Short pulse UV laser ablation of solid and liquid gallium

UV laser ablation of gallium from the solid (300 K) and liquid (330K) phase is reported. The ablation is performed by using a 248 nm excimer laser with pulse durations of 15 ns and 0.5 ps. The ablated neutrals are ionised by resonant multiphoton ionization with a time-delayed laser (605 nm, 15 ns) and are detected by time-of-flight mass spectroscopy. The ablation threshold fluences, the ablation rates, and the velocity distributions of Ga atoms from the solid and liquid phase for irradiation with 0.5 ps and 15 ns pulses are determined. The threshold fluence for 0.5 ps laser ablation is found to be 7 times lower than that for 15 ns laser ablation and is identical for the solid and liquid samples of gallium. For ns pulses, the threshold fluence is 1.5 times higher for the solid compared to the liquid phase. This difference between the sub-picosecond and nanosecond laser ablation behaviour can be explained by the longer and deeper heat diffusion during the longer laser pulses. The time-of-flight measurements show the thermal behaviour of the removed Ga atoms in both 0.5 ps and 15 ns laser ablation and are fitted to Maxwell-Boltzmann distributions giving translational temperatures of several thousand Kelvin. An increase in the temperature as a function of the ablation laser fluence is observed.