Anisotropic diffusion of n-butane on a stepped Ru(001) surface

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Abstract
The surface diffusion of n‐butane on a stepped ruthenium {Ru(S)‐[15(001)×2(100)]} surface was studied using laser‐induced thermal desorption (LITD) techniques. Large aspect ratio laser desorption areas were used to measure the anisotropy of n‐butane diffusion on the stepped Ru(001) surface. Surface diffusion coefficients at Θ=Θsat for diffusion parallel (D∥) or perpendicular (D⊥) to the steps were measured by orienting these desorption areas either perpendicular or parallel to the step edges. At 115 K, the surface diffusion coefficient was much larger in the direction parallel to the step edges and D∥/D⊥≊26. Surface diffusion coefficients for diffusion on a terrace (Dt) or over a step (Ds) were deconvoluted from the measured D∥ and D⊥ values. The temperature‐dependent D∥ and D⊥ values yielded an activation barrier for n‐butane diffusion on the Ru(001) terrace at Θ=Θsat of Et=2.8±0.4 kcal/mol and a diffusion preexponential of D0,t=1.6×10−2±0.2 cm2/s. For diffusion over a step at Θ=Θsat, the surface diffusion barrier was Es=4.8±0.5 kcal/mol and the diffusion preexponential was D0,s=6.1×10−1±0.3 cm2/s. The measured corrugation ratio for n‐butane on the Ru(S)‐[15(001)×2(100)] surface was Ω≡Edif/Edes=0.24 for diffusion on the terrace and Ω=0.41 for diffusion over a step. The surface corrugation ratio on the terrace of Ru(S)‐[15(001)×2(100)] was similar to Ω=0.29 measured previously for n‐butane on Ru(001). This similarity indicated that step defects did not dominate previous LITD measurements of alkanes on Ru(001).