The first layers of water on Ru(001)

The initial growth of water molecules to form the first bilayer and then ice layers on Ru(001) was studied utilizing work function change (Δφ), temperature programmed desorption (TPD), and supersonic atomic beam-collision-induced desorption (CID) measurements. A kinetic model that reproduces the first bilayer growth, as determined by the Δφ measurements, was developed. It indicates that monomers dominate the cluster size distribution at low coverages, but at high coverages, tetramers gradually, become the dominant clusters. Small contributions to Δφ suggest that tetramers are cyclic at the adsorbed state with inclined dipoles. CID measurements of H₂O and D₂O at coverages near one bilayer reveal strong selectivity to the removal of molecules in the A₂ adsorption sites over those in the icelike C sites and the A₁ sites. Soft removal rates of thicker ice layers as a result of CID with energetic Kripton atoms were then studied as a function of the ice layer thickness. Near the completion of the third bilayer, a sharp stabilization of the ice structure occurs, which leads to two concomitant effects: (a) a significant decrease in the CID removal rate of the ice layers, and (b) caging of adsorbed nitrogen followed by an extremely sharp desorption of the trapped molecules near 165 K. This happens at the onset of the ice desorption temperature. These effects are discussed in terms of the structure of the first layers of ice which grow on the surface of a Ru(001) single crystal and are consistent with recent model molecular dynamics simulations of such a system.