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 H2 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 H2 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. @S1050-2947~96!08810-5#