Role of Rotations on Surface Diffusion of Water Trimers on Pd\{111\}

TL;DR

This study uses ab-initio calculations to analyze how rotations influence the surface diffusion of water trimers on Pd(111), revealing a model where rotations enable faster diffusion than single molecules due to geometric and energetic factors.

Contribution

It introduces a rotational diffusion model explaining the enhanced mobility of water trimers on Pd(111) compared to single molecules.

Findings

Water trimers diffuse faster than single water molecules by a factor of approximately 100.

Rotations around specific axes facilitate diffusion while maintaining strong surface interactions.

The model explains diffusion through differences in adsorption geometry and thermal fluctuations.

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 the trimer based in rotations yields a simple explanation of why the cluster can diffuse faster than a single water molecule by a factor $\approx 10^{2}$. This model is based on the differences between the adsorption geometry for the three monomers forming the cluster. 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 monomer. Translations of the whole trimer imply breaking the strong interaction of the d monomer with the surface. Alternatively, thermal fluctuations exchange the actual monomer sitting closer to the surface with a lower energetic cost. Rotations around different axis introduce a diffusion mechanism where a strong interaction is kept along the diffusion path between the water molecule defining the axis of rotation and the Pd underneath.