Atomic nitrogen is formed in the high-temperature shock layer of hypersonic vehicles and contributes to the ablation of their thermal protection systems (TPSs). To gain atomic-level understanding of the ablation of carbon-based TPS, collisions of hyperthermal atomic nitrogen on representative carbon surfaces have recently be investigated using molecular beams. In this work, we report direct dynamics simulations of atomic-nitrogen (N( 4 S)) collisions with pristine, defected, and oxidized graphene. Apart from non-reactive scattering of nitrogen atoms, various forms of nitridation of the graphene were observed in our simulations. Furthermore, a number of gaseous molecules, including the experimentally observed CN molecule, have been found to desorb as a result of N-atom bombardment. These results provide a foundation for understanding the molecular beam experiment and for modeling the ablation of carbon based TPSs and for future improvement of their properties.

We report here an extensive direct dynamics study on the collisions of hyperthermal (14.9 kcal mol–1) atomic oxygen with a variety of graphene surfaces to explore possible reaction channels. Several models, ranging from pristine graphene to graphene with defects and with different extents of oxidation and nitridation, are investigated. The impinging oxygen atom is found to form various surface oxygenated species, such as epoxides, ethers, and lactones, as well as gaseous species, such as CO, CO2, O2, N2, CN, and NO. Some of the gaseous species have been detected in recent molecular beam studies, and our simulations help to reveal their formation mechanisms. The comparison with previous dynamical studies for a much higher O-atom incident energy (120 kcal mol–1) finds similar reactive channels and reaction mechanisms, with quantitatively different product branching ratios.

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Nonstatistical dynamics is important for many chemical reactions. The Rice-Ramsperger-Kassel-Marcus (RRKM) theory of unimolecular kinetics assumes a reactant molecule maintains a statistical microcanonical ensemble of vibrational states during its dissociation so that its unimolecular dynamics are time independent. Such dynamics results when the reactant's atomic motion is chaotic or irregular. Intrinsic non-RRKM dynamics occurs when part of the reactant's phase space consists of quasiperiodic/regular motion and a bottleneck exists, so that the unimolecular rate constant is time dependent. Nonrandom excitation of a molecule may result in short-time apparent non-RRKM dynamics. For rotational activation, the 2J + 1 K levels for a particular J may be highly mixed, making K an active degree of freedom, or K may be a good quantum number and an adiabatic degree of freedom. Nonstatistical dynamics is often important for bimolecular reactions and their intermediates and for product-energy partitioning of bimolecular and unimolecular reactions. Post–transition state dynamics is often highly complex and nonstatistical.

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