Observation of a Universal Aggregation Mechanism and a Possible Phase Transition in Au Sputtered by Swift Heavy Ions

Two exponents, $\delta$, for 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.