The main objective of this study is to explain the experimental observations. To simulate material ablation, plume
formation and its evolution, we developed a combined molecular dynamics (MD) and direct simulation Monte Carlo
(DSMC) computational study of laser ablation plume evolution. The first process of the material ablation is described by
the MD method. The expansion of the ejected plume is modelled by the DSMC method. To better understand the
formation and the evolution of nanoparticles present in the plume, we first used separate MD simulations to analyse the
evolution of a cluster in the presence of background gas with different properties (density, temperature). In particular, we
examine evaporation and growth reactions of a cluster with different size and initial temperature. As a result of MD
calculations, we determinate the influence of the background gas parameters on the nanoparticles. The reactions rates
such as evaporation/condensation, which are obtained by MD simulations, are directly transferred to the DSMC part of
our combined model. Finally, several calculations performed by using MD-DSMC model demonstrate both plume
dynamics and longer-time cluster evolution. Calculations results are compared with experimental findings.
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.
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