Mechanisms of translational and rotational energy transfer in (N_2)_n cluster-surface scattering

摘要

Molecular dynamics simulatons were employed to investigate the dynamics of surface-induced (N_2)_n cluster fragmentation. The calclated translational and rotational state distributions of the monomer products of (N_2)_n clusters scattered off a hard surfaceindicate that the translational states follow a single Maxwell-Boltazmann distribution, whereas the rotational state distributions re best represented by a sum of two distinct Boltzmann distributions, in agreeemnt with previous experimental findings obtained with a graphite surface. Analysis of the scattering dynamics provides insight into a molecular-level explanation for the differing behaviors of enregy transfer to the translational and rotational modes of the monomer products. Our simulation results indicate that transitional excitation of scattered products depends on the instantaneous cluster temperature at which the monomers evaporate. The obtention of two rotational distributions indicates that two rotational excitation mechanisms occur during the scattering event. The first mechanism is related to evaporation during cluster-surface impact of moleculse lying at the outskirts of the cluster. The degree of rotational excitation of these molecules is shown to depend both on the degree of cluster heating during impact and on the degree of rotational hindrance imparted on a given molecule prior to evaporation. The second mechanism is related to evaporation of molecules that were caged within the interior of the cluster during surface impact. These moleculse evaporate after the molecules lying at the outskirts with a degree of excitation that depends on the stabilized instantaneous cluster temperature.