Features of plasma plume evolution and material removal efficiency during femtosecond laser ablation of nickel in high vacuum

We present an experimental characterization describing the characteristics features of the plasma plume dynamics and material removal efficiency during ultrashort, visible (527 nm, ≈ 300 fs) laser ablation of nickel in high vacuum. The spatio-temporal structure and expansion dynamics of the laser ablation plasma plume are investigated by using both time-gated fast imaging and optical emission spectroscopy. The spatio-temporal evolution of the ablation plume exhibits a layered structure which changes with the laser pulse fluence F. At low laser fluences (F< 0.5 J/cm2) the plume consists of two main populations: fast Ni atoms and slower Ni nanoparticles, with average velocities of ≈ 104 m/s for the atomic state and ≈ 102 m/s for the condensed state. At larger fluences (F> 0.5 J/cm2), a third component of much faster atoms is observed to precede the main atomic plume component. These atoms can be ascribed to the recombination of faster ions with electrons in the early stages of the plume evolution. A particularly interesting feature of our analysis is that the study of the ablation efficiency as a function of the laser fluence indicates the existence of an optimal fluence range (a maximum) for nanoparticles generation, and an increase of atomization at larger fluences.