Electronic response of C-60 in slow collisions with highly charged ions - art. no. 022712

We present measurements of projectile angular differential cross sections, d sigma/d theta, and mean projectile energy gain or loss, Delta E-mean, as functions of the number s of electrons stabilized on the projectile in 16- and 26.4-keV Ar8+ + C-60 --> Ar(8-s)+ + C-60(r+) + (r-s)e(-) collisions. Th ese results are discussed in view of two models of the electronic response of C-60. In the infinitely conducting sphere model the charge mobility is s ufficiently high in order to average out all effects of localization of ind ividual charge carriers. In the movable-hole model "positive holes" are ass umed to be localized as point charges in their equilibrium positions on the "molecular surface" within thr times (down to 10(-16) s) between sequentia l over-the-barrier electron transfers. The two sets of predictions for thet a are close for r less than or equal to 8, and for r less than or equal to 5 they are also in agreement with experimental results indicating ultrafast electronic response of ionized C-60. For r > 5, both models underestimate theta and therefore we have developed Monte Carlo calculations for close co llisions with individual carbon atoms in C-60. The energy gain first increa ses with s, has a flat maximum around s = 4 and yields mean energy loss Del ta E-mean = -20 +/- 5 eV for s = 7. The measured fragmentation spectra thet a(s) and Delta E-mean(s) may be partially rationalized by combining each of the two smooth-sphere models with the Monte Carlo calculations for close c ollisions.