Microscopic mechanisms of thermal and driven diffusion of non rigid molecules on surfaces

TL;DR

This paper investigates the microscopic mechanisms of how non-rigid molecules, like dimers, diffuse on surfaces, highlighting the importance of vibrations and intramolecular dynamics in their mobility and activation barriers.

Contribution

It introduces a detailed Langevin-based analysis of driven and undriven diffusion of non-rigid molecules, emphasizing the role of intramolecular vibrations and challenging the traditional activation barrier concept.

Findings

Dimer mobility exceeds monomer mobility below the natural stretching frequency.

Vibrations significantly influence the activation barrier, causing it to vary with temperature.

Complex interplay between vibrations and intramolecular length affects diffusion behavior.

Abstract

The motion of molecules on solid surfaces is of interest for technological applications such as catalysis and lubrication, but it is also a theoretical challenge at a more fundamental level. The concept of activation barriers is very convenient for the interpretation of experiments and as input for Monte Carlo simulations but may become inadequate when mismatch with the substrate and molecular vibrations are considered. We study the simplest objects diffusing on a substrate at finite temperature $T$, namely an adatom and a diatomic molecule (dimer), using the Langevin approach. In the driven case, we analyse the characteristic curves, comparing the motion for different values of the intramolecular spacing, both for T=0 and $T\ne 0$. The mobility of the dimer is higher than that of the monomer when the drift velocity is less than the natural stretching frequency. The role of intramolecular excitations is crucial in this respect. In the undriven case, the diffusive dynamics is considered as a function of temperature. Contrary to atomic diffusion, for the dimer it is not possible to define a single, temperature independent, activation barrier. Our results suggest that vibrations can account for drastic variations of the activation barrier. This reveals a complex behaviour determined by the interplay between vibrations and a temperature dependent intramolecular equilibrium length.