Collective effects in the energy loss of large hydrogen clusters

The energy loss of large molecular-hydrogen clusters incident on aluminum and amorphous carbon targets is analyzed as a function of the cluster size and velocity, using a dielectric formalism to describe the electronic interactions between the projectile and the target. The dependence of the energy loss with the cluster size predicts a marked different behavior for aluminum or amorphous carbon. The energy loss of H-2 clusters in aluminum indicates that for each cluster velocity there is a ''resonant'' cluster size for which the intermolecular contribution to the energy loss is maximum. On the other hand, the intermolecular contribution to the energy loss of H-2 clusters in amorphous carbon saturates with the cluster size. The origin of this behavior lies in the low-energy plasmon present in the energy loss function of amorphous carbon. We also discuss the consequences of a proper description of the dielectric properties of the target.