Using pulsed atomic beam technique and a surface ionization ion microscope, the desorption kinetics and the surface diffusion of the alkalis potassium, rubidium and cesium were investigated on a Si(111)7 × 7-surface at extremely low alkali coverages.
In the temperature range 1120 … 800 K, the mean adsorption lifetime τ(T) = τ0 · exp(Edesi/kT) and the mean diffusion length x(T) - defined in the equilibrium between adsorption, diffusion and desorption - were measured. From these data the diffusion constant D(T) = D0 · exp(-Ediff/kT) was obtained as D = x−2/τ. For temperatures T ⩽ 750 K, the diffusion constant was calculated from nonstationary alkali concentration profiles using the Boltzmann-Matano method.
From the temperature dependence of these quantities the parameters of desorption (Edes,i τ0) and surface diffusion (Ediff, D0) for K, Rb and Cs on Si(111) were obtained. The values of Ediff and D0 are comparably high and may be interpreted by non-localized diffusion according to a model proposed by Bonzel (Surf. Sci. 21 (1970) 45).