Signal enhancement in underwater long-pulse laser-induced breakdown spectroscopy for the analysis of bulk water

Abstract

Long ns pulses have been proven to be effective in enhancing the LIBS signals of submerged solids. However, the enhancement effect on the liquid detection has yet to be further revealed, which is also of great significance for the application of long-pulse LIBS in underwater chemical analysis. In this work, we investigated the temporal emission characteristics of underwater laser-induced plasma under different pulse duration conditions by using time-resolved spectroscopy and imaging techniques. It is shown that short pulses lead to the formation of an elongated and short-lived plasma with weak emission due to multi-site underwater breakdown, and a rapid decrease of line intensity with delay time. Long pulses with a pulse duration of 180 ns, on the other hand, produce a compact plasma with strong emission and longer lifetime. Such an enhancement is more pronounced for the spectral line with a high-energy transition level, whose line intensity can be increased by more than 7 times. For this reason, the clear atomic lines of O in water are also detected, which are difficult to be observed with short pulses. This is also related to the changes in the physical properties of the plasma, i.e., the long pulses enable the plasma to maintain higher electron density and plasma temperature for a longer period of time. The present results suggest that long ns pulses can be of great advantage in underwater LIBS detection as well as capable of performing important roles in in-situ chemical applications.