Two exponents $\delta$ for the size distribution of $n$-atom clusters, $Y(n)\sim n^{-\delta }$, have been found in Au clusters sputtered from embedded Au nanoparticles under swift heavy ion irradiation. For small clusters, below 12.5 nm in size, $\delta$ has been found to be $3/2$, which can be rationalized as occurring from a steady state aggregation process with size independent aggregation. For larger clusters, a $\delta$ value of $7/2$ is suggested, which might come from a dynamical transition to another steady state where aggregation and evaporation rates are size dependent. In the present case, the observed decay exponents do not support any possibility of a thermodynamic liquid-gas-type phase transition taking place, resulting in cluster formation.

• Received 7 January 2008

DOI:https://doi.org/10.1103/PhysRevLett.100.245501