Adsorbate-assisted adsorption: Trapping dynamics of Xe on Pt(111) at nonzero coverages

The trapping dynamics of Xe on Pt(111) has been probed as a function of Xe coverage with supersonic molecular-beam techniques. Adsorption probabilities were directly measured at a surface temperature of 95 K at coverages ranging from zero to monolayer saturation at incident translational energies between 6 and 63 kJ/mol and incident angles between 0° and 60°. In apparent agreement with the predictions of the original Kisliuk model, the adsorption probability at the lowest incident translational energy (6 kJ/mol) remains almost constant with coverage up to near monolayer saturation. However, in contradiction to the original Kisliuk model, at higher incident translational energies, the trapping probability increases nearly linearly with xenon coverage up to near monolayer coverage. For example, the trapping probability increases from 0.06 to 0.42 for an incident translational energy of 63 kJ/mol at normal incidence as the coverage is increased from zero to saturation monolayer coverage. This behavior can be explained adequately by a model that incorporates enhanced trapping onto the monolayer compared to the clean surface, a property of the model that is confirmed directly by experiments presented herein. The angular dependence of the adsorption probability shows progressive deviation from normal energy scaling with increasing Xe surface coverage, proving that the degree to which parallel momentum participates in the adsorption process increases with adsorbate coverage. The initial trapping probability of Xe onto the monolayer is independent of incident angle indicating total-energy scaling. The above findings are qualitatively identical to our previous results for the molecular adsorption of ethane on the same surface, suggesting that these phenomena occur, in general, for weak molecular adsorption regardless of molecular shape and internal degrees of freedom, at least for small molecules.