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

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 ($p=D_A\mathrm{/}{D}_B<1$), 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 $c_{As}\mathrm{/}{c}_e$ ($c_e$ being the steady state concentration at $t\to \mathrm{\infty }$) a maximum appears, the amplitude of which grows both with $I$ and with $D_B\mathrm{/}{D}_A$ ratio. In the diffusion-controlled limit $I\gg I_c$ at $p\ll 1$ the reaction “selects” the {\it universal laws} for the particles number growth ${\mathcal{N}}_A={\mathcal{N}}_B\propto t^{1\mathrm{/}4}$ 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$(c_{As}\mathrm{/}{c}_e)\propto I^{1\mathrm{/}6}$, the amplitude of which is $p$-independent, in the second one $c_{As}$ goes through a {\itplateau-like} max$(c_{As}\mathrm{/}{c}_e)\propto p^{-1\mathrm{/}4}$, 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 $\mathcal{K}=p^{3\mathrm{/}2}J$ ($J$ being the reduced flux density) and is described by the set of {\it scaling laws}, which we study in detail analytically and numerically.