Experimental and theoretical investigations of femtosecond laser ablation of aluminum in vacuum

We used time-gated optical emission spectroscopy to investigate the characteristics of aluminum plumes and their vacuum expansion after femtosecond laser ablation at different fluences. The prominent feature is the presence of two main classes of species in the plume: very fast Al atoms and ions preceding the plume bulk essentially constituted of much slower Al nanoparticles expanding with a ten times smaller average velocity. Atomic force microscopy of deposited Al nanoparticles evidenced an average size of about 10 nm with a pretty narrow size distribution. These results and the peculiar feature of nanoparticle formation during femtosecond laser irradiation of matter were very satisfactorily interpreted and reproduced by molecular-dynamics simulation of the process. Finally, the analysis of the dependence on laser fluence of the ablation process showed an initial logarithmic increase of ablation yield, up to about 500 mJ/cm2, followed by a sudden and very steep increase at higher fluences. According to our numerical calculations, this latter feature can be ascribed to the increase of the overheated material volume due to electron heat diffusion.