Simulating cluster-ion impacts

Although cluster-ion interactions may superficially appear to resemble those in nuclear physics, there are important differences. First, at the energies of interest the deBroglie wavelength of an atom in the cluster is so short compared to atomic dimensions that the atom's trajectory in a solid is classical. Thus, one may use the Born-Oppenheimer approximation in which the particles all obey Newton's equations of motion and only the interaction potentials rellect the quantum mechanical character of the system. Thus, we can treat many atomic interaction processes in the semi-classical limit; in this paper I shall, for example, indicate how in this way one can include the effect of atomic excitation in collisions. Second, the cross sections involved are much larger than those for nuclear interactions, i.e., the mean free paths of the "reaction products" are so short that large collective effects always occur. This means that the calculations must deal with large numbers of interacting particles. For the case of cluster ions at MeV energies this has required us to develop new computational strategies, e.g., the use of massively parallel computers. By using such simulations strategically we are able both to optimize the design of experiments and to interpret their results less ambiguously.