The dissociation probability of N 2 on Ru(001) increases from 5 × 10 -7 at a kinetic energy of 0.15 eV to 10 -2 at 4.0 eV. Vibrational excitation of the impinging nitrogen molecules enhances the dissociation more than the equivalent energy in translation. Its relative importance increases as the incident kinetic energy grows. The dissociation was found to be surface temperature independent at all incident kinetic energies, in agreement with theoretical predictions based on quantum mechanical nonadiabatic calculations. These simulations reproduce accurately the kinetic energy dependence of S 0 over the entire energy range, suggesting that N 2 tunnels from the molecular to the adsorbed atomic state through an effective barrier of 2.2 eV.

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Classical molecular dynamics simulations have been performed to study the details of collision-induced desorption (CID) of nitrogen molecules adsorbed at low coverages on Ru(001). Semiempirical potential energy surfaces (PES) were used to describe the movable two layers of 56 ruthenium metal atoms each, the nitrogen adsorbate, the Ar and Kr colliders, and the interactions between them. An experimentally measured threshold energy for the CID process of 0.5 eV and the dependence of the cross section σdes on incidence energy and angle of incidence have been precisely reproduced in the energy range of 0.5–2.5 eV. Strong enhancement of the σdes is predicted as the angle of incidence increases. Kinetic energy and angular distributions of the scattered rare gas and the desorbing nitrogen were determined as a function of the dynamical variables of the collider. It is predicted that half of the collision energy is transferred to the solid and the other half is shared among the two scattered species. While no vibrational excitation is observed, efficient rotational energy excitation is predicted which depends on both incident energy and angle of incidence. Polar and azimuthal angular distributions were found to be strongly dependent on the incidence angle and energy of the colliders. These results suggest a new CID mechanism for the weakly chemisorbed nitrogen molecules on Ru(001), based on extensive analysis of individual trajectories. According to this mechanism, the CID event is driven by an impact excitation of frustrated rotation or tilt motion of the adsorbed molecule as a result of collision with the energetic rare gas atom. In addition, lateral motion along the surface is also excited. Strong coupling of these two modes with the motion in the direction normal and away from the surface eventually leads to desorption and completes the CID process. The efficiency of this coupling is dictated by the details of the corrugation of the Ru–N2 PES. It is concluded that the simple hard cube–hard sphere model, frequently used to analyze CID processes, is insufficient for the description of this system. While reasonably well predicting threshold energy, it cannot explain the full dynamical picture of the CID event.

An extremely large isotope effect [Ieff=Pdiss(15N2)/Pdiss(14N2)], has been measured in the dissociative chemisorption of nitrogen molecules over Ru(001). It varies from unity at kinetic energies above 2 eV to 0.2 at Ek=1.4 eV. These observations are consistent with a barrier for direct dissociation of 1.8 eV, in agreement with previous experiments and recent ab initio density functional theory calculations. It supports earlier studies that proposed tunneling as the dissociation dynamics mechanism.

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.