Observation of a universal aggregation mechanism and a possible phase transition in Au sputtered by swift heavy ions

Two exponents delta for the size distribution of n-atom clusters, Y(n) approximately 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.