Nitrogen recombination dynamics at Cu(111): Rotational energy release and product angular distributions

Nitrogen atoms adsorbed on Cu(111) desorb thermally from an ordered Cu(100)−c(2×2)N phase in a sharp, zero order desorption feature near 700 K with an activation barrier of 143 kJ mol−1. Detailed N2 product rovibrational state distributions have been measured following recombinative desorption from a 700 K Cu(111) surface exposed to a N atom beam, with an equilibrium N coverage θN⩽10−2 ML. Although desorbing N2 is translationally and vibrationally hot, with a vibrational temperature of 5100 K and 4.2 eV of translational excitation perpendicular to the surface, rotation is excited with a temperature of just 910(±50) K for the vibrational ground state and 840(±250) K for (v=1). The energy released during recombinative desorption channels effectively into translational and vibrational motion, but not into rotational excitation. The angular distribution of recombinatively desorbed N2 is sharply peaked along the surface normal, P(θ)=cos(28±1) θ, indicating a mean energy release of 0.28 eV into translation parallel to the surface. This is inconsistent with 1D models of the translational energy release based on thermal motion parallel to the surface and a repulsive energy release directed along the surface normal. The dynamics can be described by a direct, repulsive model with a transition state at extended N2 separation, similar to the models developed for H2 dissociation on the same surface. We discuss the application of detailed balance to determine N2 sticking functions S(E,v,J) and, using a simple model for these functions, estimate a rotational efficacy of ∼0.23 for sticking of N2(v=0, J⩽24) and a vibrational efficacy of 0.7 for N2(v=1). The dynamics are compared to the models developed for H2 dissociation and the role of molecular chemisorption states and the local desorption site discussed.