Collisions of fast clusters with solids and related phenomena

Fast ions, impinging on solid targets at a few MeV per atom, deposit energy in the material through electronic excitation processes. The relaxation of this energy induces emissions of photons, electrons, ions and neutral species from the target. The detailed study of these ejecta can give insight on the energy deposition and relaxation steps of the ion-solid interaction. Fast polyatomic projectiles allow to deposit very high energy densities (at the surface and in the bulk) well beyond that being possible with any single ion impact. As a result, new and unexpected phenomena are oberved which will be discussed: large non linear emission of ions and cluster ions, very high sputtering yields, production of giant tracks and craters in various irradiated materials including metals. When entering the solid, the atomic constituents of the projectile remain in a close proximity for a certain distance, and the energy density deposited in the overlapping trajectories region is high enough to induce collective effects. Secondary ion emission experiments, performed with various carbon cluster beams delivered by the Tandem accelerator in Orsay, aim to determine this depth of spatial correlation by probing these collective effects inside the solid. The influence of the proximity of the cluster constituents on some of their properties such as their charge state distributions inside the solid has been studied and it has been shown that the mean charge state of the constituents is significantly smaller than when the atoms penetrate the target independently.