DELAYED EMISSION OF ELECTRONS FROM C-60 FOLLOWING ENERGETIC IMPACT AGAINST GRAPHITE

The yield of C60+ ions reflected from the surface of graphite is found to be a bimodal function of the energy of impact. Below 140 eV (6 km/s), the yield decreases with energy and the scattering time-of-flight peak is sharp. Above 170 eV, the yield increases, but the peak is broad and asymmetric, corresponding to delayed electron emission from an impact-heated, neutralized cluster. There is also evidence for fragmentation to C58 and C56. The emission process may be analogous to that reported for scattered C60-. Application of statistical rate theory to either observation (C60+/-) gives an estimated 25-30 percent transfer of impact energy into cluster internal modes. The decreasing yield at highest energies (> 400 eV) may result from adhesion processes (C60-surface bonding) that could also account for the absence of delayed electron emission in C60-silicon collisions.