Initial reaction probability and dynamics of ozone collisions with a vinyl-terminated self-assembled monolayer

Abstract

The gas-surface reaction dynamics of ozone with a model unsaturated organic surface have been explored through a series of molecular beam scattering experiments. Well-characterized organic surfaces were reproducibly created by adsorption of C=C-terminated long-chain alkanethiols onto gold, while the incident molecular beams were created by supersonic expansion of ozone seeded in an inert carrier gas to afford control over collision energy. Time-of-flight distributions for the scattered molecules showed near complete thermal accommodation of ozone for incident energies as high as 70 kJ/mol. Reflection-absorption infrared spectroscopy, performed in situ with ozone exposure, revealed that oxidation of the double bond depends significantly on the translational energy of O 3. For energies near room temperature, 5 kJ/mol, the initial reaction probability (γ 0) for the formation of the primary ozonide was determined to be γ 0 = 1.1 × 10 -5. As translational energy increased to 20 kJ/mol, the reaction probability decreased. This behavior, along with a strong inverse relationship between γ 0 and surface temperature, demonstrates that the room-temperature reaction follows the Langmuir-Hinshelwood mechanism, requiring accommodation prior to reaction under nearly all atmospherically relevant conditions. However, measurements show that the dynamics transition to a direct reaction (analogous to the Eley-Rideal mechanism) for elevated translational energies. © 2011 American Chemical Society.

Publication Title

Journal of Physical Chemistry C

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