Electronic excitation in H+-C-60 collisions: Evaporation and ionization

We have studied energy transfer, fragmentation and ionization in collisions between protons and C-60 molecules. The collision energy is varied between 1 and 300 keV and we thus cover a velocity range from 0.2 to 3.5 atomic units. The distributions of intact and fragmented C-60 ions have been measured by means of time-of-flight mass spectrometry using pulsed beams or emitted electrons to clock the collision events. The processes leading to multiple ionization and evaporation of neutral dimers are analyzed as functions of the projectile velocity. In contrast to recent findings for He+-C-60 collisions by Schlatholter et al., our results show that the cross sections for ionization and evaporation first increase with velocity, have maxims at around 1-2 a.u., and decrease again for higher velocities. This velocity dependence closely resembles the one for the electronic stopping power indicating that electronic excitations are the main means of energy transfer to C-60 and that this energy is rapidly distributed among the vibrational degrees of freedom. This conclusion is supported by Monte Carlo calculations of nuclear and electronic energy loss for protons passing through and closely outside the C-60 cage.