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Scattering-Angle Randomization in Nonthermal Gas–Liquid Collisions

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journal contribution
posted on 11.09.2019, 04:13 by Eric J. Smoll, Timothy K. Minton
Our ability to characterize the physical and chemical properties of gas–liquid interfaces is limited by the information content of surface-sensitive methods. Dynamical observables from gas–liquid scattering experiments are sensitive to liquid–vacuum interfacial structure but remain difficult to interpret without the aid of simulation. In particular, the interpretation of subtle changes in flux angular distributions is underdeveloped relative to the information content of this observable. We present a new analysis of published data on noble gas-atom scattering from three low vapor pressure liquids (squalane, perfluoropolyether, and liquid gallium) in high vacuum. If θi and θf are the incident and final angles of the scattered product, our analysis suggests that the shapes of the flux angular distributions collected at a fixed θi  under a variety of conditions appear to have a simple relationship. Specifically, f(θf) ≈ c1·g(θf) + c2·h(θf), where h(θf) = A·cos­(θf), f and g are any two flux angular distributions, and c1, c2, and A are constants. Also, the relative cos­(θf) character of total flux angular distributions from the liquid–vacuum interfaces of two liquids, squalane and perfluoropolyether, appears to be independent of gas-atom identity and θi, suggesting that this metric is an intrinsic property of the liquid pair. Although more research is needed, the experimental data currently available suggest that cos­(θf) character in total flux angular distributions is a result of scattering-angle randomization from multiple-collision scattering trajectories induced by atomic-scale corrugation at the liquid–vacuum interface.