  Nanoparticle formation by femtosecond laser ablation is investigated for
different experimental conditions. Dynamics of the laser plume expansion is
visualized based on numerical simulations and the possibility of primary
nanoparticle formation is analyzed. Calculations are performed for metals under
different background conditions. The effects of the background environment are
considered, including initial expansion stage of the ejected atoms and
clusters, nucleation, collisions, and longer scale nanoparticle evolution.
Nanoparticle size distribution is shown to evolve from a decreasing function to
the commonly observed bell-shaped distribution as early as at 50 microseconds
after the beginning of the laser pulse. However, at the end of the initial
plume expansion stage, the supersaturation parameter can be sufficiently high
for a much longer diffusion-driven nanoparticle growth to enter into play. In
this case, the major nanoparticle formation process is based on the so-called
catastrophic nucleation accompanied by collisional growth, which determines the
final size distribution. The obtained calculation results explain numerous
experimental findings and help to predict both nanoparticle plume evolution and
changes in its size distribution.
