Stereodynamics at the Gas−Liquid Interface: Orientation and Alignment of CO2 Scattered from Perfluorinated Liquid Surfaces

Abstract

Rotational orientation/alignment dynamics of CO(2) scattered from a perfluorinated polyether (PFPE) liquid surface has been investigated via direct absorption experimental studies and theoretical molecular dynamics (MD) simulations. Experimentally, polarization modulation of a single mode diode laser is combined with lock-in detection to measure circular/linear IR polarizance due to CO(2) scattering from the surface at theta(inc) = 60 degrees and E(inc) = 10.6(8) kcal/mol and probed over a series of final scattering angles. The differential absorption intensities are related through Fano-Macek theory to the three lowest multipole moments (A(0), A(2+), and O(1-)) which describe collisionally induced orientation and alignment at the liquid surface. The total scattering population reflects both trapping-desorption (TD) and impulsive scattering (IS) components, with a strong positive anisotropy in the M(J) distribution that indicates preferential CO(2) scattering from the surface with a forward (i.e., "topspin") sense of end-over-end tumbling. Theoretical trajectory simulations provide 3D CO(2) flux and J state distributions scattering from fluorinated self-assembled monolayers (F-SAMs) and are compared with experimental results as a function of final rotational state. Specifically, trends in the theoretical orientation/alignment moments are in remarkable agreement over the full range of J states but with values consistently overpredicted by nearly 2-fold, which may reflect a higher level of local ordering for F-SAMS vs a gas-PFPE liquid interface.