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 bac kground. Although relatively simple in structure, it gives excellent f its 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 formulat ions. The plume is broken into orders corresponding to the number of c ollisions made with the background. The first order reaches the detect or without any scattering, the second order undergoes one scattering e vent, and so forth. While particles can only be transferred between th e various orders by collisions, the densities in the individual orders propagate to give the overall plume expansion. The propagation is det ermined by the usual equations for the conservation of mass and moment um. Representative results from the calculations are presented, and th e probable role of the background gas in nanoparticle formation is dis cussed. Published by Elsevier Science B.V.