Seven-dimensional microcanonical treatment of hydrogen dissociation dynamics on Cu(111):Clarifying the essential role of surface phonons

摘要

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,E_0 =79 kJ/mol,and the number of surface oscillators involved in the localized H_2/Cu(111) collision complex,s=1.The two-parameter MURT quantitatively predicts much of the varied behavior observed for the H_2 and D_2/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 approx <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 approx 30% of the energy required to surmount the activation barrier to H_2 dissociation under thermal equilibrium conditions.The MURT with s= 1 is used to analytically confirm the experimental finding that partial deriv E_a(T_s)"/ partial deriv E_t=-1 for eigenstate-resolved dissociative sticking at translational energies E_t