Sputtering of Surfaces by Ion Irradiation: A Comparison of Binary Collision Approximation and Molecular Dynamic Models to Laboratory Measurements

Surface sputtering by solar-wind ion irradiation of airless planetary bodies is important for understanding the body’s surface and exospheric compositions. Laboratory simulations are both complex and expensive. Hence, theoretical sputtering models are used to describe the behavior of the incoming ions, impacted surface, and sputtered atoms. Results from these models provide inputs for exospheric models. The most common sputter models use the binary collision approximation (BCA), such as SDTrim.SP. While these models are typically accurate for high impact energies, the approximations can potentially introduce error for low energy impacts or crystalline targets. In contrast, Molecular Dynamics (MD) simulations remove the BCA approximation and are able to simulate the entire collision cascade, albeit at a high computational cost. MD simulations also allow one to control the properties of the initial substrate, including crystalline structure, temperature, crystal damage, and roughness. However, MD simulations can require several user specific choices, such as interatomic potential, that can affect accuracy. Before either BCA or MD models can be confidently used to simulate planetary surface ion impacts, it is important to verify their ranges of applicability and limitations by comparison to pre-existing experimental values. Specifically, we consider a copper substrate impacted by argon ions with kinetic energies between 200-1000 eV and compare the SDTrim.SP and MD results to the experimental values for the resulting sputtering yield and energy distributions. In addition, we compare results with the commonly used Thompson model for the energy distributions. Overall, our findings highlight the potential advantages for MD simulations when specific substrate and simulation parameters are desired.