Dynamics of Inelastic Scattering of OH Radicals from Reactive and Inert Liquid Surfaces

Abstract

The inelastic scattering of gas-phase OH radicals from a liquid hydrocarbon and a liquid perfluorinated polyether (PFPE) has been investigated. The surfaces examined were the potentially reactive, branched hydrocarbon squalane (C 30 H 62 , 2,6,10,15,19,23-hexamethyltetracosane) and the inert PFPE Krytox 1506 (F-[CF(CF 3 )-CF 2 O] 14ave -CF 2 CF 3 ). Superthermal OH was formed by 355-nm laser photolysis of a low pressure of HONO above the liquid surface. Laser-induced fluorescence (LIF) was used to determine the relative yields and nascent translational and rotational distributions of OH (V′ ) 0). The time-of-flight profiles from both liquids can be resolved, at least empirically, into two components. The dominant, faster component is consistent with direct, inelastic scattering. It has a higher average translational energy from PFPE than from squalane. This faster OH also has a higher Boltzmann-like rotational temperature for PFPE (655 ( 45 K) than for squalane (473 ( 27 K), in both cases considerably hotter than the incoming OH. For both liquids, there is also a slower component, with characteristics consistent with a thermalized, trapping-desorption mechanism. This is a higher proportion for squalane (0.22 ( 0.02) than for PFPE (0.09 ( 0.01). These results are consistent with squalane being the "softer" surface, exhibiting more efficient momentum transfer than PFPE, and more able to temporarily trap OH. Relative to PFPE, around half (0.49 ( 0.04) of the OH molecules that collide with squalane are lost, presumably due to reaction forming H 2 O. These results are compared with previous studies of the scattering of inert gas species from both squalane and PFPE. The reactive branching fraction of OH on squalane is discussed in the context of previous observations of enhanced reactivity at the gas-liquid interface.

The heterogeneous reaction of hydroxyl radicals with sub-micron squalane particles: a model system for understanding the oxidative aging of ambient aerosols

“…Due to gas-phase diffusion limitations the uncertainty in this measurement was relatively large, such that γ OH values ranging from 0.16 to 1.0 could not be ruled out. In addition, Bagot et al (2008) recently reported γ OH to be 0.49±0.04 for the reaction of superthermal OH on a squalane surface at low pressure. The slightly larger value for γ OH from this measurement is potentially due to the increased translational energy of the OH radicals resulting in a larger reaction probability.…”

Dynamics of the Reaction of O(3P) Atoms with Alkylthiol Self-assembled Monolayers

“…43,44,[46][47][48][49]62 A schematic of the revised apparatus is shown in Figure 1. It is a dual-chamber design, with the subsidiary, turbopumped chamber allowing SAM specimens to be stored rapidly under vacuum on removal from their solutions.…”

“…This conclusion was corroborated by our subsequent work on the inelastic scattering of OH radicals themselves from liquid surfaces. 62 We have also gone on to study the relative reactivity of O( 3 P) and OH-product energy partitioning for a range of different hydrocarbons. It is believed on the basis of molecular dynamics (MD) simulations 37,45,49,[63][64][65] that for squalane all three C-H bond types (primary, secondary, and tertiary) are available for abstraction.…”

Quantifying the reactive uptake of OH by organic aerosols in a continuous flow stirred tank reactor

“…By measuring the gas phase loss of OH they found that γ is greater than 0.1. Using a molecular beam scattering technique, Bagot et al 2008 measured an uptake coefficient of γ = 0.49 ± 0.04 by colliding superthermal OH radicals with a liquid squalane surface. 19 Rather than measuring the gas phase loss of OH, the reactive uptake coefficient can be quantified by monitoring the removal of an organic molecule in the particle phase.…”