MD-Based Transport and Reaction Model for the Simulation of SIMS Depth Profiles of Molecular Targets

We present a novel theoretical approach allowing us to model erosion and chemical alteration of organic samples during depth profiling analysis by secondary-ion mass spectrometry with cluster projectile ion beams. This approach is able to take into account all of the cumulative phenomena occurring during such analysis, including ion-beam-induced reactions and atomic/molecular mixing by means of the numerical solution of an advection-diffusion-reaction (ADR) differential equation. The results from the single-impact molecular dynamics computer simulations are used as a source for the input parameters into the ADR model. Such an approach is fast and allows turning the phenomenological model into a more quantitative tool capable of calculating molecular secondary-ion mass spectrometry depth profiles. The model is used to describe phenomena taking place during depth profiling of polystyrene samples by 20 keV C60, Ar872, and Ar1000 projectiles. It is shown that theoretical findings are in good agreement with the experimental results. The model is also used to determine the overall efficiency of nitrogen monoxide molecules in eliminating the radicals responsible for polystyrene cross-linking induced by analyzing ion beams.