Formation of secondary cluster ions during sputtering of silver and copper

By simultaneous measurements of neutral and ionized clusters sputtered from polycrystalline silver and copper, the absolute ionization probabilities ${\mathrm{\alpha }}_{\mathit{X}\mathit{n}}^{+}$ for the formation of the corresponding secondary ions ${\mathit{X}}_{\mathit{n}}^{+}$ (n=1,...,4) are determined. The alternating abundance of ${\mathrm{Ag}}_{\mathit{n}}^{+}$ and ${\mathrm{Cu}}_{\mathit{n}}^{+}$ is discussed quantitatively in terms of microscopic models for surface ionization. At least for silver clusters this effect is ascribed to an alternation of the ionization probability induced by the different ionization potentials of the sputtered clusters, which were also measured.