Classical stereodynamics in Ar plus NO inelastic collisions

The stereodynamics of the Ar + NO ( j = 0) rotational inelastic excitation has been investigated at 66 meV by means of quasiclassical trajectories on a recent ab initio potential energy surface. A marked correlation between the preferred sense of rotation of NO and the scattering plane is obtained for the highest rotational levels accessible, which are excited in strong repulsive collisions. This result is in qualitative agreement with recent quantum mechanical calculations and experimental measurements. For the lower rotational levels, where the interactions are not so repulsive, the preferred sense of rotation is found to oscillate with scattering angle, but the intensity of the oscillations is small and their angular range is not entirely coincident with those from quantum mechanics and experiment. Classical dynamics, even including attractive interactions, cannot account properly for the mentioned oscillatory behaviour. Secondary encounters between the outgoing Ar atom and NO molecule, giving rise to 'chattering', are found to be relatively frequent, leading to a decrease in the final rotational energy of NO with respect to that attained in the first encounter. Chattering trajectories are defined and their mechanism is characterized.