Research of wake effects on the charge of dissociation fragments of fast molecular ions in a thin sheet

When molecular ions are injected into a solid target the bonding electrons are removed and the atomic components propagate through the medium as clusters of correlated charged particles. Their propagation is influenced by the interaction of the ions with the medium, by their mutuel ''Coulomb'' repulsion and by the effects of the electronic wake trailing each ion. This wake can be described as follows: when a fas charged particle is moving faster than the Fermi velocity in a solid target, the target electrons tend to screen the ion charge and an oscillatory charge density, which is stationary in the projectile frame, trails the ion. The reality of this oscillatory behaviour has been already demonstrated in several experiments using molecular projectiles. We have done an experimental study where we tried to observe those oscillations by measuring the neutral hydrogen component emerging at O0 when 100 keV/amu HeH+ and NH+ molecular ions are incident into thin carbon foils. We have not observed any effect related expected to the oscillations of the electronic density at the back surface of the foil. But we have observed significant over production of neutral hydrogen when the cluster is aligned with the beam direction. This suggests that the probability of electronic capture increases when the proton is aligned in the track of its heavy partner (N ou He)