Scattering of Ice Particles from a Graphite Surface:  A Molecular Dynamics Simulation Study

Abstract

Large-scale classical trajectory calculations of (H 2 O) n , n e 25 159, colliding with a graphite surface have been carried out in order to relate the phenomenon of direct scattering to the initial conditions of the collision. The collisions were performed at normal incidence with the incident velocity ranging from 50 to 2000 ms -1 and at surface temperatures between 300 and 1400 K. Upon impact, the cluster is deformed elastically (reversibly) and plastically (irreversibly), and if the elastically stored energy is larger than the binding energy between the cluster and the surface, the cluster scatters directly from the surface. The partitioning between elastic and plastic deformation is governed by the initial conditions (cluster temperature, incident velocity, incident cluster size, and surface temperature). At low incident velocities the scattering probability is controlled by adhesion and at high incident velocities by plastic deformation, and the direct scattering is thus confined to a narrow range of incident velocities. The results are in qualitative agreement with recent experimental studies of water clusters scattering from graphite.

Collision dynamics of large water clusters on graphite

“…This behavior can be explained by the observation that formation of charged clusters mainly is associated with evaporationmediated cluster emission. 28 Figure 4͑d͒ shows the final kinetic energy distributions in the scattered flux for incident beams with different average cluster sizes. As expected, the final kinetic energy increases with increasing incident cluster size.…”

“…We recently reported that in addition to the evaporation-mediated emission of cluster fragments, large water clusters may also undergo direct inelastic scattering. 28 The evaporation-mediated process was present at surface temperatures above 700 K, while direct scattering of clusters was observed also at lower surface temperatures.…”

“…[20][21][22]28,32 The emission of monomers and small fragments has been studied during collisions between (H 2 O) n ͑average incident cluster size n р4000) and graphite. 21 At surface temperatures of 800-1400 K, fragments evaporate from parent clusters gliding along the surface plane, while at lower temperatures the clusters are slowed down by friction forces before complete evaporation takes place.…”

“…The use of large incident angles from the surface normal direction prevents the clusters from being destroyed upon surface impact. Under these conditions neutral 28 as well as negatively and positively charged clusters 20,22,28 are emitted in sharply peaked angular distributions close to the surface tangential direction. In related work, Vostrikov and co-workers 4 -9 studied scattering of H 2 O clusters from several surface materials.…”

“…Our previous studies of the (H 2 O) n /graphite system [20][21][22]28 indicate that the collision processes are complex, and a better characterization of the dynamics is required in order to improve the understanding of the cluster charging mechanisms and the onset of direct cluster scattering as the cluster size is increased. The present paper examines the dynamics of large water clusters colliding with graphite surfaces, focusing on the flux of large neutral clusters from the surface.…”