Characteristics of laser-induced plasma generated in water and in air with different nanosecond laser pulse durations

The formation of laser-induced plasma on a solid target depends strongly on the laser pulse parameters and the ambient conditions. In this work, the characteristics of laser-induced plasma generated in water and in air were investigated with two laser pulse durations of 6 ns and 17 ns. It was shown that the responses of laser-induced breakdown spectroscopy (LIBS) signals on the laser pulse duration in water and in air are different. In water, the 17 ns laser induces a brighter plasma and a stronger LIBS signal, whereas in air, the 6 ns laser is better. Such differences were attributed to the different plasma growth mechanisms in water and in air. The plasma growth in air is driven by strong laser-supported absorption waves, where the 6 ns laser with a higher peak irradiance corresponds to a stronger shock wave and more efficient heating on the plasma. Whereas for plasma growth in water, a great amount of laser energy is consumed in the material phase transitions, and in the mechanical effects of bubble expansion and shock wave propagation. The underwater plasma is far less heated by the laser pulse, and more easily suffers from the multiple breakdowns of water producing multiple shock waves and multi-plasmas. By using a longer laser pulse, which has a lower peak irradiance, the underwater plasma can absorb more laser energy over a longer laser pulse duration, and the multiple breakdown phenomenon can be suppressed. This results in a higher quality of underwater LIBS signals. The present work gives insights into the mechanism of plasma growth in water and in air, and provides suggestions for the choice of laser pulses in the relevant LIBS analyses. The characteristics of laser-induced plasma generated in water and in air were compared with two laser pulse durations of 6 ns and 17 ns.