Rotationally excited NO molecules incident on a graphite surface: In- and out-of-plane angular distributions

In-plane and out-of-plane angular distributions of NO molecules scattered from an atomically flat graphite surface have been measured as functions of the incident rotational and translational energy, and the surface temperature T-s. The specularly scattered molecules exhibit a lobular angular distribution which can be described by a fit with a cos (5-20)theta dependence for the in-plane and by cos(15-80)phi for the out-of-plane direction. The diffusely scattered molecules show an unexpected asymmetric behavior: the in-plane distribution is cosine-like, suggesting trapping-desorption dynamics but the out-of-plane angular distribution narrows from cos(1) phi to cos(6) phi with increasing either the surface temperature or the incoming translational or rotational energy. The latter dependence leads to a decrease of the fraction of diffusely scattered molecules with increasing incoming rotational energy, as expected qualitatively from a rotational state-dependent sticking factor. The molecular scattering dynamics can be attributed to a transition from scattering with a very small number of bounces (specular lobe) at high T-s to multiple scattering (a few collisions, asymmetric diffuse distributions) at medium T-s to trapping/desorption at very low T-s, characterized by symmetric cosine distributions. (C) 1997 Elsevier Science B.V.