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

Inverted vibrational distributions from N2 recombination at Ru(001): Evidence for a metastable molecular chemisorption well.
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by M. J. Murphy, J. F. Skelly, A. Hodgson, and B. Hammer.

J. Chem. Phys. **110**, 6954 (1999).

Abstract
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We have measured translational and internal state distributions for
N/sub 2/ desorbed from a Ru(001) surface following NH/sub 3/ 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, T/sub
rot/(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 N/sub 2/(v=0) and 0.61 eV for (v=1). The product state
distributions indicate a preferential energy release into the N/sub 2/
stretching coordinate with a relatively weak N/sub 2/-surface repulsion.
Density functional calculations for N/sub 2/ dissociation on Ru(001) and
Cu(111) have been performed to compare the shape of the potentials in
the N/sub 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/sub 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/sub 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/sup -9/) on Ru(001) at thermal energies but
in disagreement with recent molecular beam adsorption measurements