We have measured translational and internal state distributions for N2 desorbed from a Ru(001) surface following NH3 cracking at 900 K. Nitrogen is formed with a vibrational population inversion, P(v=1)/P(v=0)=1.4, but a subthermal rotational energy release, Trot(v=0)=630 K. The translational energy distributions show a peak at low energy with a tail extending up to ∼2 eV and a mean energy release of 0.62 eV for N2(v=0) and 0.61 eV for (v=1). The product state distributions indicate a preferential energy release into the N2 stretching coordinate with a relatively weak N2–surface repulsion. Density functional calculations for N2 dissociation on Ru(001) and Cu(111) have been performed to compare the shape of the potentials in the N2 stretching (d) and translational (Z) coordinates. These reveal a sharp curvature of the surface for Ru, the energy release occurring close to the surface over a narrow range of Z. We suggest that this behavior is the result of the presence of a metastable molecular state, bound close to the surface with a short N2 bond, as predicted by Mortensen et al. [J. Catalysis, 169, 85 (1997)]. We contrast the dynamics on Ru with that observed for N recombination on Cu(111) [Murphy et al., J. Chem. Phys. 109, 3619 (1998)], where the potential energy surface shows no evidence for a molecular chemisorption well. Detailed balance arguments predict that N2 dissociation on Ru(001) is highly activated, S(E) increasing by nine orders of magnitude between 0.1 and 2 eV translational energy. The vibrational population inversion implies that vibration promotes dissociation more efficiently than translational excitation, sticking having a vibrational efficacy of 1.3. The predicted S(E) are consistent with reports of a very low sticking probability (S<10−9) on Ru(001) at thermal energies but in disagreement with recent molecular beam adsorption measurements.

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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.

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The recombination of surface and subsurface D atoms on Ni(111) has been studied using resonance-enhanced multiphoton ionisation (REMPI) to measure the internal state and translational energy distributions of the desorbing product. By detecting formed D 2 during temperature-programmed desorption we were able to examine the reaction between subsurface and surface D atoms, and the recombination of two D atoms chemisorbed on the surface. Translational energy distributions for formed by D 2 recombination of surface D are very sensitive to coverage. Desorption from a low coverage surface produced a translational energy release of 2.6 kT , but a thermal rotational distribution, reÑecting an entrance channel barrier to dissociative chemisorption on the clean Ni(111) surface. Sticking probabilities predicted from detailed balance are consistent with molecular beam adsorption measurements. Desorption from D coverages above 0.5 ML resulted in a sub-thermal energy release, desorption being mediated by a molecular precursor state with dissociation occurring via a non-activated, D 2 trappingÈdissociation channel. In contrast, the reaction of subsurface D produces translationally hot with a mean energy approaching 8 at 180 K. This is consistent D 2 , kT s with the energetics for direct recombination of a chemisorbed D atom with a metastable subsurface D atom, which overcomes an activation barrier to resurface of between 0.35 and 0.47 eV depending on D concentration. The energy release decreases at higher temperature, probably as a result of a reduction in the energy of resurfacing D as the subsurface D concentration drops. This low energy component is attributed to accommodation of resurfacing D which is unable to react directly, followed by slow thermal desorption via the high coverage, surface D recombination channel. No internal rotational or vibration excitation was found in formed by reaction of subsurface D. D 2