Several distinct mechanisms of femtosecond laser ablation of thin Ag films from a silica substrate are established in large-scale atomistic simulations and are mapped to the space of film thickness and absorbed fluence. For a fixed film thickness, the increase in fluence results in sequential transitions from melting with no ejection of the film, to film splitting or spallation, to an explosive decomposition of the top part of the film and generation of a residual layer in the lower part of the ablation plume, and to a complete phase decomposition of the film into small droplets and vapor. To facilitate the experimental validation of the computational predictions, the variation of the scattering and reflectivity of the ablation plume is calculated from atomic configurations predicted in the simulations and related to the results of pump-probe optical imaging of the ablation plume.
The mechanisms of picosecond laser fragmentation of gold nanoparticles in water are investigated in a closely integrated atomistic simulations and time-resolved X-ray probing. The results of this joint computational and experimental study reveals a sequence of nonequilibrium processes triggered by the laser irradiation, from heating, melting, and resolidification of nanoparticles proceeding under conditions of strong superheating and undercooling, to evaporation of Au atoms followed by condensation into atomic clusters and small satellite nanoparticles, and to the regime of rapid (explosive) phase decomposition of superheated nanoparticles into small liquid droplets and vapor phase atoms. The transition to the phase explosion fragmentation regime is signified by prominent changes in the small-angle X-ray scattering profiles measured in experiments and calculated in simulations. A good match between the experimental and computational diffraction profiles gives credence to the physical picture of the cascade of thermal fragmentation regimes revealed in the simulations.
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