Strong Forward-Backward Asymmetry of Water Ionic Fragments by Slow Highly Charged Ions Impact

Fragmentation of molecules subsequent to the interaction with ions has been studied intensively in the last decade. If the projectile is fast that the interaction time is orders of magnitude smaller than the dissociation time, the collision can be considered as a two-step process . On the other hand, for slow (v < 1 a. u.), highly charged projectiles the energy and angular distribution of fragments is determined by the energy and momentum transferred from the projectile, in addition to the energy gained by the repulsive dissociation of the molecule.
We present the results of the fragment ion spectroscopy studies of water molecules subsequent to impact of highly charged 1-220 keV Xeq+ ions (q=10, 15 and 22). We observed a strong forwardbackward asymmetry in the emission of the ionic fragments. For example, the ion yield in the forward direction is almost completely suppressed for the impact of below 10 keV Xe22+ ions.

These results are compared with our previous studies of water fragmentation induced by He1,2+ and Ne(3-9)+ ions . For projectiles with low charge state, the final kinetic energy is apparently determined by the related Franck-Condon transition. In order to guide the interpretation of the experimental data, we performed a classical trajectory simulation within the framework of a Coulomb Explosion model wherein a satisfactory agreement is achieved. Finally, the integrated cross sections over all fragmentation channels are compared with results of the semiempirical scaling law deduced from a multi-electron capture model.