Direct Gas−Liquid Interfacial Dynamics:  The Reaction between O(³P) and a Liquid Hydrocarbon

Abstract

We report the first measurements of internal energy distributions of the OH produced via a direct mechanism, isolated from other components on the basis of time-of-flight, in the interfacial reaction between gas-phase O((3)P) atoms and the liquid hydrocarbon squalane, C(30)H(62). O((3)P) atoms were generated by laser photolysis of NO(2) above the liquid. Resulting hydroxyl radicals that escape from the surface were detected by laser-induced fluorescence. Time-of-flight profiles demonstrate that the kinetic energy of the fastest OH (nu' = 1) is lower than that of (nu' = 0). Rotational distributions were measured at the rising edge of their appearance for both OH (nu' = 0) and (nu' = 1). They were found to differ substantially more than at the peak of their profiles. They were also less dependent on the bulk liquid temperature. We conclude that the new data confirm strongly that at least two mechanisms contribute to the production of OH. The higher-velocity component has translational and rotational energy distributions, observed cleanly for the first time, consistent with a direct mechanism. The close correspondence of these rotational distributions to those from the corresponding homogeneous gas-phase reaction of O((3)P) with smaller hydrocarbons suggests a very similar, near collinear direct abstraction. This is accompanied by a slower component with kinetic energy and rotational (but not vibrational) distributions reflecting the temperature of the liquid, consistent with a distinct trapping-desorption mechanism.

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

“…As noted above, we also reconfigured the chamber to allow direct comparison with the appearance profiles for O( 3 P) reaction with the previously most heavily studied [43][44][45][46]48 liquid, squalane. We took particular care to ensure that distance from the laser axis to the surface of the liquid wheel was the same as that for the SAMs.…”

“…Although as yet not precisely quantified, we find that the reactivity is also not greatly different from that of the "benchmark" liquid, squalane. [43][44][45][46][47][48] These observations suggest that the O( 3 P) must be able to penetrate beyond the terminal -CH 3 groups of the alkyl chains. It is very well-known from the gas phase 79,80 that the reactivity of H-C bonds with O( 3 P) is inversely correlated with the H-C bond strength and, hence, increases markedly from primary to tertiary.…”

“…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.…”

“…[43][44][45][46][47][48][49] We use laser-induced fluorescence (LIF) spectroscopy to detect the hydroxyl radicals formed at the liquid surface, yielding information on their translational and internal state distributions. This is a development of the earlier work of McCaffery, who was the first to use LIF spectroscopy to detect I 2 inelastically scattered from a range of liquid surfaces.…”

Stereodynamics in state-resolved scattering at the gas–liquid interface

“…Recent developments in laser-based detection (13)(14)(15)(16)(17)(18)(19) have provided complementary information about the internal states of the scattered molecules and the role they play in the dynamics. The prevailing paradigm for gas-liquid dynamics involves a projectile colliding with the surface through two distinct pathways: (i) trapping-desorption (TD) and (ii) impulsive scattering (IS).…”