Reduction of secondary-electron yields by collective electric fields within metals

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Abstract
A fast ion passing through a metal target drives electrons out of its path by means of Coulomb scattering. Some of these electrons escape the target and are denoted as secondary electrons. The rapid displacement of electrons from the ion’s path produces a charge separation in the metal, which gives rise to an electric field in the wake of the ion. For z>1 ions, the wake fields can be so strong that they retard the movement of electrons away from the ion’s path, which reduces the number of secondary electrons that escape the target. The time-dependent electric fields behind fast ions in metals are modeled and the motion of Coulomb-scattered electrons in these fields is examined. The fraction of the electrons that are trapped by the wake is estimated and from this the reduction in the yield of secondary electrons is quantified. The results are in agreement with measurements of secondary-electron emission by fast, high-z ions hitting metal targets.