Inverted vibrational distributions from N_2 recombination at Ru (001): Evidence for a metastable molecular chemisorption well

摘要

We have measured translational and internal state distributions for N_2 desorbed from a Ru(001) surface following NH_3 cracking at 900 K. Nitrogen is formed with a vibrational population inversion, P(#upsilon# = 1)/P(#upsilon# = 0) = 1.4, but a subthermal rotational energy release, T_(rot)(#upsilon# = 0) = 630 K. The translational energy distributions show a peak at low energy with a tail extending up to approx 2 eV and a mean energy release of 0.62 eV for N_2(#upsilon# = 0) and 0.61 eV for (#upsilon# = 1). The product state distributions indicate a preferential energy release into the N_2 stretching coordinate with a relatively weak N_2-surface repulsion. Density functional calculations for N_2 dissociation on Ru(001) and Cu(111) have been performed to compare the shape of the potentials in the N_2 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 N_2 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 N_2 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.