Potential electron emission induced by multiply charged ions in thin film tunnel junctions

Thin film metal-insulator-metal tunnel junctions are used to investigate the electronic excitation process induced by the impact of multiply charged ions onto a metallic surface. Hot charge carriers (electrons and holes) generated by the dissipation of the kinetic and potential energies of the projectiles are detected as an ion induced internal emission current from the bombarded “top” metal film into the “bottom” substrate electrode. Results are presented for Ar^q+ ions with a kinetic impact energy of 1 keV and charge states q=1–8 impinging onto an Ag-AlO_x-Al junction. It is shown that the internal emission yield exhibits an approximately linear dependence on the potential energy of the projectile. At low potential energy, a bias voltage applied between the two metal films is found to strongly influence the internal emission current, whereas this influence becomes much weaker with increasing projectile charge state. The results are shown to be qualitatively well described in the framework of a thermodynamical free-electron model.