SURFACE-DIFFUSION OF POTASSIUM ON RU(001)

The surface diffusion of potassium on Ru(001) was studied using laser-induced thermal desorption (LITD) techniques. The potassium surface mobility was very dependent on the potassium surface coverage. The coverage dependent surface diffusion coefficients were obtained from an iterative analysis based on surface diffusion measurements performed at different initial coverages. At 220 K, the surface diffusion coefficient increased dramatically as the potassium coverage increased from theta(K) = 0.08 to 0.33 ML. The surface diffusion coefficients also displayed Arrhenius behavior at various temperatures and potassium coverages. As the potassium coverage increased from theta(K) = 0.15 to 0.33 ML, the surface diffusion activation energy decreased slightly and the diffusion pre-exponential increased by a factor of 35. The diffusion activation energies were E(dif) = 9.0 +/- 0.4, 8.5 +/- 0.3 and 8.1 +/- 0.4 kcal/mol and the corresponding pre-exponentials were D0 = 0.40 +/- 0.06, 4.4 +/- 0.3 and 14.0 +/- 0.5 cm2/s at theta(K) = 0.15, 0.25 and 0.33 ML, respectively. The coverage dependence of the potassium surface diffusion on Ru(001) was attributed to strong repulsive interactions between potassium adatoms. These interactions most likely originate from repulsions between dipoles formed by partially ionized potassium adatoms and their surface image charges.