Inelastic scattering of OH radicals from organic liquids: isolating the thermal desorption channel

Abstract

Inelastic scattering of OH radicals from liquid surfaces has been investigated experimentally. An initially translationally and rotationally hot distribution of OH was generated by 193 nm photolysis of allyl alcohol. These radicals were scattered from an inert reference liquid, perfluorinated polyether (PFPE), and from the potentially reactive hydrocarbon liquids squalane (C30H62, 2,6,10,15,19,23-hexamethyltetracosane) and squalene (C30H50, trans-2,6,10,15,19,23-hexamethyltetracosa-2,6,10,14,18,22-hexaene). The scattered OH v = 0 products were detected by laser-induced fluorescence. Strong correlations were observed between the translational and rotational energies of the products. The high-N levels are translationally hot, consistent with a predominantly direct, impulsive scattering mechanism. Impulsive scattering also populates the lower-N levels, but a component of translationally relaxed OH, with thermal-desorption characteristics, can also be seen clearly for all three liquids. More of this translationally and rotationally relaxed OH survives from squalane than from squalene. Realistic molecular dynamics simulations confirm that double-bond sites are accessible at the squalene surface. This supports the proposition that relaxed OH may be lost on squalene via an addition mechanism.

Atomic and Molecular Collisions at Liquid Surfaces

“…It is clear that to fully describe the interfacial dynamics at the microscopic level, one needs to go beyond the simple two-channel IS/TD model. Inelastic scattering of OH radicals from hydrocarbon and polymer surfaces strengthened this argument (60,75,76). The translationally hot but rotationally nearthermal OH produced by HONO photolysis scattered with at least partial IS characteristics.…”

“…Inelastic scattering of OH from different liquids offered further insights into its loss mechanisms at the surface (60,75,76). The reactive uptake of OH by the liquid, derived from the survival probability of the incident OH, was larger for the unsaturated liquid squalene compared to the fully saturated squalane.…”

Influence of Molecular Structure and Chemical Functionality on the Heterogeneous OH-Initiated Oxidation of Unsaturated Organic Particles

“…Molecular dynamics simulations of liquid squalene show that squalene molecules are isotropically oriented at the liquid surface. 49 Analogous to the O 3 + methyl oleate reaction, 46 this arrangement of squalene molecules within the particle may reduce the overall probability of a surface OH reaction with a CC double bond.…”

Heterogeneous chemistry and reaction dynamics of the atmospheric oxidants, O3, NO3, and OH, on organic surfaces

“…Two key studies by the McKendrick group have implemented laser-induced fluorescence techniques in vacuo to probe the collision dynamics between hydroxyl radicals and liquid organic thin films of squalane. In the first published study, 145 the objective was to examine a novel photolytic source of hydroxyl radicals and determine the contributions of impulsive scattering and thermal desorption mechanisms to the final energy distribution of OH radicals scattered from the surface. Separate trajectory calculations complemented this work by revealing the importance of the impulsive scattering and thermal desorption in collisions of OH with a perfluorinated SAM surface.…”