{#sectiondynamics}
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The long time-scale rearrangement of heavy atoms on a-C substrates has been observed previously.[@Utla80] Here we present a detailed investigation of pattern formation of Au on a-C substrates. Figure \[expfig1\](a) shows a typical TEM image of a sample prepared by deposition of 3000 shots of Au clusters on a 10 nm a-C film. The image displayed was recorded one day after the preparation of the sample. As expected, one observes a statistical particle distribution. The histogram included shows the size distribution. Au clusters smaller than 1 nm in diameter could not be discerned from the 10 nm carbon substrate background owing to their low contrast. However, ion mobility measurements have revealed[@GWF+02] that clusters in this range of sizes are widely present in cluster ion beams. This suggests that a significant number of small clusters is also present on the substrate, albeit unresolved in the TEM.

Island formation {#sectionIsland}
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After the sample is kept at room temperature and in absence of inert conditions for four months, TEM studies yield images as displayed in Fig. \[expfig1\](b). The previously statistically distributed clusters are now collected in agglomerates referred to as islands. The vast majority of these islands is of similar 