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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. [ABSTRACT FROM AUTHOR]

We describe the real-space imaging of the products of molecular scattering, applied to collisions of hydroxyl radicals with low-vapor-pressure-liquid surfaces. A pulsed molecular beam of OD (for technical reasons) with a mean laboratory-frame kinetic energy of 29.5 kJ mol−1 was directed at continually refreshed surfaces of the representative liquids perfluoropolyether, squalane, and squalene. Laser-induced fluorescence (LIF) was excited by pulsed laser light shaped into a planar sheet, tuned to selected rovibronic transitions in the OD A–X band. The LIF emission was imaged and intensified before being captured by an external camera. Sequences of images allowed the evolution of the incident packet and scattered plumes of OD molecules to be observed. The results confirm previous observations of the internal-state distributions of the scattered OD and its differential survival probability on different liquid surfaces. New measurements of the angular distributions found them all to be broad and approximately symmetric, independent of the angle of incidence. This is interpreted as implying a high degree of atomic-scale roughness, rather than a predominant trapping-desorption mechanism, because of the other observed signatures of impulsive scattering; these include the degree of OD rotational excitation, superthermal speeds, and the correlation of speed with scattering angle. This approach has considerable potential to be applied in related gas-surface scattering experiments. It is immune from the difficulties of some other imaging methods that involve charged-particle detection and allows a spatially extended region of the scattering plane perpendicular to the surface to be imaged. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]