Heather Abbott-Lyon - 7-dimensionsal microcanonical treatment of hydrogen dissociation dynamics on Cu(111): Clarifying the essential role of surface phonons

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      Publication Details (including relevant citation   information):

      H.L. Abbott and I. Harrison, Journal of   Chemical Physics, 125, 024704 (2006).

      Abstract:

      A simple picture of the hydrogen dissociation/associative   desorption dynamics on Cu(111) emerges from a two-parameter, full   dimensionality microcanonical unimolecular rate theory (MURT)   model of the gas-surface reactivity. Vibrational frequencies for   the reactive transition state were taken from density functional   theory calculations of a six-dimensional potential energy surface   (Hammer et al., Phys. Rev. Lett. 73, 1400 (1994)). The two   remaining parameters required by the MURT were fixed by   simulation of experiments. These parameters are the dissociation   threshold energy, E0 =79 kJ/mol, and the number of surface   oscillators involved in the localized H2/Cu(111) collision   complex, s=1. The two-parameter MURT quantitatively predicts much   of the varied behavior observed for the H2 and D2/Cu(111)   reactive systems, including the temperature-dependent associative   desorption angular distributions, mean translational energies of   the associatively desorbing hydrogen as a function of   rovibrational eigenstate, etc. The divergence of the statistical   theory’s predictions from experimental results at low rotational   quantum numbers, J<~5, suggests
      that either (i) rotational steering is important to the   dissociation dynamics at low J, an effect that washes out at high   J, or (ii) molecular rotation is approximately a spectator degree   of freedom to the dissociation dynamics for these low J states,   the states that dominate the thermal reactivity. Surface   vibrations are predicted to provide ~30% of the energy required   to surmount the activation barrier to H2 dissociation under   thermal equilibrium conditions. The MURT with s=1 is used to   analytically confirm the experimental finding that   d“Ea(Ts)”/dEt=−1 for eigenstate-resolved dissociative sticking at   translational energies Et<E0−Ev−Er. Explicit treatment of the   surface motion (i.e., surface not frozen at Ts=0 K) is a   relatively novel aspect of the MURT theoretical approach.

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