ATOM EJECTION FROM A FAST-ION TRACK - A MOLECULAR-DYNAMICS STUDY

As a model for atom ejection from fast-ion tracks, molecular-dynamics simulations of a cylindrical track of energized particles are performed. An idealized situation is studied where every atom in a cylindrical track of radius R(o) is energized with energy E(o). The emission yield Y(E(o),R(o)) shows the existence of two ejection regimes. If the particle energy E(o) is below the sublimation energy U of the material, a threshold regime is seen in which Y rises roughly like the third power of E(o); for high-energy densities E(o) greater than or similar to U, the yield rises much more slowly, roughly linearly. In both cases, ejected particles mostly originate from the track, rather than from its surroundings, and from the first or the first few monolayers. The behavior found is interpreted here in terms of emission due to a pressure-driven jet (linear regime) or due to a pressure pulse (threshold regime). These both behave differently from the often-used thermal-spike sputtering model.