A Theoretical Investigation of the Yield-to-Damage Enhancement with Polyatomic Projectiles in Organic SIMS

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, 12 Si(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.