Reactive Uptake via Inelastic Scattering of CN Radicals at a Liquid Hydrocarbon Surface

Abstract

Collisions of ground-electronic-state CN radicals with the surface of a prototypical saturated-hydrocarbon liquid have been studied experimentally. A molecular beam of CN(X²Σ⁺) with a mean laboratory-frame kinetic energy of 44 kJ mol^-1 was directed at normal incidence at a continually refreshed liquid squalane (2,6,10,15,19,23-hexamethyltetracosane) surface. The incident and scattered CN radicals were detected in a range of rotational states by multi-pass frequency-modulated absorption spectroscopy on selected lines of the CN(A-X) transition. The ratio of scattered-to-incident CN populations for squalane were compared with those obtained previously for a reference liquid, perfluoropolyether (PFPE), which is assumed to be inert. The overall survival probability, summed over the significantly populated rotational levels, of CN on squalane was found to be 0.15 ± 0.04. The 85% that is lost is inferred, on energetic grounds, to produce HCN via H-atom abstraction. The surviving CN has a significantly superthermal rotational distribution and a hot, non-thermal velocity distribution in the direction perpendicular to the surface normal. These dynamical attributes are characteristic of impulsive scattering. However, we conclude that the low survival probability is not compatible with a simple, ‘single-bounce’ mechanism and hence that multiple-encounter trajectories must contribute significantly to CN reactive loss. We find no evidence of a distinct trapping-desorption component, corresponding to full thermal accommodation, in the surviving CN from either squalane or PFPE at these collision energies.