Dynamics of energy transfer in collisions of O(3P) atoms with a 1-decanethiol self-assembled monolayer surface

Chemical dynamics simulations are reported of energy transfer in collisions of O(³P) atoms with a 300 K 1-decanethiol self-assembled monolayer (H-SAM) surface. The simulations are performed with a nonreactive potential energy surface, developed from PMP2/aug-cc-pVTZ calculations of the O( ³P) + H-SAM intermolecular potential, and the simulation results represent the energy transfer dynamics in the absence of O(³P) reaction. Collisions energies Eⁱ of 0.12, 2.30, 11.2, 75.0, and 120.5 kcal/mol and incident angles 6, of 15, 30, 45, 60, and 75° were considered in the study (θ = 0° is the surface normal). The translational energy distribution of the scattered O(³P) atoms, P(E_f), may be deconvolved into Boltzmann and non-Boltzmann components, with the former fraction identified as f_B. The trajectories are also analyzed in terms of three types; that is, direct scattering from and physisorption on the top of the H-SAM and penetration of the H-SAM. There are three energy regimes in the scattering dynamics. For the low E_i values of 0.12 and 2.30 kcal/mol, physisorption is important and both f_B and the average final translational energy of the scattered O(³P) atom, 〈 E _f〉, are nearly independent of the incident angle. The dynamics is much different for hyperthermal energies of 75.0 and 120.5 kcal/mol, where penetration of the surface is important. For hyperthermal collisions, the penetration probability decreases as θ_i is increased, with a significant transition between θ_i of 60 and 75°. Hyperthermal penetration occurs upon initial surface impact and is more probable if the impinging O(³P) atom may move down a channel between the chains. For E_i = 120.5 kcal/mol, 90% of the trajectories penetrate at θ_i = 15°, while only 3% penetrate at θ_i = 75°. For the former θ_i, the energy transfer to the surface is efficient with 〈E_f〉 = 4.04 kcal/mol, but for the latter θ_i, 〈E_f〉 = 85.3 kcal/mol! Particularly interesting penetrating trajectories are those in which O(³P) is trapped in the H-SAM for times exceeding 60 ps, linger near the Au substrate, and strike the Au substrate and scatter directly. For E_i = 11.2 kcal/mol, there is a transition between the scattering dynamics for the low and hyperthermal collision energies. Additional detail in the energy transfer dynamics is obtained from the final polar and azimuthal angles, the residence time on/in the H-SAM, the minimum height with respect to the Au substrate, and the number of inner turning points in the O-atom's velocity. Calculated values of 〈E_f〉 vs the final polar angle, of, are in qualitative agreement with experiment. The O(³P) + H-SAM nonreactive energy transfer dynamics, for E_i of 11.2 kcal/mol and lower, are very similar to previously reported Ne + H-SAM simulations.