Ion beam generation from a protic ionic liquid source with an externally wetted tungsten needle

Abstract

Ionic liquid ion sources (ILISs) are similar to liquid metal ion sources and use ionic liquids (i.e., room temperature molten salts) instead of liquid metals. Aprotic ionic liquids have been used in most cases, whereas protic ion liquids have hardly been used thus far. In this study, propylammonium nitrate (PAN), which is a protic ionic liquid, was tested as the liquid material for an ILIS. Beam generation was investigated in DC operation using a tungsten needle externally wetted with PAN. Experimental results showed that a stable ion beam was generated without special measures and that the ion beam contained cluster ions with m/z values on the order of 103. Additionally, electrochemical experiments were performed in vacuum to investigate electrochemical reactions involved in beam generation. Residual gas analysis revealed that H2O and O2 were electrochemically produced at a tungsten electrode immersed in PAN. These products will probably result from electrochemical reactions of water in PAN because protic ionic liquids contain a small amount of water even in vacuum. The stability of the ion beam can be explained by the effect of water in PAN.