Measurement of Non-Equilibrium Diffusion from Two-Dimensional Ordering Kinetics

Abstract

The thermodynamic behavior of an overlayer on a surface is determined by the microscopic interactions between the adsorbate atoms and between adsorbate and substrate atoms. A determination of these microscopic interactions can be made at equilibrium or also as the system evolves toward equilibrium. The usual equilibrium determination is a measurement of the temperature-coverage phase diagram of the overlayer, for example, with a diffraction or specific-heat experiment. Using a statistical model with a few local interaction parameters and an underlying lattice geometry, the phase boundaries are fitted to determine the magnitude of the interaction parameters. Nonequilibrium determinations of the same interactions can also be made. A system that is initially in an equilibrium state Xi is suddenly forced into a different state (for example, by a temperature or chemical potential quench) where its equilibrium state is Xf. The evolution of the system toward Xf requires mass transport, which can be monitored through a measurement of the change in order of the system with time. The temperature and coverage dependence of the ordering provides information on the microscopic interactions that govern non-equilibrium diffusion in the system.