Molecular Dynamics Study of the Collision-Induced Reaction of H with CO on Small Water Clusters

The successive hydrogenation of CO is supposed to be the main mechanism leading to the formation of complex oxygenated species in the interstellar medium, possibly mediated by ice layers or ice grains. In order to simulate the dynamical influence of a water environment on the first step of the hydrogenation process, we perform molecular dynamics simulations of the reactive collision of H with CO adsorbed on water clusters in the framework of the self-consistent-charge density functional based tight-binding approach (SCC-DFTB) to calculate potential energy surfaces. The reaction probabilities and the reactive cross sections are determined for water cluster sizes up to ten water molecules. The collision results are analyzed in terms of different reaction pathways: reactive or nonreactive, sticking or desorption of the products or reactants. We show that the HCO radical, although potentially formed as an intermediate regardless of the size of the water cluster, is significantly stabilized for cluster sizes larger than one water molecule and may remain adsorbed on water clusters with more than three molecules. This behavior is shown to be linked with the dissipation of the collision energy into vibrational excitation of the water cluster.