Observation of metastable atomic nitrogen adsorbed on Ru(0001)

Exposing a Ru(0001) surface to an atomic beam of N produces a series of different states of atomic N adsorbed on the surface. For low atom doses, well-known low coverage states are produced, but for higher atom doses, several previously unknown higher coverage states are sequentially filled. These states exhibit well defined temperature programmed desorption (TPD) peaks which shift to considerably lower temperatures with N coverage. The highest N coverage obtainable is almost 1 ML N/Ru. Recent density functional calculations demonstrate that the N-Ru energy decreases significantly with N coverage, and in fact predict that N adsorbate states are not thermodynamically stable relative to associative desorption at high coverage. The observed high coverage states must, therefore, be metastable with lifetimes determined by the height of the barrier between gas phase N-2 and the adsorbed atomic states. Simple analysis of the TPD features in combination with the theoretical adsorption energies allows us to estimate these coverage dependent barrier heights. We find that the barrier heights increase significantly with coverage, and this is important in the metastability of the adsorbed states. A comparison of nitrogen adsorption on Ru(0001) with oxygen adsorption on Ru(0001) surface is stressed throughout. (C) 2000 American Institute of Physics. [S0021-9606(00)70705-7].