| dc.description.abstract | This thesis presents a new experimental method for studying the dynamics of collisions between the gas-phase radical OH (or OD for technical reasons) and an organic liquid surface acting as proxy for the surface of aerosol particles. This new experimental method consists of two configurations. The first introduces a molecular beam source of OD radicals, generated via an electric discharge, and probes them via conventional laser-induced fluorescence (LIF) measurements. This allows the user to set the angle of incidence and control the speed, and thus the collision energy, with which the radicals hit the liquid surface. The second arrangement is an extension of the first, in which OD is detected via planar-LIF. This allows images of OD fluorescence from the plane of detection to be acquired, thus providing information on the speed and angular distribution of the radicals scattered off the surfaces.
The collision-energy dependence of the inelastic scattering dynamics and survival probability of OD from the long-chain, saturated hydrocarbon, squalane (C30H62), and its partially unsaturated analogue, squalene (C30H62), was studied with the molecular beam set-up. The results were compared with scattering off a liquid perfluoropolyether (PFPE) used as an inert reference. The molecular-beam source allowed collision energies close to atmospheric conditions to be accessed, for the first time. The results provide the first direct evidence of OD uptake on squalene being negatively activated due to addition reactions of OD with the unsaturated sites. More surprisingly, the positively activated behaviour expected for squalane on the basis of analogous OH + alkane gas phase reactions was not observed, which is speculated to be the result of trapping-enhanced reactivity at lower collision energies.
The collision-energy and angle-of-incidence dependence of the scattering dynamics of OD off liquid surfaces was investigated using, for the first time in gas-liquid scattering experiments, the planar-LIF image acquisition approach. Preliminary findings show that OD scatters off all surfaces with an unexpectedly broad angular distribution. Nevertheless, the observed above-thermal speeds and hot rotational distributions of the scattered OD confirm that a substantial fraction of the scattering is directly impulsive at an incident translational energy of 30 kJ mol-1. More subtle differences indicate that impulsively scattered radicals are still present, even at the lower collision energy of 7.5 kJ mol-1 | en |
