COLLECTIVE EFFECTS IN THE ENERGY-LOSS OF LARGE HYDROGEN CLUSTERS

The energy loss of large molecular-hydrogen clusters incident on alumi num and amorphous carbon targets is analyzed as a function of the clus ter size and velocity, using a dielectric formalism to describe the el ectronic interactions between the projectile and the target. The depen dence of the energy loss with the cluster size predicts a marked diffe rent behavior for aluminum or amorphous carbon. The energy loss of H-2 clusters in aluminum indicates that for each cluster velocity there i s a ''resonant'' cluster size for which the intermolecular contributio n to the energy loss is maximum. On the other hand, the intermolecular contribution to the energy loss of H-2 clusters in amorphous carbon s aturates with the cluster size. The origin of this behavior lies in th e low-energy plasmon present in the energy loss function of amorphous carbon. We also discuss the consequences of a proper description of th e dielectric properties of the target.