Molecular dynamics simulations have been used to model the kiloelectronvolt particle bombardment of organic layers on metal substrates such as occurs in the analytical techniques of secondary ion mass spectrometry and fast atom bombardment mass spectrometry. Vignettes of insights gained from the simulations along with comparisons to experimental data are presented in this Account. Topics include intact molecular ejection vs fragmentation, prediction of reaction pathways, influence of the substrate, and quantitative predictions of energy and angular distributions.
In this theoretical investigation, we combine the results of molecular dynamics (MD) simulations with a simple statistical sputtering model (SSM) in order to understand the factors limiting the optimum depth resolution achievable in sputter depth profiling experiments. The advantage of the SSM model is that it can be used to extrapolate the MD simulations toward the regime of high projectile fluence. First, a recently developed scheme [
Experiments have shown that the use of polyatomic projectiles in secondary ion mass spectrometry (SIMS)
increases the secondary yield of molecular ions by an order of magnitude or more. This observation, coupled
with the availability of an SF5
+ source, has sparked renewed interest in SIMS measurements for characterizing
a wide range of molecules. In this paper, we present the results of molecular dynamics simulations with Xe
and SF5 projectiles that show that the molecular ion yield from bombarded organic surfaces is enhanced by
the use of polyatomic projectiles. The model systems consist of a monolayer of twenty biphenyl molecules
on two different substrates, Cu(001) and Si(100), and are designed as a prototype for experimentally studied
systems. Our results show that the structure of the lattice is the critical factor. The breakup of the SF5 cluster
within the more open lattice of the Si(100) substrate initiates collision cascades that lead to substrate atoms
hitting the biphenyl molecules from below, which results in a greater yield of ejected molecules. The results
are important because they predict that the nature of the substrate or matrix is a critical factor in maximizing
the molecular ion yield.
Experiments show that polyatomic projectiles have the potential to improve the sensitivity of organic secondary
ion mass spectrometry by increasing the yield without a comparable increase in damage to the sample. Molecular
dynamics simulations of the high energy bombardment of an organic film have been performed with the
purpose of understanding how the yield-to-damage ratio is enhanced with polyatomic projectiles. The model
systems consist of 0.6 keV Xe and SF5 projectiles bombarding a monolayer of biphenyl molecules on two
different substrates, Cu(100) and Si(001). The yield-to-damage ratio is the ratio of the yield, defined as the
number of molecules ejected stable and intact, to the damage, defined as the sum of molecules ejected either
fragmented or unstable. To have a quantity that is comparable to the experimental definition of damage cross
section, a yield-to-disappearance ratio, defined as the ratio of the yield to the sum of the yield and damage,
is also calculated. The enhancements in both the yield-to-damage and yield-to-disappearance ratios show the
same trends, with the greatest enhancement on the substrate with the open lattice structure and the lighter
mass atoms, 12Si(100). Polyatomic projectiles are able to increase the yield more than the damage because
different types of motion are responsible for the production of the two types of molecules. The yield is
enhanced when the polyatomic projectile deposits energy into upward moving substrate atoms over a wider
surface area, which leads to a greater number of intact and stable molecules ejected from the surface. Damage
to molecules is caused primarily by the impact of the bombarding projectile.
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