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.

We use helium spin-echo spectroscopy (HeSE) to investigate the dynamics of the diffusion of benzene adsorbed on Cu(111). The results of these measurements show that benzene moves on the surface through an activated jump-diffusion process between the adsorption sites on a Bravais lattice. Density Functional Theory (DFT) calculations with van der Waals (vdW) corrections help us understand that the molecule diffuses by jumping through non-degenerate hollow sites. The results of the calculations shed light on the nature of the binding interaction between this prototypical aromatic molecule and the metallic surface. The highly accurate HeSE experimental data provide a quantitatively stringent benchmark for the vdW correction schemes applied to the DFT calculations and we compare the performances of several dispersion interaction schemes.

Measurement of the phason dispersion of misfit dislocations on the Au(111) surface.

We report measurements of the acoustic and optical phason dispersion curves associated with the lattice of partial dislocations on the reconstructed (111) surface of gold. Our measurements of these low energy (<0.5 meV) weakly dispersive modes have been enabled by the very high resolution of the novel helium spin-echo technique. The results presented here constitute the first measurement of the phason dispersion of misfit dislocations, and possibly of excitations associated with any type of crystalline dislocations.