Isotopic Enrichment Resulting from Differential Condensation of Methane Isotopologues Involving Non-equilibrium Gas-Surface Collisions Modeled with Molecular Dynamics Simulations

We employ molecular dynamics simulations to understand the energy transfer processes involved during the collisions of CH₄ and CD₄ with CH₄ layered surfaces at 20 K in order to explain our experimental finding of preferential adsorption of CD₄ compared to CH₄. There is good agreement between our MD simulations and our experimental results. We find that gas-surface collisional energy accommodation is dominated by exchange involving the translational degrees of freedom of the incident molecule and intermolecular vibrations of the interface. This observation allows us to understand that the cause of CD₄ preferential sticking arises from its propensity to lose more energy during its first impact with the surface, inducing longer residence times and leading to increased probability of becoming trapped and condensed onto the surface. Systematic trends are seen for sticking probabilities and energy transfer when we explore the behavior of the other H/D-substituted isotopologues of methane. These molecular insights provide context into the adsorption behavior occurring on icy dust grains in our solar system. Because adsorption is often the first step, trapping efficiency differences between isotopologues have notable implications for condensed phase reaction probabilities involving isotopically substituted species and subsequent events leading to increased molecular complexity. Aside from astrophysical significance, our findings have direct implications for novel isotope enrichment mechanisms under non-equilibrium conditions involving the preferential condensation of heavier isotopes and isotopologues during gas-surface collisions under specifically selected substrate, gas mixture, and incident kinematic conditions.