Coverage dependence of activation barriers:: Nitrogen 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 up to a N coverage of almost 1 ML, N/Ru. These states show well-defined temperature programmed desorption (TPD) peaks. Recent density functional calculations demonstrate that the N-Ru bond strength decreases significantly with N coverage, and in fact that high coverage N adsorbate states are not thermodynamically stable relative to associative desorption. The observed high coverage states must, therefore, be only metastable, with lifetimes determined by the height of the barrier between gas phase N-2 and the adsorbed atomic state. Analysis of the TPD in combination with the theoretical adsorption energies allows us to estimate these coverage dependent barriers. Independent measurements of the coverage dependent barrier heights were also obtained via the technique of laser assisted associative desorption i.e., by measuring the translational energy distribution of desorbing N-2 via time of flight techniques induced by a short laser induced temperature jump. A barrier increases of greater than or equal to 1 eV with N atom coverage was observed by both methods. (C) 2000 American Vacuum Society. [S0734-2101(00)04004-5].