THE INFLUENCE OF SURFACE RECONSTRUCTION ON THE COLLECTIVE MOTION OF ADSORBED ATOMS

The effect of adsorbate-induced surface reconstruction on the motion o f adsorbed particles is analyzed by means of Monte Carlo modeling. The reconstruction is assumed to be the order-disorder phase transition t hat is described by the two-position model introduced by A.V. Myshlyav tsev et al. [J, Chem. Phys. 92 (1990) 3909]. The lattice gas model is used to mimic the hydrogen adsorption on the (001) face of tungsten [H /W(001)], which presents a c(2 x 2) structure below the critical tempe rature, T-c. The phase diagram of the real system is reproduced only a t low coverages (0<0.25). One of the principal features of. the model is that the critical temperature can either increase or decrease with coverage, depending on the relation of the parameters which describe t he system. In addition, different ordered phases, such as c(2 x 2) and (2 x 1) can be present according to the characteristics of the metal- metal, J(NN), adsorbate-adsorbate, epsilon(NN), and adsorbate metal, l ambda(NN), interactions. Utilizing Monte Carlo simulations we characte rize how the behavior of the chemical, D, the jump, D-J, and the trace r, D, diffusion coefficients depend on the different ordered phases p redicted by the model. In order to calculate the chemical diffusion co efficient we apply the fluctuation and Kubo-Green methods. Surface dif fusion is strongly anisotropic for the (2 x 1) ordered phase, indicati ng that it is Facilitated in one direction and reduced in another. (C) 1998 Elsevier Science B.V. All rights reserved.