Effects of Pressure and Electric Field Strength on the Emission Rate for a Molecular Dynamics Model of [EMIM][BF4] Ionic Liquid

Through an all-atom molecular dynamics approach, a model of a bulk ionic liquid in the presence of an applied electric field is developed to study the relationship between the pressure associated with a mass flow through an emitter, the applied electric field strength, and the resulting ion emission rates for 1-Ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]). In these simulations, a force is applied to a rigid wall located below the bulk ionic liquid placed in a domain that is laterally periodic and fixed in the Z-direction. A nanopore placed above the bulk ionic liquid allows molecules to form a convex meniscus due to the intrinsic surface tension of the ionic liquid. When an electric field is applied to the ionic liquid, ion emissions are produced from the meniscus as long as the force on the liquid due to the field is able to overcome the surface tension of the [EMIM][BF4]. As a pressure differential is applied and the electric field strength is varied, the emission rate changes, and a relationship is developed for the adjustable parameters and the resulting flow of ions. Experimental work on secondary mass flux was carried out in conjunction with the simulations. A diagnostic for measuring ejected mass at a polar angle from a bombarded target was developed and measurements were made to validate the methodology and characterize the energy dependence and angular profile. Mass flux yields from a high voltage primary plume dropped from 7.304×10−14 ng ion-sr at a polar angle of 35◦ to 3.645×10−14 ng ion-sr at 80◦. When spraying a low voltage plume, the yields were relatively constant across the set of polar angles, with values ranging from 7.391×10−14 ng ion-sr to 8.828×10−14 ng ion-sr. Results indicate that the high energy plume produces a secondary plume with less mass than atomic carbon sputtering nickel, but more mass than if the carbon atoms simply bounced off the surface of the target.