Steric effects in scattering and adsorption of NO at Ag(111)

Abstract

Scattering experiments have been performed using orientated beams of NO molecules in the Π_½-electronic ground state and J=M_J=½. In direct inelastic scattering we observe that with the O-end of the molecule preferentially orientated towards the surface, the angular distribution shifts away from the specular direction towards the surface. In the experiments on trapping/desorption it appears that trapping is enhanced for the O-end towards the surface. Both results can be interpreted in terms of larger rotational excitation of the molecule with the O-end down than in case of the N-end down. Since the rotational excitation will mainly occur at the expense of the normal component of the incident velocity, this can easily explain the experimental results. This is confirmed using classical trajectory calculations. The interaction between NO and Ag(111) seems therefore to be governed by the anisotropy of the repulsive interaction, which is stronger for the O-end region.