Theoretical and experimental studies of energy transfer dynamics in collisions of atomic and molecular species with model organic surfaces

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Virginia Tech

A full understanding of chemical reaction dynamics at the gas/organic-surface interface requires knowledge of energy-transfer processes that happen during the initial gas/surface collision. We have examined the influence of mass and rovibrational motion on the energy-transfer dynamics of gas-phase species scattering from model organic surfaces using theory and experiment. Molecular-beam scattering techniques were used to investigate the rare gases, Ne, Ar, Kr, and Xe, and the diatomics, N2 and CO, in collisions with CH3- and CF3-terminated self-assembled monolayer (SAM) surfaces. Complementary molecular-dynamics simulations were employed to gain an atomistic view of the collisions and elucidate mechanistic details not observable with our current experimental apparatus. We developed a systematic approach for obtaining highly accurate analytic intermolecular potential-energy surfaces, derived from high-quality ab initio data, for use in our classical-trajectory simulations. Results of rare gas scattering experiments and simulations indicate mass to be the determining factor in the energy-transfer dynamics, while other aspects of the potential-energy surface play only a minor role. Additionally, electronic-structure calculations were used to correlate features of the potential-energy surface with the energy-transfer behavior of atoms and small molecules scattering from polar and non-polar SAM surfaces. Collisions of diatomic molecules with SAMs are seen to be vibrationally adiabatic, however translational energy transfer to and from rotational modes of the gas species, while relatively weak, is readily apparent. Examination of the alignment and orientation of the final rotational angular momentum of the gas species reveals that the collisions induce a stereodynamic preference for the expected "cartwheel" motion, as well as a surprising propensity for "corkscrew" or "propeller" motion. The calculated stereodynamic trends suggest that the CH3-SAM is effectively more corrugated than the CF3-SAM. Finally, the feasibility for collisional-energy promoted, direct gas/organic-surface reactions was interrogated using the 1,3-dipolar azide-alkyne cycloaddition reaction. We found that geometrical constraints prevented the reaction from proceeding at the probed conditions.

stereodynamics, classical-trajectory simulations, molecular-beam scattering, potential-energy surface derivation, self-assembled monolayers, rotational and vibrational energy transfer