Surface processes induced by collisions

Energetic gas-phase particles that collide with adsorbed species on solid surfaces induce a variety of processes. Collision induced processes (CIP) are important and may play a central role in the mechanism governing heterogeneous catalytic reactions at high pressures and elevated temperatures. A number of different CIPs are discussed in this article with a strong emphasis on the utilization of molecular dynamics (MD) simulations as a tool for gaining molecular level mechanistic and dynamic insight into chemical events under investigation. Collision induced desorption (CID) is the simplest CIP to be discussed. The CID of N-2 from Ru(001) is described as a test case for the effect of collisions on a polarized adsorbate and has been studied at both low and high coverages. The interpretation of the experimental data at low coverage using MD simulations has led to the introduction of a new desorption mechanism. It involves strong coupling of surface corrugation and adsorbate frustrated rotation that lead to a normal motion away from the surface. Another system for which CID was shown to provide unique information is that of water on Ru(001). Here, the enhanced CID rate was demonstrated to be selective for a specific adsorption site/structure on the surface (the A(2) site), providing a new insight into the structure of water on this surface, recently recalculated by employing ab initio methods. At the multilayer coverage range, a remarkable stability was found of the ice layer against CID, suggesting particularly efficient dissipation of the collider energy within the hydrogen bonded network at an ice thickness of 3 bilayers and above. A unique CIP to be discussed is collision induced migration (CIM), a new phenomenon that has never been considered before. Based on MD simulations, it is shown that CIM may result in migration distances of more than 150 A at very low coverage, whereas at high coverage, these displacements are shortened significantly. The potential importance of this process for inducing novel catalytic routes on surfaces is discussed. A related example involves MD simulations that address the relation between tracer surface diffusion and the pressure of collider from the gas phase. It is predicted that by increasing the pressure in the range 0-500 atm. significant changes in adsorbates surface diffusivity should take place as a result of collision induced migration. Finally, CID within the O-2/Ag(110) system arising from photodissociation of adsorbed molecular oxygen is described. MD simulations were used to explain the experimentally determined coverage dependent phenomena such as desorption yield and angular distribution of desorbates.