Electron emission mechanism for impact of C_N^- and Sl_N^- clusters

Surface collisions of negatively charged atomic clusters C_N^- and Si_N^-, N < 15, have been investigated by time-of-flight methods. At low impact velocities (<10 km/s), the intact scattered ion and its charged fragments are clearly observed, along with ejected electrons. The time profile for the electron is broad and asymmetric, proving that it arises from delayed emission from the intact scattered negative ion. This provides a simple explanation for the low-energy emission found by Achiba and co-workers which they attribute to cluster electronic excitation. The relative yield of ions and electrons suggests that fragmentation and electron emission are competing processes for cooling the hot scattered ion.