Post-dissociation Dynamics of N₂ on Ru(0001): How Far Can the "Hot" N Atoms Travel?

Due to the high barrier and large exoergicity, the dissociation of N-2 impinging on Ru(0001) produces ballistic N atoms that can travel significant distances from the impact site, as shown by a recent scanning tunneling microscopy study [Wagner, J. et al. J. Phys. Chem. C 2022, 126, 18333-18342]. In this work, the "hot" nitrogen atom dynamics following N-2 dissociation is investigated theoretically on a high-dimensional potential energy surface based on a neural network representation of density functional theory data. Quasi-classical trajectory simulations for N-2 dissociation with several initial conditions revealed that typically only one N atom undergoes significant migration, while the other is often trapped near the impact site. Regardless of the initial condition, the average final separation between the two N atoms is typically less than 10 angstrom, about 1 order of magnitude less than the experimental report (66 +/- 28 angstrom). The relatively short migration distance of the hot N atom found in our simulations is attributed to a high diffusion barrier and fast energy dissipation to surface phonons. The theory-experiment discrepancy presents a challenge to the quantitative understanding of hot atom dynamics on metal surfaces.