Dynamics of Nitrogen Scattering off N-Covered Ag(111)

We analyze the reflection and adsorption dynamics of N atoms on a (1 X 1) N-covered Ag(111) surface, using an ab initio three-dimensional potential energy surface (3D PES) and classical molecular dynamics (MD) in the frozen and vibrating surface regimes. Our calculations reveal strong changes in the PES upon atomic N adsorption, which becomes much more corrugated than that of the clean Ag(111) surface. This apparently contradicts a key experimental finding made for atoms with incident average energy < E-i > = 4.3 eV, namely that the N reflection dynamics on Ag(111) at N saturation coverage are quantitatively similar to those of clean Ag(111). In good agreement with the experiments, we find that the stronger PES corrugation of the N-covered Ag(111) surface does not affect the angular distribution of the scattered N atoms with that < E-i > value. However, discrepancies are found in the final-to-initial average energy ratios, < E-f >/< E-i >, at grazing outgoing angles. Upon examination of the adsorption trajectories, it can be inferred that gas N is likely to react with adsorbed N. MD shows that this "pickup" mechanism is particularly effective for slow atoms and could be behind the experimental < E-f >/< E-i > values.