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 CH 4 and CD 4 on CH 4 and CD 4 ices under nonequilibrium flow conditions using a combination of experimental methods and numerical simulations. The experimental methods include time-resolved 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 CH 4 and CD 4 have the same incident velocity but different kinetic energies and momenta. We found that as the incident velocity of CH 4 and CD 4 increases, the sticking probabilities for both molecules on a CH 4 condensed film decrease systematically, but that preferential sticking and condensation occur for CD 4 . These observations differ when condensed CD 4 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 CH 4 and CD 4 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 CD 4 projectile and the CH 4 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.

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

“…In general, we find that the CD 4 and CH 4 sticking probabilities are identical for the lowest velocity beam before diverging for the higher velocity beams as more CD 4 trajectories become preferentially trapped on the surface. For comparison, the VENUS calculations are overlaid with our experimental King and Wells sticking probability measurements (solid circles) . Although the theoretical sticking probabilities are slightly lower than the experimental values, the trend and differential sticking between CH 4 and CD 4 at each initial condition are in excellent agreement (Figure b).…”

“…Sticking probabilities (a) calculated from the number of CH 4 and CD 4 direct and physisorption scattering trajectories (open circles) on a CH 4 surface at 20 K. Error bars represent the standard error of a binomial distribution of at least 1900 trajectories for each velocity. The experimental sticking probabilities (solid circles) are from at least three King and Wells measurements on at least three different days. The differential sticking probability (b) is the ratio of the CH 4 /CD 4 sticking probability at the CD 4 initial velocity.…”

“…We next examine the translational, vibrational, and rotational energy transfer upon impact with the CH 4 surface because trapping only occurs when a molecule loses enough energy to the surface (Figure ). The incident projectiles contain no rotational energy (to be consistent with our supersonic beam expansion conditions) so we calculated the rotational energy gain due to collisions with the surface; the vibrational and translational energy losses are calculated by taking the initial – final energy (in electronvolts) for each scattered trajectory. With increasing initial incident velocity, more energy (translational and vibrational) is lost to the surface while nevertheless still leading to the overall decrease in sticking probability (Figure ).…”

“…Understanding energy-transfer dynamics at the gas–surface interface helps to accurately model a variety of chemical and physical phenomena. , The energy transfer that occurs when a gas molecule collides with a condensed surface is complicated and a result of many factors such as the gas–surface potential, translational energy and incident kinematics of the projectile, gas–surface mass ratio, density of states of the interface’s degrees of freedom, surface morphology, and surface temperature. We present detailed molecular dynamics simulations to explicitly calculate energy transfer with the surface in order to understand our experimental observation that preferential adsorption occurs for CD 4 when striking the CH 4 surface compared to CH 4 under non-equilibrium gas–surface collision conditions.…”

“…In general, adsorption is a key process in both astrophysical and terrestrial environments because it is the first step in many gas–surface interactions. − In extraterrestrial environments where chemical species are scarce, adsorption onto an interstellar grain, planetesimal, or other larger body dictates whether more complex organic molecules can form. − In order to accurately model chemical abundances, we previously examined isotopologues to understand how differences in mass between the projectile and the surface influence the ability of the species to adsorb and thus lead to observed relative isotopic abundances. , We studied methane and its isotopologue deuterated methane due to methane’s interstellar abundance in both the gaseous and solid forms − and its ability to form larger hydrocarbon species through addition reactions . We determined, through two independent experimental methods (monitoring reflected species as well as those species adsorbed on the surface), that preferential sticking and condensation occurred for CD 4 compared CH 4 under specific non-equilibrium gas–surface collision conditions …”