium. Methods. The induced modifications in the chemical composition and the bonding configuration of the upper and the near surface regions were monitored in situ by X-ray photoelectron spectroscopy (XPS). The associated morphological evolution of the samples was studied by atomic force microscopy (AFM). Results. Results show that the surface chemistry evolves during irradiation with a noticeable Mg enrichment relative to Si. This evolution is interpreted as coming from magnesium atom diffusion driven by the electric field caused by the positive ion implantation. The iron valence state is also strongly affected by irradiation, with reduction occurring at relatively high fluxes. However, at low fluxes the iron is found to oxidise from Fe 2+ to Fe 3+ due to a charge transfer between the incident positive ions and the iron in the sample. The atomic force microscopy results show that the surface roughness tends to increase with irradiation and that this roughness influences the surface chemical reactivity of the grains, as shown by the enhanced formation of carbonate on the surfaces when they are exposed to a CO 2 atmosphere. The implications for the evolution of interstellar dust include an enhanced dust catalytic activity. These effects would arise from modification under irradiation of the surface reactivity and an increase in the available grain surface area.

Competing Mechanisms of Molecular Hydrogen Formation in Conditions Relevant to the Interstellar Medium

“…An amorphous olivinetype silicate film was kindly provided by Dr. D'Hendecourt (IAS Orsay) by thermal evaporation of San Carlos olivine (Djouadi et al 2005) onto a gold-coated substrate. Its properties have been checked by infrared spectroscopy on a KBr substrate coated by a silicate film at the same time as the copper sample (Davoisne et al 2008).…”

Olivine reactivity with CO2 and H2O on a microscale: Implications for carbon sequestration

“…This interpretation is consistent with previous reports of carbon observed with XPS on olivine surfaces. This carbon signal could be diminished by polishing or cleaning (Mathez, 1987;Seyama et al, 1996;Davoisne et al, 2008). A smaller amount of carbon is also found on new, fresh fractures both immediately after they are formed and after the samples has been exposed to liquid, such as we see on the crushed powder (Table 3).…”

“…Among those, AFM has been applied to monitor the real time dissolution rate of olivine in nitric acid (Heaton and Engstrom, 1994). In more recent work, olivine was ion irradiated to simulate the effects of interstellar gas-grain interactions and its morphological evolution was investigated with AFM (Davoisne et al, 2008).…”

The 10 μm band in amorphous MgSiO3: the influence of medium-range structure, defects and thermal processing

“…For T Tauri and Herbig Ae/Be stars, variations in the shape and strength of the 10 μm feature are often interpreted as a measure of grain growth (Natta et al 2007) since the feature should theoretically become wider and weaker with increasing grain size. Once injected in to the ISM, any crystalline silicates formed in circumstellar regions are thought to undergo amorphization by structurally disruptive processes such as ion implantation (Demyk et al 2001(Demyk et al , 2004Brucato et al 2003;Jäger et al 2003;Bringa et al 2007;Davoisine et al 2008).…”