An interplay between the processes of condensation and ordering in open submonolayers of adsorbed atoms

We study the coupled processes of condensation and ordering in open atomic submonolayers adsorbed on the (001) crystal plane of a bcc alloy. The system of interstitial adsorption sites is viewed as consisting of two equivalent square sublattices. It is established that at sufficiently low substrate temperatures in a certain range of pressures of the surrounding vapor and under the repulsive character of lateral adatom –adatom interaction the adlayer may exhibit positional long-range order. A P–T diagram is constructed showing the stability region of the ordered surface phase. A system of kinetic equations is derived, which permits to analyze numerically the simultaneous evolution of the long-range order parameter and the adsorbate film coverage in the course of establishment of thermodynamic equilibrium. The processes of adsorption and surface ordering prove to be substantially interdependent. Evolution of the long-range order parameter is shown to bear a distinct threshold character. The equilibrium ordered state is attained via formation of a kinetically-slowed disordered surface phase. In evolutionary curves this is manifested by the two-stage variation of coverage. Reasons for such type of the coverage temporal evolution are suggested.