Dynamics of adsorbed atoms under non-equilibrium conditions

This paper deals with the determination of the tracer (D-T) and chemical (D -ch) surface diffusion coefficients in the presence of a gradient of the co verage Theta. The lattice gas model and quasi-chemical approximation are ta ken as accepted in the theory. The results are discussed with respect to th e interaction energy of particles, which influences the equilibrium energy of atoms as well as the saddle-point energy. Such interactions break the sy mmetry of jumps in the systems with gradient grad Theta. This model predict s a decrease of D-T with the square of the gradient of the coverage Theta. D-ch depends on the coefficient of proportionality of the difference betwee n the mean jump rate in the direction of grad Theta and that in the opposit e direction. It has been found that, in the case of repulsive interaction, the coverage dependencies of D-T and D-ch have local maxima, whose position s depend on the rates of change of the saddle-point energy and of the equil ibrium energy of the atoms due to the interaction. For attractive interacti on, D-T either decreases with Theta or increases depending on the saddle-po int energy changes. At low temperatures our results differ substantially fr om those of the calculations made within dynamical mean-field theory.