Stereodynamical effects in the adsorption of ethylene molecules onto metal surfaces

The role of molecular alignment of nonpolar molecules in gas-surface interactions has so far escaped direct experimental investigations. We have employed a novel technique to prepare highly aligned supersonic beams and we show that ethylene molecules that impinge on silver surfaces precovered with molecular oxygen as "helicopters" have a higher sticking probability than "cartwheels" (Figure 1). The sticking probability, S, of reactant gas-phase molecules impinging on a metal surface is a key parameter for the quantitative understanding of heterogeneous catalysis at the gas-solid interface. Herein we report the dependence of S on molecular alignment for ethylene, an apolar molecule, interacting with a metallic surface, specifically O-recovered Ag (001). The observed steric effect is large, thus strongly influencing adsorption. The study of such a prototype system is very important to clarify the dynamics in the entrance channels of the potential energy surfaces controlling the catalytic epoxidation of unsaturated hydrocarbons.