  We develop a version of the vacancy mediated tracer diffusion model, which
follows the properties of the physical system of In atoms diffusing within the
top layer of Cu(001) terraces. This model differs from the classical tracer
diffusion problem in that (i) the lattice is finite, (ii) the boundary is a
trap for the vacancy, and (iii) the diffusion rate of the vacancy is different,
in our case strongly enhanced, in the neighborhood of the tracer atom. A simple
continuum solution is formulated for this problem, which together with the
numerical solution of the discrete model compares well with our experimental
results.
