Atomic scale friction of molecular adsorbates during diffusion

TL;DR

This study investigates the microscopic origins of increased friction experienced by molecular adsorbates during surface diffusion, highlighting the role of internal degrees of freedom and rotation modes through experiments and simulations.

Contribution

It reveals that internal and external degrees of freedom, especially rotation modes, significantly contribute to molecular adsorbates' friction on surfaces, advancing understanding of surface diffusion mechanisms.

Findings

Friction coefficients > 2 ps$^{-1}$ for all studied molecules.

Internal and external degrees of freedom influence friction.

Rotation modes are primary contributors to friction.

Abstract

Experimental observations suggest that molecular adsorbates exhibit a larger friction coefficient than atomic species of comparable mass, yet the origin of this increased friction is not well understood. We present a study of the microscopic origins of friction experienced by molecular adsorbates during surface diffusion. Helium spin-echo measurements of a range of five-membered aromatic molecules, cyclopentadienyl (Cp), pyrrole and thiophene, on a copper(111) surface are compared with molecular dynamics simulations of the respective systems. The adsorbates have different chemical interactions with the surface and differ in bonding geometry, yet the measurements show that the friction is greater than 2 ps$^{-1}$ for all these molecules. We demonstrate that the internal and external degrees of freedom of these adsorbate species are a key factor in the underlying microscopic processes and identify the rotation modes as the ones contributing most to the total measured friction coefficient.