The irregular surface you see here was imaged using the scanning tunneling microscope (the region shown is 900 Å by 900 Å ). This irregular surface was created by noble gas ion beam bombardment of the gold (111) surface, which creates both pits and islands. The different shades of grey correspond to different layers of the surface; individual atomic-height steps are visible. Thus, the arrows show two inner pits (two layers deep) inside lighter grey (one layer deep) outer pits. Selecting the image will download an mpeg movie showing the evolution of this initial surface with time. Watch carefully and you'll see the islands and pits evolve: the smaller features will disappear completely and the larger ones should smooth out. In particular, the lower right inner pit shown by the arrow disappears almost immediately when the outer pits merge: it's being absorbed by the upper left inner pit via vacancy diffusion or being filled in by mobile atoms from the layer immediately above or a combination of the two. The surface is trying to flatten out: atomic diffusion, detachment of atoms and vacancies from steps, and perhaps the effects of the STM imaging are mediating the flattening process.
To develop a microscopic understanding of the surface evolution we've been modelling the decay of islands and pits using a Monte Carlo simulation. We use ``continuous time'' Monte Carlo, to speed up the same; the simulation shown is on a square lattice (more appropriate for the (100) surface than for the (111) surface).
We found experimentally (as seen in the first few frames of the STM movie) that pits inside other pits were stable until the outer pits merged: then the smaller inner pit disappeared. Selecting this image will show a simulation which exhibits the same behavior. In this simulation, we forbid atoms from moving from one layer to another, so that the decay of pits is driven by vacancy diffusion.
This research was paid for by THE US GOVERNMENT through the National Science Foundation (NSF DMR-#9121654 and GER-#9022961).