Langmuir probe investigation of plasma expansion in femto- and picosecond laser ablation of selected metals

A time resolving Langmuir probe was used to study the plasma plumes produced by the ablation of Ag, Ni and Al targets with laser pulses of different pulse durations (0.2 - 10 ps). These metals were chosen because their electron-phonon relaxation times, tau(e-ph), are of the order of the pulse durations used. The time of flight (TOF) signals have been used to establish the threshold fluences and plume expansion dynamics of the laser produced plasmas for the different pulse durations. The angular dependence of the magnitude of the ion flux was analysed on the basis of Anisimov's self-similar model of the plasma expansion. The amount of charge in the ablation plume is compared for the different pulse durations.