Motion of small gold clusters in the electron microscope

The rapid motion of small (∼1000 atom) gold clusters observed with atomic resolution in the electron microscope is argued to be the result of brief melting events, triggered by the Auger decay of an M-shell vacancy created in a gold atom within the cluster. Each molten episode lasts ∼10 ps, too short for noticeable evaporation. The observed frequency of motion events is shown to be consistent with ionization cross sections for the gold M shell and the electron current densities employed. It is argued from the lack of observable evaporation that the steady-state temperature of the clusters is ≤1100 K, well below the melting point. The model correctly predicts the cessation of motion at a particle diameter ∼8 nm, when a single Auger decay can no longer melt the cluster.