Short-pulse UV laser ablation of solid and liquid metals: Indium

Laser-ablation experiments on solid (T = 300 K) and liquid (T = 600 K) indium are reported. The ablation was performed under high vacuum conditions with UV laser pulses at 248 nm and pulse durations of 15 ns and 0.5 ps. The ablated neutral indium atoms were resonantly ionized with a second laser pulse 28 mm above the sample surface and detected in a time-of-flight mass spectrometer. The ablation threshold fluence for solid indium decreases by a factor of 40 from 100 mJ/cm(2) to 2.5 mJ/cm(2) when a 0.5 ps pulse is used instead of a 15 ns laser pulse. Measurements on liquid indium show a different behavior. With 15 ns laser pulses the threshold fluence is lowered by a factor of similar to 3 from 100 mJ/cm(2) for solid indium to 30 mJ/cm(2) for liquid indium. In contrast, measurements with 0.5 ps laser pulses do not show any change in the ablation threshold and are independent of the phase of the metal at 2.5 mJ/cm(2). This behavior could be explained by thermal diffusion and heat conduction during the laser pulse and demonstrates in an independent way the energy lost into the material when long laser pulses are applied. Time-of-flight measurements to investigate the underlying ablation mechanism show thermal behavior of the ablated indium atoms for both ps and ns ablation and can be fitted to Maxwell-Boltzmann distributions.