Chemical dynamics simulations of CO₂ in the ground and first excited bend states colliding with a perfluorinated self-assembled monolayer

Energy transfer in collisions of CO₂, in the ground (00 ⁰0) 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 CO ₂(00⁰0) + F-SAM, and it decreases linearly for CO ₂(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(00 ⁰0) + F-SAM collisions. The J quantum number distributions P(J) of the scattered CO₂ molecules in the (00⁰0) and (01 ₁0) states compare very well with experimental results of CO ₂ 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.