Water cluster collisions with graphite surfaces: Angular-resolved emission of large cluster ions

We present experimental studies of negative cluster ions formed during scattering of (H2O)(n) ((n) over bar less than or equal to 4600) from graphite surfaces. Angular distributions are measured using surface temperatures of 1280-1450 K and an incident cluster velocity of 1380 m/s. Large cluster ions are found to be emitted in sharply peaked distributions close to the tangential direction, except for normal incidence which produces distributions that are peaked in the surface normal direction. Time-of-flight measurements, combined with energy analysis of the emitted negative ions, are used to determine the final velocity and size of the cluster ions. The clusters are concluded to conserve 65-85% of their velocity parallel to the surface plane during a surface interaction, while the velocity component in the normal direction is 75-100 m/s independent of incident angle. The clusters undergo considerable fragmentation in surface contact, and 15-25% of a cluster survives as one unit for an initial cluster size of a few thousand molecules. The experimental findings are compared with results from earlier studies, and the detailed collision dynamics and conditions under which cluster charging takes place are discussed.