The Dynamics of Gas-Surface Energy Transfer in Collisions of Rare Gases with Organic Thin Films

Understanding mechanisms at the molecular level is essential for interpreting and predicting the outcome of processes in all fields of chemistry. Insight into gas-surface reaction dynamics can be gained through molecular beam scattering experiments combined with classical trajectory simulations. In particular, energy exchange and thermal accommodation in the initial collision, the first step in most chemical reactions, can be probed with these experimental and computational tools.

There are many questions regarding the dynamic details that occur during the interaction time between gas molecules and organic surfaces. For example, how does interfacial structure and density affect energy transfer? What roles do intramonolayer forces and chemical identity play in the dynamics? We have approached these questions by scattering high-energy, rare gas atoms from functionalized self-assembled monolayers. We used classical trajectory simulations to investigate the atomic-level details of the scattering dynamics. We find that approximately six to ten carbon atoms are involved in impulsive collision events, which is dependent on the packing density of the alkyl chains. Moreover, the higher the packing density of the alkyl chains, the less energy is transferred to the surface on average and the less often the incident atoms come into thermal equilibrium with the surface. In addition to the purely hydrocarbon monolayers, organic surfaces with lateral hydrogen-bonding networks create more rigid collision partners than surfaces with smaller inter-chain forces, such as van der Waals forces. Finally, we find some interesting properties for organic surfaces that possess fluorinated groups. For argon scattering, energy transfer decreases with an increasing amount of surface fluorination, whereas krypton and xenon scattering transfer most energy to monolayers terminated in CF₃ groups, followed by purely hydrocarbon surfaces, and then perfluorinated surfaces.