Enhancing Ion Emission: Insights from Molecular Dynamics and Monte Carlo Simulations

Gas cluster ion beam (GCIB) guns have found several applications in science and industry, such as surface smoothing or depth profiling and surface analysis in conjunction with secondary ion mass spectrometry (SIMS). The former application is severely hindered by the low amount of ejected secondary ions, which can be boosted by more than an order of magnitude by properly selecting the size of cluster projectiles and changing their constituent particles from argon to water. The mechanism of this phenomenon is still unknown. By combining molecular dynamics (MD) and Monte Carlo (MC) simulations with experimental results, we posit that the increase in ion yield can be attributed to proton transfer in long-lived complexes of sample molecules and hydronium (H₃O⁺) ion from the projectile. The number of molecule-water complexes formed in simulations is directly proportional to the experimental signal intensity, with a small deviation for projectiles containing more than 7000 water molecules.