Finalist EPFL doctorate Award 2006 - Maroni Plinio

Bond- and mode-specific reactivity of methane on Ni(100)

In this work, the state-resolved reactivity of methane excited to different C-H stretch vibrations have been measured on a Ni(100) surface. Two kinds of experiments have been performed. In the first series of experiments, we have measured the reactivity of dideutero methane (CD2H2) excited in two different C-H stretch vibrational states which are nearly iso-energetic, but have different vibrational amplitudes. We observed that CD2H2 excited with two quanta of vibrational energy in one C-H bond were more reactive (by as much as a factor 5) than molecules excited with one quantum in each of two C-H bonds. This was the first time that state specificity has been observed in a gas-surface reaction. Our results clearly exclude the possibility of statistical models correctly describing the mechanisms of the methane chemisorption and highlight the importance of the dynamical calculations. We rationalize our results in terms of a spectator model and bond-specific reactivity, where the laser excited bond is broken in the reaction with the surface and the difference in reactivity of the two vibrational states is explained in terms of vibrational energy localized in a single C-H bond. Additionally, we have measured the state-resolved reactivity of CH4 in its totally symmetric C-H stretch vibration (ν1) on Ni(100). The methane molecules were excited to ν1 by stimulated Raman pumping prior the collision with the surface. We observed that the reactivity of the ν1 excited CH4 is about an order of magnitude higher than that of methane excited to the isoenergetic antisymmetric stretch (ν3) 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 (2ν3). Since all four bonds initially carry vibrational amplitude for both ν1 and ν3, the difference in reactivity between the symmetric and antisymmetric vibrations cannot simply be explained in terms of bond-specific laser excitation. We refer to this reactivity difference as mode-specific. In this case, the relative reactivity between two different vibrational states does not only depend on the quantity of vibrational energy contained in each bond, but it is also influenced by the symmetry of the vibrational state excited. Our results are consistent with predictions of a vibrationally adiabatic model of the methane reaction dynamics [Halonen et al., J. Chem. Phys. 115, 5611 (2001)].