DISSOCIATION DYNAMICS OF DIATOMIC-MOLECULES EMBEDDED IN IMPACT HEATEDRARE-GAS CLUSTERS

Molecular dynamics simulations demonstrate facile dissociation of halo gen molecules embedded in rare gas clusters upon impact at a surface a t collision velocities up to 10 km/s. Two pathways are discerned: a he terogeneous dissociation of the molecule on the surface and a homogene ous mechanism where rare gas atoms which have rebounded from the surfa ce cause the translational-vibrational coupling. The total yield of di ssociation of the clustered molecule can reach up to 100%, whereas the yield of dissociation of the bare, vibrationally cold molecule satura tes below 40%. A systematic study of the role of different conditions is made possible by not accounting for the atomic structure bf the sur face. The role of dissipation at the surface is found, however, to be quite important and is allowed for. Larger clusters, clusters of the h eavier rare gases and a more rigid surface, all favor the homogeneous mechanism. Evidence for a shock front which, upon the initial impact, propagates into the cluster; the binary nature of the homogeneous diss ociation process; and the absence of a dominant cage effect are discus sed. A quantitative functional form of the velocity dependence of the yield of dissociation, which accounts for the size of the cluster, the rigidity of the surface and other attributes, is used to represent th e data. The physics of the processes within the cluster is dominated b y the novel dynamical features made possible when the duration of the atom-molecule collisions is short compared to the vibrational period. This ''sudden'' regime is sudden with respect to all modes of the nucl ear motion and provides a hitherto unavailable tool for examination of reaction dynamics under extreme conditions.