The effects of surface temperature on the gas-liquid interfacial reaction dynamics of O(3P)+squalane

Abstract

OH/OD product state distributions arising from the reaction of gas-phase O(3P) atoms at the surface of the liquid hydrocarbon squalane C30H62/C30D62 have been measured. The O(3P) atoms were generated by 355 nm laser photolysis of NO2 at a low pressure above the continually refreshed liquid. It has been shown unambiguously that the hydroxyl radicals detected by laser-induced fluorescence originate from the squalane surface. The gas-phase OH/OD rotational populations are found to be partially sensitive to the liquid temperature, but do not adapt to it completely. In addition, rotational temperatures for OH/OD(v'=1) are consistently colder (by 34+/-5 K) than those for OH/OD(v'=0). This is reminiscent of, but less pronounced than, a similar effect in the well-studied homogeneous gas-phase reaction of O(3P) with smaller hydrocarbons. We conclude that the rotational distributions are composed of two different components. One originates from a direct abstraction mechanism with product characteristics similar to those in the gas phase. The other is a trapping-desorption process yielding a thermal, Boltzmann-like distribution close to the surface temperature. This conclusion is consistent with that reached previously from independent measurements of OH product velocity distributions in complementary molecular-beam scattering experiments. It is further supported by the temporal profiles of OH/OD laser-induced fluorescence signals as a function of distance from the surface observed in the current experiments. The vibrational branching ratios for (v'=1)/(v'=0) for OH and OD have been found to be (0.07+/-0.02) and (0.30+/-0.10), respectively. The detection of vibrationally excited hydroxyl radicals suggests that secondary and/or tertiary hydrogen atoms may be accessible to the attacking oxygen atoms.

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

“…They are qualitatively similar to those in our earlier work on liquid surfaces. [43][44][45][46][47][48][49] They consist of a characteristic "dead time" after the photolysis pulse, followed by a returning wave of OD. They show the variations expected with the distance between the common laser axis and the surface.…”

“…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.…”

“…44 The SAM stage was replaced by a stainless-steel wheel of 50 mm diameter. This was mounted on an axle, rotating at 0.5 Hz through the hydrocarbon liquid of interest contained in a copper bath, which in the current work remained at room temperature.…”

“…[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.…”

“…To extract nascent populations, the spectra were compared with those from a known "thermalized" distribution, as we have described in our previous work. 44 The thermal spectra were obtained by photolyzing the precursor molecule HONO (or DONO for OD radicals) at 355 nm. Gaseous HONO was prepared by adding NO 2 (containing some NO impurity) to a flask containing a few milliliters of H 2 O (or D 2 O-Sigma Aldrich, 99% purity) and then leaving the mixture to stand overnight.…”