Dynamics of plume propagation, splitting, and nanoparticle formation during pulsed-laser ablation

A new approach has been developed for modelling the expansion of laser-generated plumes into low pressure gases, where, initially, the mean free path may be long enough for interpenetration of the plume and background. Although relatively simple in structure, it gives excellent fits to new experimental data for Si in He and Ar, and provides for the first time a detailed, coherent explanation of the observed splitting of the plume into a fast and a slow component. The model is based on a combination of multiple elastic scattering and hydrodynamic formulations. The plume is broken into orders corresponding to the number of collisions made with the background. The first order reaches the detector without any scattering, the second order undergoes one scattering event, and so forth. While particles can only be transferred between the various orders by collisions, the densities in the individual orders propagate to give the overall plume expansion. The propagation is determined by the usual equations for the conservation of mass and momentum. Representative results from the calculations are presented, and the probable role of the background gas in nanoparticle formation is discussed. Published by Elsevier Science B.V.