Low-Energy CO Scattering at the Gas–Liquid Interface: Experimental/Theoretical Evidence for a Novel Subthermal Impulsive Scattering (STIS) Channel

• Published in: Journal of physical chemistry. C Vol. 124; no. 51; pp. 28006 - 28017
• Main Authors: Livingston Large, Timothy A, Nesbitt, David J
• Format: Journal Article
• Language: English
• Published: American Chemical Society 24.12.2020
• Subjects: C: Surfaces, Interfaces, Porous Materials, and Catalysis
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.0c06400

Molecular beams of supersonically cooled (T rot ≈ 10 K) carbon monoxide (CO) have been scattered from three low-vapor-pressure liquids (PFPE, squalane, and glycerol) over a range of surface temperatures (253–303 K), with the final rovibrational distributions probed by shot-noise-limited direct IR laser absorption methods. Specifically, the present work focuses on quantum-state-resolved scattering at low incident energies (E inc ≤ 1.0 kcal/mol), which would normally be expected to yield pure trapping desorption (TD) dynamics with CO in complete thermal equilibrium with the liquid (T rot ≈ T S). By way of contrast, the nascently scattered CO­(J) exhibits both rotational (T rot) and Doppler translational (T Dopp) distributions distinctly colder than T S, a phenomenon which is systematically reiterated over a wide range of liquid temperatures. To help identify the relevant collision physics responsible for this surprising subthermal behavior in CO, high-level ab initio potentials and detailed molecular dynamics simulations are explored for a series of projectiles (CO, DCl, and CO2) with varying strengths of interaction with the liquid. At low incident energies, each of the more strongly interacting DCl and CO2 projectiles is found to thermalize with the liquid interface (T rot ≈ T S), while CO is predicted to emerge colder than the surface (T rot < T S) and in remarkably quantitative agreement with experiment. Statistical analysis of the trajectories identifies that CO spends substantially less time and penetrates less deeply into the surface compared to DCl/CO2 projectiles due to a combination of a shallow van der Waals well and a steep repulsive wall. The simulations reveal that low-energy CO does not undergo conventional trapping desorption (TD) at the gas–liquid interface but instead exhibits incomplete warming from its jet-cooled value (T rot ≈ 10 K) via an unexpected subthermal impulsive scattering (STIS) pathway. The data suggest that non-equilibrium IS dynamics at low energies may play a crucial role in inelastic energy transfer and thermal accommodation at the gas–liquid interface for weakly interacting collision systems.