Strong velocity effects in collisions of He+ with fullerenes

We have studied fragmentation and ionization of C-60 by He+ impact over a v elocity range from 0.1 to 1 a.u. where a transition from vibrational to ele ctronic excitation is predicted. With increasing velocity we observe a stro ng decrease of evaporative processes (C-60-2m(r+) peaks) and a linearly inc reasing yield of small fragments (C-n(+), n < 15). Apparently direct vibrat ional excitation leads preferentially to evaporation, whereas electronic ex citation is responsible for multifragmentation. In the intermediate range w e find indications for a "transparency window" similar to that predicted re cently in simulations of Na-9(+)-He collisions. [S0031-9007(98)08105-8].