Distinguishing Mechanisms for Reactive Uptake at Liquid Surfaces via Angular Distributions of Inelastically Scattered Molecules.

Angular distributions of OH inelastically scattered from the surfaces of the reactive hydrocarbon liquids squalane (fully saturated) and squalene (partially unsaturated) have been measured. A pulsed, rotationally cold molecular beam (Ei = 35 kJ mol-1) of OH was scattered from refreshed liquid surfaces in a vacuum. Spatially and temporally resolved OH number densities were measured by pulsed, planar laser-induced fluorescence. Results are compared with those for the inert liquid perfluoropolyether. The clearly asymmetric distributions for 45° incidence add to the weight of evidence for predominantly impulsive scattering from all three liquids. However, we propose that significant differences in their shapes may be diagnostic of contrasting reaction mechanisms. Direct, near-specular trajectories survive preferentially on squalene, consistent with an addition mechanism removing those at more backward angles. This trend is reversed for squalane, as expected for direct abstraction. The results reinforce the need to consider the effects of composition-dependent contributions from different reaction mechanisms in the modeling of OH-aging of atmospheric aerosol particles.

Inelastic scattering of OH from a liquid PFPE surface: Resolution of correlated speed and angular distributions.

Inelastic collisions of OH with an inert liquid perfluoropolyether (PFPE) surface have been studied experimentally. A pulsed molecular beam of OH with a kinetic energy distribution peaking at 35 kJ mol-1 was directed at a continually refreshed PFPE surface. OH molecules were detected state-selectively with spatial and temporal resolution by pulsed, planar laser-induced fluorescence. The scattered speed distributions were confirmed to be strongly superthermal, regardless of the incidence angle (0° or 45°). Angular scattering distributions were measured for the first time; their reliability was confirmed through extensive Monte Carlo simulations of experimental averaging effects, described in Paper II [A. G. Knight et al., J. Chem. Phys. 158, 244705 (2023)]. The distributions depend markedly on the incidence angle and are correlated with scattered OH speed, consistent with predominantly impulsive scattering. For 45° incidence, the angular distributions are distinctly asymmetric to the specular side but peak at sub-specular angles. This, along with the breadth of the distributions, is incompatible with scattering from a surface that is flat on a molecular scale. New molecular dynamics simulations corroborate the roughness of the PFPE surface. A subtle but unexpected systematic dependence of the angular distribution on the OH rotational state was found, which may be dynamical in origin. The OH angular distributions are similar to those for kinematically similar Ne scattering from PFPE and hence not strongly perturbed by OH being a linear rotor. The results here are broadly compatible with prior predictions from independent quasiclassical trajectory simulations of OH scattering from a model-fluorinated self-assembled monolayer surface.

Quantifying the dynamical information content of pulsed, planar laser-induced fluorescence measurements.

We have analyzed the effects of the spreads in experimental parameters on the reliability of speeds and angular distributions extracted from a generic surface-scattering experiment based on planar laser-induced fluorescence detection. The numerical model assumes a pulsed beam of projectile molecules is directed at a surface. The spatial distribution of the scattered products is detected by imaging the laser-induced fluorescence excited by a thin, pulsed sheet of laser light. Monte Carlo sampling is used to select from realistic distributions of the experimental parameters. The key parameter is found to be the molecular-beam diameter, expressed as a ratio to the measurement distance from the point of impact. Measured angular distributions are negligibly distorted when this ratio is <∼10%. Measured most-probable speeds are more tolerant, being undistorted when it is <∼20%. In contrast, the spread of speeds or of corresponding arrival times in the incident molecular beam has only very minor systematic effects. The thickness of the laser sheet is also unimportant within realistic practical limits. These conclusions are broadly applicable to experiments of this general type. In addition, we have analyzed the specific set of parameters designed to match the experiments on OH scattering from a liquid perfluoropolyether (PFPE) surface in the Paper I [Roman et al., J. Chem. Phys. 158, 244704 (2023)]. This reveals that the detailed form of the molecular-beam profile is important, particularly on apparent angular distributions, for geometric reasons that we explain. Empirical factors have been derived to correct for these effects.