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 $n<\mathrm{}70\mathrm{}$ can be thought of as loosely bound assemblies of small strongly bound fragments (such as ${\mathrm{Ge}}_7$ and ${\mathrm{Ge}}_{10}$). It appears that the structural transition at ∼70 atoms may reflect a change to a more bulklike bonding arrangement.