Projectile atomic-number effect on ion-induced fragmentation and ionization of fullerenes

The delocalized \ensuremath{\pi} electrons of a ${\mathrm{C}}_{60}$ cluster can be well described as an electron gas. Electronic friction experienced by a multicharged ion colliding with a fullerene might then be modeled in terms of the electronic stopping power. We investigated such collisions for projectile atomic numbers Z ranging from 2 to 18 and charge states $q=2,3.$ Direct mass-spectrometric evidence of the clear oscillatory behavior of electronic-stopping-related processes such as multifragmentation and ionization of ${\mathrm{C}}_{60}$ was observed as a function of Z. From the positions of the maxima of the corresponding cross sections, two classes of trajectories can be distinguished: close collisions through the high electron density of the cage inducing multifragmentation, and glancing collisions through the lower electron densities of the fullerene ``atmosphere'' inducing direct ionization.