Scattering of NO Molecules from a Graphite Surface: Selectivity of the Rotational Excitation by Inelastic Collisions.

The scattering of NO molecules from a graphite surface has been investigated using molecular dynamics simulations in conjunction with a new potential energy surface that properly accounts for the basic role of long-range interactions on the collision dynamics. NO molecules impinge the surface in selected low-medium roto-vibrational states with collision energies ranging from subthermal to hyperthermal. The initial vibrational state is preserved in the triggered elementary processes while the molecules scatter into rotational states that follow well-defined distributions. For medium-low values of the initial rotational state, molecules are mostly excited, whereas for medium values, there is a slight quenching effect. Moreover, for medium-high collision energies, the rotational distributions reveal a unique feature: two secondary peaks appear in the region of final high rotational states. This phenomenon relates to the configuration of the molecule as it approaches the surface. Moreover, when NO molecules at low collision energy collide with the O-end facing the surface, the scattering predominantly occurs through a direct mechanism. Conversely, when the N-end is directed toward the surface, the scattering involves an indirect mechanism, characterized by multiple bounces on the surface, accompanied by significant energy exchanges between the molecular internal degrees of freedom and the surface itself.