HICS: Highly charged ion collisions with surfaces
Abstract The layout of a new instrument designed to study the interaction of highly charged ions with surfaces, which consists of an ion source, a beamline including charge separation and a target chamber, is presented here. By varying the charge state and impact velocity of the projectiles separately, the dissipation of potential and kinetic energy at or below the surface can be studied independently. The target chamber offers the use of tunable metal–insulator–metal devices as detectors for internal electronic excitation, a time-of-flight system to study the impact induced particle emission and the possibility to transfer samples in situ to an ultra high vacuum (UHV) scanning probe microscope. Samples and detectors can be prepared in situ as well. As a first example data on graphene layers on SrTiO3 which have been irradiated with Xe36+ are presented.
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{sup q+} ions with a kinetic impact energy of 1 keV and charge states q=1-8 impinging onto an Ag-AlO{sub 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.
Angle resolved electron spectra from grazing incidence He2+/Cu(110) collisions
Electron spectra from keV collisions of He2+ with a single crystal Cu(110) surface are reported. The spectra are obtained for well defined angles of incidence and detection. From the spectra and from their variation with detection angle the following conclusions are drawn. At vertical velocities of the order of 0.01 atomic units the He2+ neutralizes in two successive steps while it approaches the surface. In the first step He2+ is converted to He+ in the ground state by two competing processes, namely, double capture followed by autoionization or Penning ionization, and direct Auger capture. In the second step He+ is neutralized by Auger capture into ground state He. A model is developed that allows one to calculate electron spectra due to those processes using model functions for distance-dependent transition rates and transition energies. It is shown that the main features of the observed spectra can be well reproduced by the model calculations if it is assumed that the metal does not respond adiabatically to the sudden changes induced upon the emission of fast electrons in the various spontaneous ionization processes.