Ultrafast Diffusion at the Onset of Growth: O/Ru(0001)

Nanoscopic clustering in a 2D disordered phase is observed for oxygen on Ru(0001) at low coverages and high temperatures. We study the coexistence of quasistatic clusters (with a characteristic length of $\ensuremath{\sim}9\text{ }\text{ }\AA{}$) and highly mobile atomic oxygen which diffuses between the energy-inequivalent, threefold hollow sites of the substrate. We determine a surprisingly low activation energy for diffusion of $385\ifmmode\pm\else\textpm\fi{}20\text{ }\text{ }\mathrm{meV}$. The minimum of the $\mathrm{O}\ensuremath{-}\mathrm{O}$ interadsorbate potential appears to be at lower separations than previously reported.