Fragmentation process of size-selected aluminum cluster anions in collision with a silicon surface

Dynamical processes involved in the collision of aluminum cluster anions, Al_N^- (4≤N≤25), with a silicon surface were investigated. Intact and fragment cluster anions, Al_n^- (n≤N), were produced upon the collision. The surface-tangent and surface-normal recoil velocity components of these product anions were determined. The tangential recoil velocities of the fragment cluster anions were considerably slow, ranging from 5% to 30% of the velocity of the incident parent cluster anion, while the normal velocities were conserved relatively well. These results are explained in such a manner that the fragment cluster anion is evaporated from the parent cluster anion while it interacts with the surface and loses its tangential momentum. The dynamics and the energetics derived from these results show further that the fragmentation process involves not only sequential evaporation of aluminum atoms but also simultaneous production of several small fragment clusters. Comparison of the present result with that of the collision-induced dissociation by a rare-gas atom lends a further support on this nonsequential fragmentation mechanism.