Real-space imaging of OH radicals scattering from liquid surfaces

Abstract

A novel technique for studying the dynamics of gas-liquid scattering was used to image the products of OH radical collisions with low-vapour-pressure liquids that served as proxies for the surfaces of atmospheric aerosol particles. A pulsed molecular beam of OH was aimed at layers of squalane, squalene and perfluoropolyether either along the normal to the surface or at 45° to it. The OH molecules were intercepted above the surface by pulsed laser light shaped into a sheet and probed by exciting laser-induced fluorescence. The emitted photons were collected, intensified, and finally imaged creating images in which spatial distributions of the OH number density were recorded. Controlling the delay between the production and probing of the projectiles allowed for capturing the OH pre- and post-collision. The experimental images were then used to reveal information about the dynamics of collisions between OH and organic liquid surfaces. The images allowed, for the first time ever in the field of gas-liquid interactions, the complete angular distribution of scattered products to be measured, including backward scattering, along or close to the incidence angles. The measured scattered OH angular distributions were found to be dependent on the angle of incidence and peaked around subspecular or specular final angles. The measured most-probable speeds of scattered OH were superthermal and correlated with the incident and final angles. These, and other, observations were consistent with a predominantly impulsive mode of scattering from atomically rough surfaces.