Positive and negative secondary species emission behavior for an ionic liquid electrospray

Ionic liquid electrosprays can emit a polydisperse population of charged droplets, clusters, and molecular ions at high velocity. Secondary species emission (SSE) is a term that encompasses many concurrent impact and emission phenomena that occur when electrosprayed primary species strike a surface, resulting in a diverse population of secondary electrons, ions, clusters, and droplets. This Letter examines the spatial dependency of SSE behavior across an [EMI]Im electrospray beam using microscopy of the target surface and experimental quantification of SSE yields as a function of the plume angle. Microscopy of the beam target confirms our prediction of shock-induced desorption when operating at elevated beam voltages. SSE yield measurements show that, upon impact with a surface, incident primary species that consist of entirely positive charge will produce both positive and negative SSE. Furthermore, the results show that the SSE yields for an ionic liquid electrospray have strong spatial and energy dependencies. These findings have significant implications for understanding and predicting ionic liquid electrospray thruster lifetime and performance and focused ion beam applications.