Institut des
NanoSciences de Paris

Clusters and Surfaces under Strong Excitations

Ion-solid interaction at high velocity

Study of the interaction of a fast heavy ion with the matter can be approached in two different ways. One can be interested in the target modifications, like the formation of latent track in insulators, or the fragmentation of small molecules or clusters. Another alternative is to study what happens to the projectile like its energy loss, the evolution of its charge state, the population of its excited states. Actually, these two aspects are closely related. For instance, a simple change of the projectile charge state results in a variation of its energy deposition and thus on the target modifications. Similarly, the target response to the projectile penetration has direct consequences on the incident ion itself, leading to modifications of the population of its excited states. Our studies are dedicated to the production and the transport of the projectile excited states, which are used as a “probe” of target material response to the travelling ion. The diffusion of an ion by a solid offers the quasi unique possibility to treat an isolated system (the ion) in contact with a complex reservoir (the solid). By controlling the collision conditions (i.e. atomic number, charge, velocity and excited levels of the projectile) we could explore various systems from the single collision condition to the equilibrium of populations. We showed that different experimental observables had a sensitivity more or less marked to the “solid” environment. While for the projectile charge state distribution, for example, the ion-solid interaction can be described by a succession of individual binary collisions where ion-atom cross sections of the elementary processes can be used, collective and specific effects due to the target response could be highlighted by precise measurements of n,l,m cross sections. These results initiated new theoretical developments which recently succeed to describe ion transport in matter with an unequalled precision and by using the suitable space of phases (quantum or classical) according to the experimental observable one seeks to describe. These new theories bring essential elements to describe the ion energy loss in matter by ab initio calculations and to control the production of intense beams in the new generation devices.