Coverage dependence of the sticking probability of ethylene on Ag(4Â 1Â 0)

The interaction of ethylene with Ag(4 1 0), a vicinal surface of Ag(1 0 0) characterised by a high density of open steps, is investigated. Energy and angle resolved measurements of the sticking coefficient are performed with the retarded reflector method of King and Wells using a supersonic molecular beam. We find that open steps remove the translational barrier for adsorption into the π-bonded state present for Ag(1 0 0). Steering removes the angle dependence of the initial sticking probability. The coverage dependence of the adsorption probability indicates that a precursor mechanism is present and that adsorbate assisted adsorption is important at hyperthermal energies. With increasing coverage steering becomes ineffective and rotations inhibit adsorption. Admolecules with second nearest neighbours are destabilised and these sites can be populated only by gas-phase molecules impinging along trajectories for which the step heights are exposed.