Coverage dependence of activation barriers: Nitrogen 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 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 d emonstrate that the N-Ru bond strength decreases significantly with N cover age, and in fact that high coverage N adsorbate states are not thermodynami cally stable relative to associative desorption. The observed high coverage states must, therefore, be only metastable, with lifetimes determined by t he height of the barrier between gas phase N-2 and the adsorbed atomic stat e. Analysis of the TPD in combination with the theoretical adsorption energ ies allows us to estimate these coverage dependent barriers. Independent me asurements 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 t echniques induced by a short laser induced temperature jump. A barrier incr eases 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].