Dynamics of the reaction-diffusion system $A + B \to 0 $ with input of particles

Abstract

We study dynamics of filling of an initially empty finite medium by diffusing particles $A$ and $B$, which arise on the surface upon dissociation of $AB$ molecules, impinging on it with a fixed flux density $I$, and desorb from it by the reaction $A + B\to AB\to 0$. We show that once the bulk diffusivities differ ($p=D_{A}/D_{B}<1$), there exists a critical flux density $I_{c}(p)$, above which the relaxation dynamics to the steady state is qualitatively changed: on time dependencies of $c_{As}/c_{e}$ ($c_{e}$ being the steady state concentration at $t\to \infty$) a maximum appears, the amplitude of which grows both with $I$ and with $D_{B}/D_{A}$ ratio. In the diffusion-controlled limit $I \gg I_{c}$ at $p \ll 1$ the reaction &#34;selects&#34; the {\it universal laws} for the particles number growth ${\cal N}_{A}={\cal N}_{B}\propto t^{1/4}$ and the evolution of the surface concentrations $c_{As}\propto t^{-1/4},c_{Bs}\propto t^{1/4}$, which are approached by one of the {\it two characteristic regimes} with the corresponding hierarchy of the intermediate power-law asymptotics. In the first of these $c_{As}$ goes through a comparatively {\it sharp} max$(c_{As}/c_{e})\propto I^{1/6}$, the amplitude of which is $p$-independent, in the second one $c_{As}$ goes through a {\itplateau-like} max$(c_{As}/c_{e})\propto p^{-1/4}$, the amplitude of which is $I$-independent. We demonstrate that on the main filling stage the evolution of the ${\cal N}(t)/{\cal N}_{e}, c_{As}(t)/c_{e},$ and $c_{Bs}(t)/c_{e}$ trajectories with changing $p$ or $J$ between the limiting regimes is unambiguously defined by the value of the scaling parameter ${\cal K}=p^{3/2}J$ ($J$ being the reduced flux density) and is described by the set of {\it scaling laws}, which we study in detail analytically and numerically.