HYBRID TREATMENT OF SPATIOTEMPORAL BEHAVIOR IN SURFACE-REACTIONS WITHCOEXISTING IMMOBILE AND HIGHLY MOBILE REACTANTS

For surface reactions on single-crystal substrates which involve highl y mobile adspecies, there is a vast separation in natural time and len gth scales, Adspecies hop rates can be many orders of magnitude larger than rates for other processes. Strong spatial correlations or orderi ng can exist on the atomic scale, while spatial pattern formation occu rs on a macroscopic scale due to high diffusivity. An efficient analys is of such systems is provided by a ''hybrid treatment'' which we appl y here to the monomer-dimer surface reaction model in the case of coex isting immobile dimer adspecies and highly mobile monomer adspecies. S pecifically, we combine a mean-field treatment of the ''randomized'' m obile adspecies, and a lattice-gas description of the immobile adspeci es. Monte Carlo simulations then reveal bistability and ''critical'' b ifurcation phenomena, while precisely accounting for the influence of correlations in the immobile adspecies distribution. A corresponding a nalysis of the evolution of macroscopic spatial inhomogeneities is ach ieved through parallel simulation of the distributed macroscopic point s with distinct correlated states and adspecies coverages. These simul ations are appropriately coupled to describe diffusive mass transport of the mobile adspecies. In this way, we examine for this model the pr opagation and structure of chemical waves, corresponding to interface between bistable reactive states, and thereby determine the relative s tability of these states. (C) 1995 American Institute of Physics.