N 2 dissociative adsorption on Ru(0001): The role of energy loss

New molecular beam experiments on the dissociation probability S0 for N2 on Ru(0001) are presented. These are in general agreement with prior measurements and exhibit very unusual behavior; a very slow increase of S0 with incident kinetic energy E and the fact that S0 is still only ∼10−3 at incident energies considerably above the barrier. A simple dynamical model is developed to describe this unusual sticking behavior. The key aspect is that there is considerable energy loss Δ from E upon initial impact with the surface (principally to the lattice) and only E−Δ is then available to surmount the activation barrier in the exit channel. Using experimentally measured values of Δ from scattering experiments gives good qualitative agreement of this model with the measured S0. One implication of the strong energy loss is that there is an apparent violation of detailed balance when comparing only the reactive fluxes of activated adsorption and associative desorption.