Determination of plasma ignition threshold fluence during femtosecond single-shot laser ablation on metallic samples detected by optical emission spectroscopy

The minimum laser fluence needed for plasma ignition on a sample during single-shot femtosecond laser ablation has been determined for a set of metals and alloys. Threshold fluence values ranged between 4 and 9 J cm(-2) for ablation craters with a diameter of 40 mm. The craters exhibited similar size and shape regardless of the sample type, energy range (for energies per pulse <400 mu J) and accumulated energy dosage (<500 laser shots on the same sample position). The results obtained for the metals (Pb, Ag, Zn, Sn, Cr, Cu, W, Ni and Fe) show a clear trend of the fluence threshold with the work function, evidencing the importance of the surface electron excitation in femtosecond laser-matter interactions. Experiments with the alloyed samples reveal that fluence thresholds for femtosecond plasma ignition are matrix independent.