Electronic response of C₆₀ in slow collisions with highly charged ions

We present measurements of projectile angular differential cross sections, dσ/dθ, and mean projectile energy gain or loss, ΔE_mean, as functions of the number s of electrons stabilized on the projectile in 16- and 26.4-keV Ar⁸⁺+C₆₀→Ar^(8-s)++C₆₀^r++(r-s)e^- collisions. These results are discussed in view of two models of the electronic response of C₆₀. In the infinitely conducting sphere model the charge mobility is sufficiently high in order to average out all effects of localization of individual charge carriers. In the movable-hole model “positive holes” are assumed to be localized as point charges in their equilibrium positions on the “molecular surface” within the times (down to 10^-16 s) between sequential over-the-barrier electron transfers. The two sets of predictions for θ are close for r<~8, and for r<~5 they are also in agreement with experimental results indicating ultrafast electronic response of ionized C₆₀. For r>5, both models underestimate θ and therefore we have developed Monte Carlo calculations for close collisions with individual carbon atoms in C₆₀. The energy gain first increases with s, has a flat maximum around s=4 and yields mean energy loss ΔE_mean=-20±5 eV for s=7. The measured fragmentation spectra θ(s) and ΔE_mean(s) may be partially rationalized by combining each of the two smooth-sphere models with the Monte Carlo calculations for close collisions.