We report zero-coverage reaction probabilities (S0) for HCl dissociative adsorption on Au(111) obtained by the seeded molecular beam hot-nozzle method. For measurements at normal incidence with mean translational energies ranging from 0.94 to 2.56 eV (nozzle temperatures 296 to 1060 K), S0 increased from 6 × 10(-6) to 2 × 10(-2). S0 also increased with increasing nozzle temperature for fixed incidence energy associated with the motion normal to the surface. Accounting for the influence of the vibrational state population and translational energy distributions in the incident beam, we are able to compare the experimental results to recent theoretical predictions. These calculations, performed employing 6-D quantum dynamics on an electronically adiabatic potential energy surface obtained using density functional theory at the level of the generalized gradient approximation and the static surface approximation, severely overestimate the reaction probabilities when compared with our experimental results. We discuss some possible reasons for this large disagreement.

We present measurements on the vibrational relaxation of NO(v = 2) scattered from atomically defined thin films of Ag on Au(111). The vibrational relaxation probability is strongly dependent on film thickness, increasing with each of the first three Ag monolayers. We interpret this as the influence of work function which changes with layer thickness. This supports the postulated mechanism of NO vibrational relaxation involving a transient NO − anion.

We investigated translational inelasticity in molecular beam surface scattering of NO and CO from ultrathin metallic films of Ag with atomically defined thicknesses grown on single-crystal Au(111). For both molecules, we observe a gradual decrease of the mean final translational energy for Ag film thicknesses between 0 and 3 ML after which no thickness dependence is seen. For Ag films with thicknesses greater than 3 ML, observations are indistinguishable from those of scattering experiments performed on pure Ag crystal surfaces. The similar behavior of both molecules suggests that translational inelasticity is dominated by the mechanical properties of the surface. Theory predicts a thickness-dependent trend of the phonon spectrum that can qualitatively explain the observed behavior.

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