Rotation assisted diffusion of water trimers on Pd{111}

Abstract

Diffusion barriers for a cluster of three water molecules on Pd{111} have been estimated from ab-initio Density Functional Theory. A model for the diffusion of a cluster of three water molecules (trimer) based in rotations yields a simple explanation of why the cluster can diffuse faster than a single water molecule by a factor ≈ 10² [1]. This model is based on the differences between the adsorption geometry for the three molecules forming the trimer. One member interacts strongly with the surface and sits closer to the surface (d) while the other two interact weakly and stay at a larger separation from the surface (u). The trimer rotates nearly freely around the axis determined by the d-like monomer. Translations of the whole trimer imply breaking the strong interaction of the d-like molecule with the surface with a high energy cost. Alternatively, thermal fluctuations can exchange the position of the molecule sitting closer to the surface with a lower energetic cost. Rotations around different axis yield a diffusion mechanism where the strong interaction is maintained along the diffusion path, therefore lowering the effective activation barrier.