The dissociation of methane on a nickel catalyst is a key step in steam reforming of natural gas for hydrogen production. Despite substantial effort in both experiment and theory, there is still no atomic-scale description of this important gas-surface reaction. We report quantum state-resolved studies, using pulsed laser and molecular beam techniques, of vibrationally excited methane reacting on the nickel (100) surface. For doubly deuterated methane (CD2H2), we observed that the reaction probability with two quanta of excitation in one C-H bond was greater (by as much as a factor of 5) than with one quantum in each of two C-H bonds. These results clearly exclude the possibility of statistical models correctly describing the mechanism of this process and attest to the importance of full-dimensional calculations of the reaction dynamics.

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The state-resolved reactivity of CH 4 in its totally symmetric C-H stretch vibration ( 1 ) has been measured on a Ni(100) surface. Methane molecules were accelerated to kinetic energies of 49 and 63:5 kJ=mol in a molecular beam and vibrationally excited to 1 by stimulated Raman pumping before surface impact at normal incidence. The reactivity of the symmetric-stretch excited CH 4 is about an order of magnitude higher than that of methane excited to the antisymmetric stretch ( 3 ) reported by Juurlink et al. [Phys. Rev. Lett. 83, 868 (1999)] and is similar to that we have previously observed for the excitation of the first overtone (2 3 ). The difference between the state-resolved reactivity for 1 and 3 is consistent with predictions of a vibrationally adiabatic model of the methane reaction dynamics and indicates that statistical models cannot correctly describe the chemisorption of CH 4 on nickel.Activated dissociation of molecules on a metal surface is a fundamental step in many catalytic processes. An important example is the chemisorption of methane on nickel to form surface-bound methyl and hydrogen; this reaction is the rate-limiting step in steam reforming, which is the principal process for industrial hydrogen production. The importance of this process has incited a number of studies, both theoretical and experimental, directed towards understanding the detailed mechanism of methane chemisorption [1-15]. Molecular-beam experiments have shown that methane chemisorption on nickel is a direct process, activated by translational and vibrational energy [1,2]. More recent state-resolved experiments investigating the reactivity of CH 4 excited to the antisymmetric stretch fundamental vibration ( 3 ) [6] and first overtone (2 3 ) [9] on Ni(100) have found that energy in 3 promotes the reaction with similar efficacy as kinetic energy along the surface normal. Furthermore, Juurlink et al. [6] have shown that CH 4 with excitation in 3 contributes less than 2% to the activated chemisorption of thermally excited methane [2]. They conclude that vibrational modes other than 3 must play a significant role in methane reactivity under thermal conditions. Theoretical treatments of methane chemisorption include statistical [11,12] as well as dynamical models [3,4,7,8,13]. While the statistical approach excludes the possibility of mode-specific reactivity, it has been claimed to reproduce the results of both thermally averaged and eigenstate-resolved measurements for CH 4 on Ni(100) [11,12]. On the other hand, simplified dynamical models for gas-surface reactions suggest the possibility of mode specificity [7,8]. For reactions that occur entirely in the gas phase, more realistic dynamical calculations find that the symmetric-stretch vibration is generally more efficient than the antisymmetric stretch in promoting reaction [16 -22], and this has been confirmed, in part, by experiments [18,23].We have previously reported vibrational mode-specific chemisorption of CD 2 H 2 on Ni(100), where we demon-strated the difference in re...

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