Ion tracks in solids: Sputtering and surface modification

A swift ion bombarding a solid forms a track of excitations and ionizations, creating a very high energy density over a narrow region. At the surface, this can lead to atom/molecule ejection (sputtering) and surface damage. An unexplained result for electronically induced sputtering is that the yield Y (i.e., the number of atoms/molecules ejected by a single projectile) appears to depend quadratically on the energy loss per unit path length, dE/dx, for molecular condensed gas solids at large dE/dx. This data has been parametrized using Thermal Spike models.; Using Molecular Dynamics (MD) simulations, I calculated the sputtering yield from a cylindrically energized region and found that the analytic and semi-analytic thermal spike models cannot be applied at large excitation densities. The main reasons are the following: (a) the high temperature of the track produces large density and pressure changes, which are typically neglected in these models; (b) melting also occurs and, after a pressure pulse is released, the phase change interface controls the heat flux; (c) even if the previous limitations are accounted for by using the complete set of 1-D hydrodynamic equations, the surface is not included in most models, but energy transport towards the surface enhances the sputtering. Therefore, MD simulations are required to understand sputtering at high energy densities.; The MD yields were found to be nearly linear in the energy density driving sputtering, (dE/dx)_eff, at high (dE/dx)_eff and constant initial track radius, r_cyl. Therefore, I have analyzed two explanations for the experimental yields. First, ( dE/dx)_eff could be non-linear in dE/dx if, for instance, Coulomb explosion were driving the sputtering. MD simulations of Coulomb explosion were studied, but the effectiveness of this process was shown to be very sensitive to the neutralization rate of the ions in the track. Second, as the MD yields increased with r_cyl for fixed (dE/ dx)_eff, I examined the growth of r_cyl with (dE/dx) for sputtering of condensed gas solids by MeV He+. Finally, MD calculations of cratering and surface modification produced by an incident ion were found to exhibit the same trends seen in experiments.