Temperature determination of laser-induced plasma in water

Abstract Temperature determination of laser-induced plasma generated in water is of great importance for a better understanding of underwater laser-induced breakdown spectroscopy as well as underwater plasma-based techniques. In this work, we measured the temperature of underwater plasma by using optical emission spectroscopy. The temperature was calculated respectively from the Saha–Boltzmann plot of Ca I 422.67 nm, Ca II 393.37 nm, and Ca II 396.85 nm lines, and from the OH molecular band A 2 Σ + −X 2 Π around 310 nm. It shows that in the time window from 75 ns to 700 ns, the Saha–Boltzmann temperature decreases from 12700 K to 7700 K, which is significantly higher than the molecular temperature that decreases from 11000 K to 4100 K. To verify these temperature values, shadowgraph images of the early-stage cavitation bubble were taken and the bubble temperatures were calculated based on the bubble adiabatic expansion model. We find that the OH molecular apparent temperature is compatible with the cavitation bubble temperature, whereas the Saha–Boltzmann apparent temperature turns out to be higher. The present results in this work provide suggestions in measuring the underwater plasma temperature, and also give a link between the plasma properties and the cavitation bubble dynamics for the laser-induced breakdown in liquids.