Study of the plasma plume generated during near IR femtosecond laser irradiation of silicon targets

Ti:sapphire femtosecond laser ablation of silicon has been investigated by Langmuir probe and time-gated optical emission spectroscopy. The measured spectra show the presence of a fast ion population preceding the main plume core of slow ions and neutrals produced by a thermal ablation mechanism. By analyzing the fluence thresholds for the emission of the two ion populations, we provide clear experimental evidence that fast ions are ejected non-thermally from the sample surface as a result of the Si surface supercritical state induced by the intense ultrashort laser pulse irradiation.