Observation of steric effects in gas–surface scattering

Abstract

Molecules adsorbed on a surface are known to show preferential orientations, and in particular, the interaction potential between a linear molecule and a surface depends on the orientation of the molecular axis. But the fact that the molecules eventually adsorbed are orientated with respect to the surface is not evidence that the dynamics of gas–surface interactions is governed by the initial molecular orientation in the gas phase. For example at very low velocities the molecule might achieve its optimum orientation adiabatically during its approach to the surface. Dependence of scattering dynamics on molecular orientation can also be degraded as a result of surface structure and surface vibrations. There have been, experimental studies, however, which suggest that orientational or steric effects could influence gas–surface scattering^1–4. Thus, the much larger rotational excitation for NO- than for N₂-scattering from Ag(lll) indicates that large anisotropies can occur in a potential for gas–surface dynamics (G. O. Sitz et al. manuscript in preparation). The double rotational rainbow observed in the rotational state distribution for NO/Ag(111) (refs 3 and 4) can also be interpreted as a manifestation of this anisotropy^5–9. These studies all indicate that steric effects could be important and here we report on scattering of orientated NO from Ag(111) to provide direct experimental evidence for the importance of such effects in gas–surface interactions.