Neighbor driven mobility of silver adatoms on Pd(100) measured by thermal helium scattering

Thermal helium scattering is used to characterize the deposition of silver atoms on Pd (100) in the coverage range 0 to 10% of a monolayer, at surface temperature between 80 and 160 K, the latter being the threshold of adatom mobility. The attenuation of the specular helium intensity as a function of coverage cannot be interpreted by a statistical distribution which excludes any mobility of the deposited adatoms. The data are well simulated by a model implying that an adatom is not stable if it is separated from another adparticle by only one adsorption site, but it would rather move and form a dimer or enlarge a cluster. This behavior is confirmed by Monte Carlo simulations. The cross section of the silver adatom for diffuse helium scattering is measured to be 109 AA/sup 2/.