Spatial fluctuations and anomalous reaction order in a reactive scheme involving a cooperative full desorption

Anomalous reaction rates have been found in the hydrogen desorption of H-terminated surfaces in semiconductor epitaxy, with a reaction order shifting from two to one, or even taking fractional values. We analyze the issue in terms of a cooperative full desorption (CFD) reaction A+A--k3-->S+S , coupled to an adsorption reaction S--k1-->A and an alternative desorption route A--k2-->S . Steady state properties of the three-step reactive scheme are analyzed in a one-dimensional lattice in the absence of diffusion. Microscopic Monte Carlo simulations show anomalous spatial distributions of reactants in the stationary state: depending on the reaction rate constants of the overall scheme, either a local "aggregation" or a local "dispersion" of A -particles is observed. The CFD reaction itself is well described by a fractional order kinetics that takes into account these anomalies and that depends on the kinetic rate constants of the overall adsorption-desorption reaction mechanism. The problem is addressed with an analytical approach for the active neighborhood of a reactant, which provides a closed expression of the reaction order as a function of the kinetic parameters. This approach is in excellent agreement with numerical simulations. Spatial correlations, as well as fluctuation correlations, are also formalized in terms of the kinetic constants. We discuss the results in the context of the hydrogen evolution reaction on silicon surfaces.