Size-Dependent Electron-Impact Detachment of Internally Cold C_n^- and Al_n^- Clusters

The cross sections for electron detachment of internally cold C_n^- and Al_n^- clusters were measured using an electrostatic ion beam trap fitted with an internal electron target. The measured electron-impact detachment cross sections for the C_n^- (n=1–9) clusters exhibit even-odd oscillations reflecting the binding energy trend, namely, higher cross sections for weaker binding. Surprisingly, however, these cross sections increase on the average with cluster size, n, in spite of the increase in electron binding. In contrast, the Al_n^- (n=2–5) clusters follow the known scaling laws for electron detachment. We suggest that the size-dependent polarizability of these clusters is responsible for the observed behavior.