Double and triple ionization of silver clusters by electron impact

Metal clusters are finite droplets of delocalized electrons with discrete energy levels, due to the small confinement volume, often referred to as ``artificial atoms'' or ``superatoms.'' Ionization processes involving clusters provide insight into the energetics of charging of finite quantum systems. While the evolution of the single-ionization energy with cluster size has been extensively studied, less is known about multiple ionization thresholds and efficiencies. We probed the production of several selected silver cluster cations Ag$_{n}^{2+}$ and Ag$_{n}^{3+}$ by electron impact ionization. The scaling of ionization thresholds with particle radius follows the metallic droplet model, but, curiously, with a slope which is significantly different from the previous literature values for single ionization. Another observation is that as the electron energy increases, the yield of high-charge cations grows faster than that of singly-charged Ag$_{n}^{+}$. This behavior is consistent with the power-law dependence of post-threshold ionization known for atomic systems. Mechanisms involved in multiple ionization phenomena in metal clusters appear far from completely understood and call for further experimental and theoretical examination.