Surface diffusion of H and CO on Cu/Ru(001): evidence for long-range trapping by copper islands

Abstract

Abstract The surface diffusion of H and Co on Cu/Ru(001) was studied using laser-induced thermal desorption (LITD) techniques. The surface mobilities of H and CO on the Ru(001) surface were both dramatically attenuated by submonolayer copper coverages. At 300 K and Θ H =0.25 ML, the H diffusion coefficient decreased by a factor of ∼30 from D H =8.7 × 10 −7 cm 2 /s on clean Ru(001) to D H =3.0 × 10 −8 cm 2 /s at Θ Cu = 0.09 ML. At fixed copper coverages of Θ Cu = 0.10 ML and Θ Cu = 0.25 ML, the hydrogen diffusion coefficient was negligible at low hydrogen coverages and then increased at higher hydrogen coverages. The copper coverages also increased the hydrogen diffusion activation barrier from E H = 2.8 ± 0.2 kcal mol on clean Ru(001) to E H = 9.8 ± 1.0 kcal mol at Θ Cu = 0.15 ML . The dependence of hydrogen mobility on hydrogen coverage and the increase in diffusion activation barrier at higher copper coverages suggest that hydrogen is trapped by copper on Ru(001). The copper coverage on Ru(001) also reduced the CO diffusion coefficient. At 300 K and Θ CO = 0.41 ML, the CO diffusion coefficient decreased by a factor of 2400 as the copper coverage increased from Θ Cu = 0.00 ML to 0.10 ML. Recent scanning tunneling microscope (STM) studies have revealed that copper forms islands on Ru(001). Monte Carlo simulations of these surface diffusion results are consistent with a trapping interaction between H and the copper islands. The Monte Carlo results also argue for a very long-ranged perturbation of ∼200 A induced by the copper islands. These experimental and theoretical results for dramatic and long-ranged trapping indicate that copper seriously perturbs the surface electronic properties of Ru(001).