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Mechanisms of nanoparticle formation by short laser pulses

Numerical modeling is performed to study cluster formation by laser ablation. The developed model allows us to compare the relative contribution of the two channels of the cluster production by laser ablation: (i) direct cluster ejection upon the laser-material interaction, and (ii) collisional sticking, evaporation and coalescence during the ablation plume expansion. Both of these mechanisms are found to affect the final cluster size distribution. Plume cluster composition is correlated with plume dynamics. The results of the calculations demonstrate that cluster precursors are formed during material ablation through both thermal and mechanical target decomposition processes. Then, clusters react in collisions within the plume. In vacuum, rapid plume expansion and cooling take place leading to the overall decrease in the reaction rates. In the presence of a gas, additional collisions with background gas species affect the cluster size distribution. Growth of larger clusters can be observed at this stage. Calculation results explain several recent experimental observations.