Chemical Diffusion, Wave Propagation, and Equistability in Lattice-Gas Models for Bistable Surface Reactions
The lattice-gas monomer-dimer surface reaction model exhibits a discontinuous transition between a highly reactive steady state and a near monomer poisoned state, with varying relative impingement rates of the reactant species (provided the monomer desorption rate is below a critical value). This transition is induced by fluctuations in the reactant adlayer. Increasing monomer mobility quenches these fluctuations, thus enhancing metastability near this transition, and leading to bistability in the hydrodynamic limit of high monomer mobility. Within this bistable regime, the more stable steady state displaces the less stable one separated from it by a planar interface. The interface is stationary at an equistability point corresponding to the transition in the lattice-gas model, and it broadens as the desorption rate approaches the critical value. Such behavior in the hydrodynamic limit can be described by reaction-diffusion equations. However, these must account for the influence of adlayer ordering on the reaction kinetics, and for the coverage-dependent and tensorial nature of chemical diffusion, even in the absence of interactions beyond site blocking. All these effects are assessed by applying a variety of conventional and novel Monte Carlo simulation techniques.
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