Multi-ionization cross-sections of small ionic carbon clusters by particleimpact as a tool to investigate their shapes

Single, double and triple ionization cross-sections of C-n(+) clusters in c ollisions with helium atoms at intermediate velocity (2.6 a.u.) have been m easured (n = 1 --> 5). The relative multi-ionization cross-sections (double over single and triple over single) are first increasing with n, then decr easing when going from n = 4 to n = 5. We show, on the basis of an independ ent atom and electron collisional treatment, that this effect is attributab le to a change in the cluster shape. The role of the cluster compactness on relative multi-ionization cross-sections is pointed out here for the first time.