Brann Michelle R, Hansknecht Stephen P, Ma Xinyou, Sibener S J
The James Franck Institute and Department of Chemistry, The University of Chicago, 929 E. 57th Street, Chicago, Illinois 60637, United States.
J Phys Chem A. 2021 Oct 28;125(42):9405-9413. doi: 10.1021/acs.jpca.1c07826. Epub 2021 Oct 16.
We examine the initial differential sticking probability of CH and CD on CH and CD ices under nonequilibrium flow conditions using a combination of experimental methods and numerical simulations. The experimental methods include time-resolved 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 and CD have the same incident velocity but different kinetic energies and momenta. We found that as the incident velocity of CH and CD increases, the sticking probabilities for both molecules on a CH condensed film decrease systematically, but that preferential sticking and condensation occur for CD. These observations differ when condensed CD 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 and CD 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 projectile and the CH 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.
我们结合实验方法和数值模拟，研究了非平衡流动条件下CH和CD在CH和CD冰上的初始微分黏附概率。实验方法包括用于监测表面气态凝结的时间分辨反射吸收红外光谱（RAIRS），以及用于监测黏附概率的补充性金氏和韦尔斯质谱技术，该技术通过第二种独立测量方法提供验证结果。采用种子超声束，使夹带的CH和CD具有相同的入射速度，但动能和动量不同。我们发现，随着CH和CD的入射速度增加，两种分子在CH凝聚膜上的黏附概率系统性降低，但CD会发生优先黏附和凝聚。当以凝聚的CD作为目标界面时，这些观察结果有所不同，表明薄膜的声子和振转态密度以及碰撞能量转移截面，在同位素取代的入射物种之间的微分能量调节中起作用。最后，我们采用了由CH和CD以3:1比例组成的混合入射超声束，并测量了凝聚物组成以及黏附概率。这样做时，我们在凝聚的混合膜中看到了相同的效果，支持了较重同位素的同位素富集。我们提出，入射的CD射弹与CH膜之间增强的多声子相互作用和非弹性截面，使得气体-表面能量转移更有效。VENUS代码分子动力学模拟显示，同位素异构体之间的黏附概率差异与气-表面散射实验中观察到的相同。对这些轨迹的持续分析将为入射物种与界面之间的能量和动量转移提供更多见解。这些结果为在特定选择的衬底、气体混合物和入射速度条件下，通过气体-表面碰撞过程中较重同位素和同位素异构体的优先凝聚实现同位素富集提供了一条新途径。它们还为非平衡条件下的气态凝结提供了有价值的见解，例如在低温环境中的飞机飞行中发生的情况。此外，这些结果有助于解释太阳系行星中氘丰度的增加，并可纳入星际冰尘颗粒表面过程的天体物理模型中。
