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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 Fe²⁺ to Fe³⁺ 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 CO₂ 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.