Inverted vibrational distributions from N2 recombination at Ru(001): Evidence for a metastable molecular chemisorption well

Abstract

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.