Silicon cluster ions: Evidence for a structural transition

The mobilities of size-selected silicon cluster ions in helium have been measured using injected-ion drift-tube techniques. The results suggest that a major structural transition occurs for clusters with {~}27 atoms. Clusters with up to {~}27 atoms appear to follow a prolate growth sequence, resulting in geometries with an aspect ratio estimated to be {~}3. Larger clusters appear to have more spherical shapes. For annealed clusters this structural transition occurs over a narrow size range.