Forsterite Surfaces as Models of Interstellar Core Dust Grains: Computational Study of Carbon Monoxide Adsorption

Abstract

Carbon monoxide (CO) is the second most abundant gas-phase molecule after molecular hydrogen (H2) of the interstellar medium (ISM). In molecular clouds, an important component of the ISM, it adsorbs at the surface of core grains, usually made of Mg/Fe silicates, and originates complex organic molecules through the catalytic power of active sites at the grain surfaces. To understand the atomistic, energetic, and spectroscopic details of the CO adsorption on core grains, we resorted to density functional theory based on the hybrid B3LYP-D* functional inclusive of dispersion contribution. We modeled the complexity of interstellar silicate grains by studying adsorption events on a large set of infinite extended surfaces cut out from the bulk Mg2SiO4 forsterite, the Mg end-member of olivines (Mg2xFe2–2xSiO4), also a very common mineral on the Earth’s crust. Energetic and structural features indicate that CO is exclusively physisorbed with binding energy values in the 23–68 kJ mol–1 range. Detailed analysis of data revealed that dispersive interactions are relevant together with the important electrostatic contribution due to the quadrupolar nature of the CO molecule. We performed a full thermodynamic treatment of the CO adsorption at the very low temperature typical of the ISM as well as a full spectroscopic characterization of the CO stretching frequency, which we prove to be extremely sensitive to the local nature of the surface-active site of adsorption. We also performed a detailed kinetic analysis of CO desorption from the surface models at different temperatures characterizing the colder regions of the ISM. Our computed data could be incorporated in the various astrochemical models of interstellar grains developed so far and thus contribute to improve the description of the complex chemical network occurring at their surfaces.