Quasielastic helium atom scattering measurements of microscopic diffusion of CO on the Ni(110) surface

Abstract

The microscopic diffusion of CO on the Ni(110) surface has been studied by quasielastic helium atom scattering. From the temperature dependence of the energetic broadening of the quasielastic peak measured at a parallel wave vector near the Brillouin zone boundary, the activation energies for diffusion have been determined to be Ediff=57±4 and 35±4 meV for diffusion parallel to the rows (〈11̄0〉) and perpendicular to the rows (〈001〉), respectively. The activation energies are a factor 2–6 smaller and the preexponential diffusion coefficients orders of magnitude larger than obtained in recent laser measurements of chemical diffusion coefficients, indicating that diffusion over distances from about a few Ångstroms up to 30 Å is much faster than over the much larger distances probed in macroscopic diffusion measurements. The difference is attributed to the impeding effect of step edges or impurities on the latter measurements.