Observation of metastable atomic nitrogen adsorbed on Ru(0001)

Exposing a Ru(0001) surface to an atomic beam of N produces a series of dif ferent 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 s tates exhibit well defined temperature programmed desorption (TPD) peaks wh ich shift to considerably lower temperatures with N coverage. The highest N coverage obtainable is almost 1 ML N/Ru. Recent density functional calcula tions demonstrate that the N-Ru energy decreases significantly with N cover age, and in fact predict that N adsorbate states are not thermodynamically stable relative to associative desorption at high coverage. The observed hi gh 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 theore tical adsorption energies allows us to estimate these coverage dependent ba rrier heights. We find that the barrier heights increase significantly with coverage, and this is important in the metastability of the adsorbed state s. A comparison of nitrogen adsorption on Ru(0001) with oxygen adsorption o n Ru(0001) surface is stressed throughout. (C) 2000 American Institute of P hysics. [S0021-9606(00)70705-7].