Differential Condensation of Methane Isotopologues Leading to Isotopic Enrichment under Non-equilibrium Gas–Surface Collision Conditions

Abstract

We examine the initial differential sticking probability of CH₄ and CD₄ on CH₄ and CD₄ ices under nonequilibrium flow conditions using a combination of experimental methods and numerical simulations. The experimental methods include timeresolved in situ reflection−absorption infrared spectroscopy (RAIRS) for monitoring on-surface gaseous condensation and complementary King and Wells mass spectrometry techniques for monitoring sticking probabilities that provide confirmatory results via a second independent measurement method. Seeded supersonic beams are employed so that the entrained CH₄ and CD₄ have the same incident velocity but different kinetic energies and momenta. We found that as the incident velocity of CH₄ and CD₄ increases, the sticking probabilities for both molecules on a CH₄ condensed film decrease systematically, but that preferential sticking and condensation occur for CD₄. These observations differ when condensed CD₄ is used as the target interface, indicating that the film’s phonon and rovibrational densities of states, and collisional energy transfer cross sections, have a role in differential energy accommodation between isotopically substituted incident species. Lastly, we employed a mixed incident supersonic beam composed of both CH₄ and CD₄ in a 3:1 ratio and measured the condensate composition as well as the sticking probability. When doing so, we see the same effect in the condensed mixed film, supporting an isotopic enrichment of the heavier isotope. We propose that enhanced multi-phonon interactions and inelastic cross sections between the incident CD₄ projectile and the CH₄ film allow for more efficacious gas−surface energy transfer. VENUS code MD simulations show the same sticking probability differences between isotopologues as observed in the gas−surface scattering experiments. Ongoing analyses of these trajectories will provide additional insights into energy and momentum transfer between the incident species and the interface. These results offer a new route for isotope enrichment via preferential condensation of heavier isotopes and isotopologues during gas−surface collisions under specifically selected substrate, gas-mixture, and incident velocity conditions. They also yield valuable insights into gaseous condensation under non-equilibrium conditions such as occur in aircraft flight in low-temperature environments. Moreover, these results can help to explain the increased abundance of deuterium in solar system planets and can be incorporated into astrophysical models of interstellar icy dust grain surface processes.