Electronic versus vibrational excitation in Heq+ collisions with fullerenes

Collisions of Heq+ ions with neutral fullerenes have been studied as a function of projectile velocity (v approximate to 0.1-1.0 a.u.) and charge state (q = 1, 2). With increasing velocity, two trends are observed for both charge states: The yield of C-60-2m(r+) clusters decreases with 1/v, as expected for quantities related to direct vibrational excitation. The relative cross section for multifragmentation increases linearly with v and can be associated with electronic excitations. The additional potential energy of He2+ with respect to He+ manifests in increased direct ionization cross sections thigh v) and multifragmentation cross sections (low v), respectively, revealing information about coupling times between electronic and vibrational excitation. (C) 1999 Elsevier Science B.V.