Electrochemical Reactions of Ionic Liquid in Vacuum and Their Influence on Ion-Beam Production by Electrospray

Abstract

Electrochemical reactions at interfaces between metal and ionic liquid are of great practical importance in ionic liquid ion sources, which can produce negative and positive ion beams by electrospray in vacuum. This is because electrochemical reactions involved in ion beam production can cause emitter degradation, leading to beam instabilities. Here we investigate ion beam production using a tungsten needle wetted with ionic liquid. We also investigate electrochemical reactions of ionic liquid at tungsten electrodes in vacuum with a quadrupole mass spectrometer, which allows us to measure volatile products formed via electrochemical reactions. Two types of ionic liquids were tested: propylammonium nitrate (PAN), which is protic and hydrophilic, and 1-ethyl-3-methyl imidazolium bis(trifluoromethanesulfonyl)amide ([EMIM][TFSA]), which is aprotic and hydrophobic. Beam measurements show that a negative ion beam containing large cluster ions can be produced continuously using each ionic liquid. Electrochemical measurements with PAN show that volatile products are formed via electrochemical oxidation and reduction, thereby indicating that ion beams of PAN can be produced continuously in both negative and positive ion modes. Experiments with [EMIM][TFSA] show that the electrochemical reductions of both [EMIM]+ and [TFSA]− occur and thus form volatile products, indicating that such reactions enable its continuous negative ion beam production.