Cluster impacts on solids

Polyatomic projectiles, which are now available in a large range of size and of incident energies, allow to deposit in solid samples high energy densities that induce large damage in the bulk of irradiated samples as well as very important emission of matter from the bombarded surfaces. Large secondary ion emission yields as well as total particle emission yields induced by Au n impacts were measured on various targets over a large range of incident energies covering the domain of energy deposited through atomic collisions and electronic excitations. An unexpected yield-energy dependence was observed with a strong maximum of the emission rate at moderate velocities (100–200 keV/atom) which may open promising developments in surface analysis. At higher velocity, it has been shown that the equilibrium charge state of the co-moving atoms of swift clusters in solid targets is significantly reduced by comparison with single atoms at the same velocity. This effect, due to the additional ionisation potential created by the proximity of the cluster constituents, has been investigated with carbon clusters of different size, in a velocity range between 600 keV up to 4 MeV/atom. The spatial distribution of cluster fragments exiting thin carbon targets were also measured with a multi-impact position sensitive detector. Comparison with theoretical simulations demonstrated that the in-target Coulomb explosion is strongly shielded. Large pulse height defects measured in silicon detectors are a direct consequence of the interaction of the spatially correlated atoms in the silicon material. Results on pulse height dependence on cluster size and energy will be presented for 1 and 2 MeV/atom carbon clusters. This set of results, obtained recently by the Orsay group (most of them in collaboration with other groups), illustrates different aspects of cluster collisions and/or transmission in solids.