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

Abstract

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.

Direct Analysis of Ion-Induced Peptide Fragmentation in Secondary-Ion Mass Spectrometry

“…This damage accumulates and thus the abundance of intact molecules on the sample surface drops with increasing ion fluence until the concentration of intact molecules in the sample reaches a steady state for high ion fluences. The steady state reflects a balance between the material removed from the sample and new fragments induced in the sample. , This characteristic behavior has been observed in various SIMS depth profiling experiments − and was modeled based on the erosion dynamics in an active region below the surface. , A model based on a modified diffusion equation for the concentration of intact molecules as a function of depth in the sample was applied to include these effects in the description of SIMS depth profiles of more complex systems. , …”

“…In order to gain a better understanding of how the signal evolves as a function of the primary-ion fluence and how it is linked to properties of the primary-ion species, we have developed a model similar to the transport and reaction model described in refs and . We, also, take into account the concentration of intact molecules as a function of depth in the sample in order to describe the different behavior of the DINeC and SIMS signals observed in the experiments.…”

“…33,36 A model based on a modified diffusion equation for the concentration of intact molecules as a function of depth in the sample was applied to include these effects in the description of SIMS depth profiles of more complex systems. 37,38 Figure 4 shows the signal intensity of the peak assigned to the intact angiotensin II molecule as a function of Ar 1000 + primary-ion fluence for both DINeC and SIMS measurements. The fluence dependence of the DINeC signal was determined by preparing multiple samples which were then irradiated with different ion fluences.…”