New insights on the stereodynamics of ethylene adsorption on an oxygen precovered silver surface

The control of spatial orientation of molecules has a great influence on the stereodynamics of elementary processes occurring both in homogeneous and heterogeneous phases. Nonpolar molecules have so far escaped direct experimental investigations because of their poor sensitivity to several external constraints. Recently, it has been shown that the collisional alignment produced in supersonic expansions coupled with molecular-beam velocity selection can help solve such problems. Here we show that the sticking probability of ethylene, a nonpolar molecule prototypical of unsaturated hydrocarbons, on an O-2-precovered Ag(001) surface is larger for molecules approaching in a helicopterlike motion than for those cartwheeling. A mechanism involving a weakly bound precursor state is suggested, with helicopter molecules having a lower chance of being scattered back into the gas phase than cartwheels when colliding with preadsorbed ethylene.