Single-electron tunnelling in few-atom systems: size of single atoms and geometry of few-atom clusters

We show results for tunnelling into single atoms and few-atom clusters. Sin gle-electron tunnelling with STM reveals a clear series of cluster capacita nce in a 2:3:4 ratio corresponding to single atoms or dimers, three-atom cl usters and four-atom clusters. We present results for Ag and Bi atoms, whic h have similar nearest-neighbour distances but are otherwise dissimilar. Qu alitatively, we conclude that tunnelling electrons occupy anti-bonding-like configurations on the clusters. Quantitatively our results imply an atomic diameter-or few-atom cluster bond length-of 2.6 +/- 0.1 Angstrom for these species.