Secondary electron generation by sub-keV ion and energetic neutral particle impacts on a surface

The secondary electron yield (SEY) from sub-keV ion and neutral particle impacts on a stainless steel surface has been investigated in the laboratory. Sub-keV ions were generated using a large Kauffman ion source and energetic neutral particles (ENPs) were generated from ion-neutral charge-exchange collisions. It is found that the SEY from ion impacts increases with the ion kinetic energy, reaching the yield of $\sim$0.25-0.4 by the 1000 eV ion impact, depending on the surface cleanliness. The SEY from neutral impacts is about an order of magnitude lower than from ion impacts, indicating that the electrostatic potential energy plays a bigger role in generating secondary electrons than the kinetic energy in this energy range. It is shown that the SEY is higher for impacts by lighter ion species. The effect of surface cleanliness is investigated, showing an increase in the SEY for a contaminated surface. Our results show that secondary electrons generated from sub-keV ion impacts are non-negligible and may play a more pronounced role in determining the surface charge in various space environments, such as permanently shadowed regions (PSRs) on airless bodies. The SEY measured from sub-keV ENP impacts is useful for determining the surface charge of a spacecraft moving through dense planetary atmospheres.