Dynamics of hydrogen dissociation on Pt{100}: Steering, screening and thermal roughening effects

Abstract

The normal translational energy (Ei) and surface temperature (Ts) dependence of the initial D2 sticking probability (s0) on Pt{100} has been measured using molecular beam techniques. On the hex phase s0 is found to decrease sharply with Ei between 5 and 10 meV, and to increase more gradually over the range 20⩽Ei⩽400 meV. This is interpreted as dynamical steering at low incident energies, giving way to weak translational activation at higher energies. The variation of the barrier to dissociation (Ea) across the unit cell gives rise to screening of dissociation with the impact parameter. There is less variation of Ea across the smaller (1×1) unit cell, leading to a stronger Ei dependence on this surface. On the hex phase s0 has been measured over a wide Ts range, from 150 to 1100 K. Weak thermal activation of dissociation is observed. This is well explained by a roughening model in which s is assumed to be proportional to the linear thermal displacement of surface metal atoms. The model is supported by density functional theory calculations of potential energy surfaces for a modelled rough surface. The absolute value of s0 under conditions of zero-point motion is estimated as 0.03 at Ei=69 meV and 0.06 at 342 meV.