Chemical Dynamics Simulations of CO2 in the Ground and First Excited Bend States Colliding with a Perfluorinated Self-Assembled Monolayer

Abstract

Energy transfer in collisions of CO2, in the ground (0000) state and first excited (0110) bend state, with a perfluorinated alkanethiol self-assembled monolayer (F-SAM) was studied by quasiclassical trajectory simulations employing a united-atom (UA) model to represent the F-SAM. The CO2 molecule was aimed perpendicularly to the surface at incident energies of 1.6 and 10.6 kcal/mol. The exchange of bend energy is more efficient for penetrating trajectories while almost no energy exchange was found for direct trajectories. Bend energy transfer depends on the surface residence time (τ). In particular, the average bend energy of the scattered CO2 molecules increases linearly with τ for CO2(0000) + F-SAM, and it decreases linearly for CO2(0110) + F-SAM at both incident energies. For the highest collision energy, the average bend energy of the scattered CO2 molecules also increases linearly with the angle formed between the molecular axis and the axis perpendicular to the surface in CO2(0000) + F-SAM collisions. The J quantum number distributions P(J) of the scattered CO2 molecules in the (0000) and (0110) states compare very well with experimental results of CO2 scattering off a perfluorinated liquid surface. Finally, the computed vibrational populations also compare well with the experimental distributions, providing vibrational temperatures below the surface temperature. The analysis of the computed vibrational temperatures as a function of the residence time provide us with a value of ∼50 ps for the time scale needed to achieve vibrational energy accommodation.