Scattering of OH at the gas-liquid interface

Abstract: The gas-liquid interface is central to many problems of real-world significance, but mechanisms of uptake and reaction are relatively poorly understood compared to gas-phase or gas-solid interactions. As a specific example, OH radicals are a principal oxidant of organic species at the surfaces of atmospheric aerosol particles. This 'ageing' of the aerosols has important climatic consequences. We have been studying collisions of OH molecules with proxy liquid surfaces under controlled conditions to understand the factors affecting the transfer of energy and probability of reaction. A pulsed molecular beam of OH is directed at a liquid surface in vacuum. The spatial distribution of OH in specific quantum states is detected by pulsed, planar laser-induced fluorescence. Recording a sequence of images as a function of delay allows the motions of the incident packet and of the scattered OH molecules to be tracked. The image sequence can be analysed to determine the OH survival probability and the dynamical properties of the scattered molecules, from which mechanisms of reaction and inelastic energy exchange are inferred.