Reactions in cluster-surface collisions

We provide the first report of a reaction process occurring during the collision between a cluster and a solid surface. Scattering of alkali fluoride clusters composed of >60 atoms from silicon at hyperthermal energies (0.1--0.5 eV/atom) leads to efficient F^- transfer reactions. The reaction probability approaches one-half for large clusters and newly etched Si(111)-H, following heating to desorb hydrogen. The abrupt size threshold implicates a collective mechanism, such as a shock wave in the impacting cluster.