Chemical Dynamics Simulation of Low Energy N₂ Collisions with Graphite

A chemical dynamics simulation was performed to study low energy collisions between N₂ and a graphite surface. The simulations were performed as a function of collision energy (6.34 and 14.41 kcal/mol), incident polar angle (20-70°) and random azimuthal angle. The following properties were determined and analyzed for the N₂ + graphite collisions: (1) translational and rotational energy distributions of the scattered N₂; (2) distribution of the final polar angle for the scattered N₂; (3) number of bounces of N₂ on the surface before scattering. Direct scattering with only a single bounce is dominant for all incident angles. Scattering with multiple collisions with the surface becomes important for incident angles far from the surface normal. For trajectories that desorb, the parallel component of the N₂ incident energy is conserved due to the extremely short residence times of N₂ on the surface. For scattering with an incident energy of 6.34 kcal/mol, incident polar angle of 40°, and final polar angle of 50° the percentage incident energy loss is 29% from the simulations, while the value is 27% for a hard cube model used to interpret experiment (J. Phys.: Condes. Matter 2012, 24, 354001). The incident energy is primarily transferred to surface vibrational modes, with a very small fraction transferred to N₂ rotation. An angular dependence is observed for the energy transfer, with energy transfer more efficient for incident angles close to surface normal.