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 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 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.
Highlights
Adsorption is a key process in both astrophysical and terrestrial environments as it serves as the first step in many gas−surface interactions.[1−3] In extraterrestrial environments where chemical species are scarce, adsorption onto an interstellar grain, planetesimal, or other larger body controls many combinatorial reactions
The sticking probability was previously found to be independent of ice film thickness,[33] we choose to grow films for ∼80 layers to achieve self-similarity in film physisorption trapping to occur, the CH4 or CD4 molecule must lose some initial kinetic energy when impinging upon the surface
The Journal of Physical Chemistry A VENUS calculations demonstrate a decrease in sticking probability with increasing incident velocity as well as a difference between CH4 and CD4 on a CH4 surface (Figure 6)
Summary
Adsorption is a key process in both astrophysical and terrestrial environments as it serves as the first step in many gas−surface interactions.[1−3] In extraterrestrial environments where chemical species are scarce, adsorption onto an interstellar grain, planetesimal, or other larger body controls many combinatorial reactions. Interstellar methane is the most common hydrocarbon, existing in both the gaseous and the solid form.[13−19] Methane is commonly found in the gaseous planetary atmospheres or as molecular ices intermixed with water ice matrices.[20,21] As the most basic hydrocarbon, CH4 serves as a base for addition reactions which form larger hydrocarbon species.[22] the isotopic twin of CH4, CD4, can serve as a model for understanding the effects and the abundance of deuterium within these environments.[12,23] Theoretical methods and gas chromatography have found that the isotopic difference in CH4 and CD4 stems from the difference in polarizability and length of the C−H and C−D bonds; no studies have reported how this difference might translate into its sticking probability.[12,24,25] Studying CH4 and CD4 adsorption is an excellent model system to determine how slight mass differences in the condensate and projectile can impact adsorption and surface abundance of isotopic species
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