Stopping of hypervelocity clusters in solids

Using molecular-dynamics simulations, we study the processes underlying the stopping of energetic clusters upon impact in matter. We investigate self-bombardment of both a metallic (Cu) and a van-der-Waals bonded (frozen Ar) target. Clusters with sizes up to N = 104 atoms and with energies per atom of E/N = 0.1-1600 eV atom-1 were studied. We find that the stopping force exerted on a cluster follows an N2/3-dependence with cluster size N; thus large clusters experience less stopping than equi-velocity atoms. In the course of being stopped, the cluster is strongly deformed and attains a roughly pancake shape. Due to the cluster inertia, maximum deformation occurs later than the maximum stopping force. The time scale of projectile stopping is set by t0, the time the cluster needs to cover its own diameter before impacting the target; it thus depends on both cluster size and velocity. The time when the cluster experiences its maximum stopping force is around (0.7-0.8)t0. We find that the cluster is deformed with huge strain rates of around 1/2t0; this amounts to 1011-1013 s-1 for the cases studied here. (paper)