Transport of Caesium ions through thin poly-p-xylylene films

We describe an experimental setup for investigating the transport of alkali ions through poly(p-xylylene) membranes. It consists of an ion source where a continuous beam of alkali ions is generated from a surface emitter. Via ion-optics the ions are guided to the main chamber where the interaction with the free-standing membranes of variable thickness is investigated as a function of the impact energy. For a cesium ion beam the transmission of ions exhibits a maximum at an impact energy of several hundred volts, depending on the membrane thickness. The transport of the ions most likely proceeds through pores or porosities in the membrane. At the highest impact energies employed (around 2000 eV) the transmission of electrons is observed, most likely due to collision processes causing kinetic electron emission.