Adsorption of Atoms on a Crystalline Ice Surface Model: Results from Periodic ab Initio Simulations

In cold and dense regions of the interstellar medium, such as molecular clouds, more than 200 gas-phase molecular species have been observed by means of infra-red and rotational spectroscopy techniques alongside solid sub-micrometer sized particles called dust grains. These grains are of uttermost importance because their surfaces serve as meeting points for chemical species that adsorb from the gas phase, diffuse and possibly react to form more complex molecules. These grains consist of a silicate or carbonaceous cores covered in layers of water dominated ices that contain other small volatile molecules such as CO, CO2, NH3, CH3OH. Most of the ice components originate from the adsorption and reaction of bare atoms, e.g., H, C, N and O, on the grain surfaces. An important challenge in Astrochemistry is to characterize the thermodynamics and kinetics of the main reaction steps of the bare atoms forming the ice components on the grain surfaces. As a first step previous to the reaction of these atomic species, in this paper we present results based on quantum chemistry methods on the adsorption of atomic carbon, nitrogen and oxygen on a crystalline water ice surface model mimicking the icy grain surfaces.