Dynamics of the gas-liquid interfacial reaction of O(D-1) with a liquid hydrocarbon

Carla Waring, Kerry L King, Matthew L. Costen, Kenneth G. McKendrick

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Abstract

The dynamics of the gas liquid interfacial reaction of the first electronically excited state of the oxygen atom, O(D-1), with the surface of a liquid hydrocarbon, squalane (C30H62; 2,6,10,15,19,23-hexamethyltetracosane) has been studied experimentally. Translationally hot O(1D) atoms were generated by 193 nm photolysis of a low pressure (nominally 1 mTorr) of N2O a short distance (mean = 6 mm) above a continually refreshed liquid squalane surface. Nascent OH (X-2 Pi, nu = 0) reaction products were detected by laser-induced fluorescence (LIF) on the OH A(2)Sigma(+)-X-2 Pi (1,0) band at the same distance above the surface. The speed distribution of the recoiling OH was characterized by measuring the appearance profiles as a function of photolysisprobe delay for selected rotational levels, N'. The rotational (and, partially, fine-structure) state distributions were also measured by recording LIP excitation spectra at selected photolysis-probe delays. The OH V = 0 rotational distribution is bimodal and can be empirically decomposed into near thermal (similar to 300 K) and much hotter (similar to 6000 K) Boltzmann-temperature components. There is a strong positive correlation between rotational excitation and translation energy. However, the colder rotational component still represents a significant fraction (similar to 30%) of the fastest products, which have substantially superthermal speeds. We estimate an approximate upper limit of 396 for the quantum yield of OH per 0(1D) atom that collides with the surface. By comparison with established mechanisms for the corresponding reactions in the gas phase, we conclude that the rotationally and translationally hot products are formed via a nonstatistical insertion mechanism. The rotationally cold but translationally hot component is most likely produced by direct abstraction. Secondary collisions at the liquid surface of products of either of the previous two mechanisms are most likely responsible for the rotationally and translationally cold products. We do not think it likely, a priori, that they could be produced in the observed significant yield via a statistical insertion mechanism for a molecule the size of squalane embedded in a surrounding liquid surface.

Original languageEnglish
Pages (from-to)7210-7219
Number of pages10
JournalJournal of Physical Chemistry A
Volume115
Issue number25
DOIs
Publication statusPublished - 30 Jun 2011

Keywords

  • SELF-ASSEMBLED MONOLAYER
  • EXCITED OXYGEN-ATOMS
  • INELASTIC-SCATTERING DYNAMICS
  • LASER-INDUCED FLUORESCENCE
  • O(P-3) ATOMS
  • SATURATED-HYDROCARBONS
  • O(P-3(J))+HYDROCARBON REACTIONS
  • HYPERTHERMAL REACTIONS
  • MOLECULAR-DYNAMICS
  • CHEMICAL-DYNAMICS

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