Structural Transitions in Size-Selected Germanium Cluster Ions

Injected ion drift tube techniques have been used to probe the geometries of germanium cluster ions. Clusters with {~}(10-40) atoms appear to follow a one-dimensional growth sequence to give prolate geometries. At {~}40 atoms the clusters stop following this growth sequence, and clusters with {~}(40-70) atoms appear to retain roughly the same aspect ratio. At {~}70 atoms the clusters abruptly reconstruct to a more spherical geometry. Dissociation energies, measured for the germanium clusters, suggest that clusters with $n<70$ can be thought of as loosely bound assemblies of small strongly bound fragments (such as Ge₇ and Ge₁₀). It appears that the structural transition at {~}70 atoms may reflect a change to a more bulklike bonding arrangement.