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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