MAGIC CLUSTERS IN 2 DIMENSIONS

The initial stages of growth of Pt on Pt(111) have been investigated as a function of the substrate temperature. The main result is that, while between 200 and 400 K the cluster density decreases with increasing temperature in accordance with nucleation theory, in the range of 400-500 K the density increases by more than 1 order of magnitude in obvious contradiction to expectations. At coverages below 0.1 we find at temperatures above 500 K a high density of small clusters (possibly heptamers) which appear to be stable with respect both to decomposition and to further growth.