Secondary electron emission due to atomic and molecular iodine ion bombardment

A fundamental plasma-surface interaction is the emission of secondary electrons due to ion bombardment. While secondary electron emission (SEE) yields are available for many different target materials and projectiles, limited data exists for iodine ions. Iodine is an emerging alternative propellant for electric propulsion systems due to its lower cost and attractive physical properties. However, it is reactive with some materials and displays more complex plasma collisional reaction processes that can form both atomic (I⁺) and molecular ions (I₂⁺). Here we present an electrostatic probe technique and perform measurements of the SEE yield of different target materials (including Mo, W, Al, Ti, Cu, carbon-carbon, and steel) exposed to multi-species iodine ion beams with an energy between 600-1400 eV. Ion beams are generated using a gridded ion source and the beam composition is determined using time-of-flight spectrometry. While the SEE yield is found to depend only marginally on the beam composition for some target materials, the yields for both Cu and steel (which are reactive with iodine) are strongly affected. This suggests the formation of iodide layers leading to a modification in target surface properties.