Planar laser-induced fluorescence (pLIF) used to determine the dynamics of inelastic hydroxyl radical collisions with liquid surfaces Daniel Moon 1 , Maksymilian J. Roman 2 , Adam G. Knight 1 , Paul D. Lane 1 , Matthew L. Costen 1 , Kenneth G. McKendrick 1 1 Heriot-Watt University, UK, 2 University of Liverpool, UK We present a recently developed novel spectroscopic method to image the scattering of hydroxyl radicals (OH) from liquid surfaces, which is providing insight into the dynamics of crucial oxidation reactions that occur on atmospheric organic aerosol surfaces. The method employs the pulsed, planar laser-induced fluorescence (pLIF) technique. The probe beam is shaped into a sheet and directed into an ultra-high vacuum chamber parallel to the surface of an enclosed rotating wheel submerged in a bath of the liquid of interest. A rotationally and vibrationally cold pulsed molecular beam of OH is fired at the rotating wheel with the ingoing beam and scattered OH passing through the probe region. The resulting LIF emission is projected onto an image intensifier to obtain real-space images of the scattered OH number density. The delay between firing the molecular beam and the probe sheet is varied, producing a sequence of images before and after the scattering event. The notable advantages over more established techniques(1) are that it is state-specific and scattered molecules from a broad range of angles can be detected, including those that are backscattered towards the molecular-beam source. Novel image analysis methodologies have been developed to obtain OH angular scattering distributions, correlated with speed and rotational quantum state. Integrated number densities also provide the survival probability, referenced to an inert liquid (PFPE). Initial proof-of-concept experiments that investigate the scattering of OH with a kinetic energy of ~ 30 kJ mol -1 from squalane, squalene and PFPE liquid surfaces were successful. They show that most of the inelastic scattering from all three liquids must be impulsive, based on the asymmetry in the angular distributions for non-normal incidence and super-thermal speeds. The surfaces also do not appear molecularly flat, leading to relatively broad and sub-specular scattering angular distributions. There are subtle differences between the angular distributions for different rotational levels. Systematic differences in the angular distributions for squalene relative to squalane can be interpreted as enhanced reactive loss of partially accommodated OH due to addition reactions at double- bond sites on squalene. Further experiments are in progress in a new, upgraded purpose-built apparatus, whose design was informed by extensive Monte Carlo simulations, that offers significant improvements in the range and sensitivity of experiments that can be conducted. References 1. esa-Serrate MA, Smoll EJ Jr, Minton TK, McKendrick KG. Atomic and Molecular Collisions at Liquid Surfaces. Annu Rev Phys Chem. 2016 May 27;67:515-40. doi: 10.1146/annurev-physchem-040215-112355. Epub 2016 Apr 18. PMID: 27090845.
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