Orientation in molecule surface dynamics

Abstract

Using an electrostatic hexapole focusing technique a state selected NO [2Π1/2(J=1/2, Ω=1/2, MJ=1/2)] beam is produced. This selected state has a preferential orientation of the internuclear axis with respect to an electrostatic field. By scattering this beam from a crystal placed in a homogeneous electrostatic field steric effects in gas–surface scattering can be investigated. Steric effects are measured for the direct scattering and adsorption of NO on Ag(111) and Pt(111). For direct scattering larger reflection angles are measured in case of an O‐end collision, than in case of an N‐end collision. In the experiments on adsorption/desorption it appears, that on Ag(111) adsorption is enhanced for the O‐end towards the surface, whereas on Pt(111) adsorption is enhanced for the N‐end towards the surface. The steric effects observed for NO/Ag(111) scattering, might 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 explain the larger reflection angle and adsorption probability in case of an O‐end collision. This explanation is confirmed using classical trajectory calculations. The interaction between NO and Ag(111) appears to be governed by the anisotropy of the repulsive interaction. The steric effects observed for NO/Pt(111) scattering, can be partly interpreted in terms of larger phonon excitation of the molecule with the N‐end down than in case of the O‐end down. The interaction between NO and Pt(111) appears to be governed by the anisotropy of the attractive interaction.