Reactive-atom scattering dynamics and liquid-vacuum interfacial structure
Smoll, Eric James, Jr.
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Experiments to characterize reactive and nonreactive gas-liquid scattering dynamics were carried out with the use of a crossed molecular beams apparatus configured for beam-surface scattering. In each experiment, the identity of the gas and liquid was strategically selected to reveal fundamental insights on the relationship between scattering observables and liquid-vacuum interfacial structure. This work is crucially important for the experimental advancement of liquid surface science and has the potential to impact our understanding of the chemical role of gas-liquid interfaces in the environment. An extensive literature review suggests that the inherent chemical specificity of reactive scattering is a promising probe of composition at the liquid-vacuum interface. We expand on what has been demonstrated in the literature by exploring F-atom scattering from the liquid-vacuum interface of deuterium labeled variants of the common ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 4 mim][Tf 2 N]). The experimental data and new molecular dynamics simulations provide evidence for the extreme surface specificity of reactive scattering and help quantify the relative populations of [C 4mim] + conformations at the liquid-vacuum interface. Also, at a fixed incident angle, the site-specific IS flux angular distributions from [C 4mim] + were discovered to be related by the addition or subtraction of a line-shape proportional to a cos(θf) function. To investigate this phenomenon, a separate study of noble gas scattering from the liquid-vacuum interface of other low vapor pressure liquids was carried out. Our results support the generality of the relative cos(θf) character trend and demonstrate that the relative cos(θf) character between total flux angular distributions from squalane and a perfluoropolyether is independent of gas identity and incident angle suggesting that this metric is an intrinsic property of the liquid pair. The existing evidence suggests that the relative cos(θf) character between flux angular distributions is a result of angle-randomization from multiple collision scattering trajectories induced by atomic-scale corrugation at the liquid-vacuum interface. A study on the liquid-vacuum interface structure of a solution of [RuCl 2(p-cymene)P(C 8H 17) 3] in perdeuterated 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (d 11-[C 2mim][Tf 2N]) is also discussed. The experimental data suggest that [RuCl(p-cymene)P(C 8H 17) 3] + is enriched at the liquid-vacuum interface at the expense of d 11-[C 2mim] + and the hydrocarbon chains of the Ru-complex protrude into the vacuum.