Electron Emission from Surfaces Induced by Slow Ions and Atoms

This chapter is concerned with heavy particle collisions (ions and atoms) at low energies (below a few keV) and will discuss first the physical mechanisms and then applications to electrical discharges in gases. The overall picture of electron emission induced by heavy particles is the following. Electrons are excited from the target or the projectile as a result of Coulomb interactions involving the nuclei and electrons through mechanisms that are grouped in two categories, potential and kinetic, depending on the source of the excitation energy. Such excitations occur mostly in binary collisions at the surface or very shallow depths, since the penetration depth of low-energy atomic projectiles is usually very shallow, tens of nm or less. The excited electrons can be ejected directly into vacuum or undergo a series of collisions in the target solid (electron transport) on their way to the surface. The collisions are either elastic scattering with atomic cores, which cause large deflections in trajectories, or energy loss collisions by scattering with other electrons, contingent on the availability of electronic states. As a result of such inelastic scattering, the electrons which succeed in escaping the solid come from a shallow depth, of the order of 2nm for metals and semiconductors and up to a few tens of nm for insulators. In the latter case, the depth of origin of electrons is limited, at low primary energies, by the slowing down of the projectile mainly as a result of ion-atom collisions. Electron escape involves energy loss through the surface barrier and change of momentum perpendicular to the surface.