  In this Letter, we present results of an extensive Monte Carlo study of the
O/W(110) system under non-equilibrium conditions. We study the mean square
displacements and long wavelength density fluctuations of adatoms. From these
quantities, we define effective and time-dependent values for the collective
and tracer diffusion mobilities. These mobilities reduce to the usual diffusion
constants when equilibrium is reached. We discuss our results in view of
existing experimental measurements of effective diffusion barriers, and the
difficulties associated with interpreting non-equilibrium data.
