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 understandthe energytransfer processes involved during the collisions of CH4 and CD4 with CH4 layered surfaces at 20 Kin order to explain our experimental finding of preferential adsorptionof CD4 compared to CH4. There is good agreementbetween our MD simulations and our experimental results. We find thatgas-surface collisional energy accommodation is dominated byexchange involving the translational degrees of freedom of the incidentmolecule and intermolecular vibrations of the interface. This observationallows us to understand that the cause of CD4 preferentialsticking arises from its propensity to lose more energy during itsfirst impact with the surface, inducing longer residence times andleading to increased probability of becoming trapped and condensedonto the surface. Systematic trends are seen for sticking probabilitiesand energy transfer when we explore the behavior of the other H/D-substitutedisotopologues of methane. These molecular insights provide contextinto the adsorption behavior occurring on icy dust grains in our solarsystem. Because adsorption is often the first step, trapping efficiencydifferences between isotopologues have notable implications for condensedphase reaction probabilities involving isotopically substituted speciesand subsequent events leading to increased molecular complexity. Asidefrom astrophysical significance, our findings have direct implicationsfor novel isotope enrichment mechanisms under non-equilibrium conditionsinvolving the preferential condensation of heavier isotopes and isotopologuesduring gas-surface collisions under specifically selected substrate,gas mixture, and incident kinematic conditions.