Macromolecular Sample Sputtering by Large Ar and CH4Clusters: Elucidating Chain Size and Projectile Effects with Molecular Dynamics

This article reports the latest developments of our theoretical studies of gas cluster bombardment of model macromolecular samples using molecular dynamics simulations. Here, we perform a detailed comparison of the effects of the sample molecular weight, the Ar cluster incidence angle (45° vs 0°), and the cluster nature (CH4 vs Ar) on the soft sputtering of polymeric samples. The results of Ar cluster-induced sputtering and fragmentation at 45° incidence for molecular targets with three different molecular weights (282, 1388, and 14002 amu) indicate a pronounced influence of that parameter beyond 1000 amu, which is explained by the extra energy needed to form fragments from longer chains and to overcome mechanical entanglement. An excellent agreement is found between the computed statistics of sputtering and the available experimental data for similar molecular weights. The variance of the sputtering and polymer fragmentation results with changing beam parameters is explained via the microscopic analysis of the interaction in the simulations. The influential physical quantities are identified, namely, the energy (density) deposited in the impact region, the projectile velocity, and the geometry of the impact. The lower sputtering efficiency of CH4 molecular clusters results mainly from the extra energy spent in covalent bond-breaking and vibrational excitation of the cluster constituents.