MOBILITIES OF SILICON CLUSTER IONS - THE REACTIVITY OF SILICON SAUSAGES AND SPHERES

The mobilities of size selected silicon cluster ions, Si(n)+ (n = 10-60), have been measured using injected ion drift tube techniques. Two families of isomers have been resolved by their different mobilities. From comparison of the measured mobilities with the predictions of a simple model, it appears that clusters larger than Si10+ follow a prolate growth sequence to give sausage-shaped geometries. A more spherical isomer appears for clusters with n>23, and this isomer completely dominates for unannealed clusters with n>35. Annealing converts the sausage-shaped isomer into the more spherical form for n>30. Activation energies for this "sausage-to-sphere" structural transition have been estimated for several cluster sizes and are approximately 1.2-1.5 eV. We have examined the chemical reactivity of the sausages and spheres towards both C2H4 and O2. With C2H4 large differences in reactivity of the isomers were found, with the spherical isomer often being more reactive than the sausage form by more than an order of magnitude. With O2 the variations in reactivity were smaller. Despite the substantial differences in reactivity observed for the two isomers in the cluster size regime where both forms coexist, examination of a broader range of cluster sizes shows that there is not a systematic change in reactivity associated with the geometry change.