Understanding the Enhancement of Surface Diffusivity by Dimerization

Beyond dilute coverage, the collective diffusion of molecules might enhance material transport. We reveal an enhanced mobility of molecular dimers by separating two motions, diffusion and rotation, of CO dimers on elemental Ag(100) as well as on a dilute Cu alloy of Ag(100). From time-lapsed scanning tunneling microscopy movies recorded between 15 and 25 K, we determine the activation energy of dimer diffusion on elemental Ag(100) to be, at (40±2)  meV, considerably smaller than the one for monomer diffusion, at (72±1)  meV. The alloyed Cu atoms reduce the dimer mobility facilitating to determine their rotational barrier separately to be (39±3)  meV. Disentangling different degrees of freedom suggests that a rotational motion is at the origin of enhanced dimer diffusivity.