Mass Transport in Surface Diffusion of van der Waals Bonded Systems: Boosted by Rotations?

Abstract

Mass transport at a surface is a key factor in heterogeneous catalysis. The rate is determined by excitation across a translational barrier and depends on the energy landscape and the coupling to the thermal bath of the surface. Here we use helium spin− echo spectroscopy to track the microscopic motion of benzene adsorbed on Cu(001) at low coverage (θ ∼ 0.07 ML). Specifically, our combined experimental and computational data determine both the absolute rate and mechanism of the molecular motion. The observed rate is significantly higher by a factor of 3.0 ± 0.1 than is possible in a conventional, point-particle model and can be understood only by including additional molecular (rotational) coordinates. We argue that the effect can be described as an entropic contribution that enhances the population of molecules in the transition state. The process is generally relevant to molecular systems and illustrates the importance of the pre-exponential factor alongside the activation barrier in studies of surface kinetics.

The dynamics of benzene on Cu(111): a combined helium spin echo and dispersion-corrected DFT study into the diffusion of physisorbed aromatics on metal surfaces

“…The population of the rotated states would be expected to affect the pre-exponential factor for diffusion, rather than the Arrhenius activation energy that was considered in this work. A typical molecule’s trajectory across the surface would be expected to involve rotation as well as translation, but without the degree of coupling between the two that might be described in terms of quasi-static steering 10 where the molecule must rotate to pass over the rate-limiting barrier. 42 …”

“…Benzene (>99.9% purity, Aldrich), purified by several freeze–evacuate–thaw cycles, was deposited on the surface by backfilling the chamber and monitoring the drop in the specularly reflected helium beam during adsorption. Using the same method as in our recent study of benzene on Cu(001), 10 we estimate absolute coverages of 0.03 and 0.1 monolayers (ML) for the low and high coverage measurements, respectively.…”

“…In previous works we have explored the dynamics of the cyclopentadienyl anion (Cp), pyrrole and thiophene adsorbed on the same Cu(111) surface. 6 – 9 The combination of state of the art helium spin-echo spectroscopy (HeSE) and density functional theory calculations has provided insight into the role of charge transfer and ionic binding in aromatic adsorption, 8 , 9 and the contribution of rotational, 6 vibrational 7 and other external degrees of freedom 10 to molecular transport on surfaces.…”

PIGLE — Particles Interacting in Generalized Langevin Equation simulator

“…In recent years, pioneering works to study adsorption of larger molecules have demonstrated the need to consider dynamical degrees of freedom beyond translational diffusion in a 2D PES. [12,13] Here we describe PIGLE (Langevin Based Molecular Dynamics simulator), a package to simulate co-motion of interacting particles from multiple species in 2D to 4D PES (i.e. up to three spatial degrees of freedom, and rigid body rotations).…”

Towards surface diffusion potential mapping on atomic length scale

“…Here, we focus on scanning probe microscopy techniques which have been used to characterize the surface diffusion of atoms and molecules on metals, [12][13][14][15][16][17][18] insulators 15,[19][20][21] and semiconductors 22,23 . Two main approaches have been adopted for determining associated energy barriers.…”