Laser-induced plasma generated by a 532 nm pulsed laser in bulk water: unexpected line-intensity variation with water temperature and the possible underlying physics

摘要

The effect of the matrix temperature on laser-induced plasma generated in bulk water by using a 532 nm pulsed laser beam has been studied. Ca I and II emission line intensities were recorded for an aqueous solution of CaCl2 in the temperature range of 7 degrees C-70 degrees C. The emission line intensities did not follow the matrix temperature in our experiments. Maximum intensities were observed at similar to 18 degrees C for both lines. Herein, a possible mechanism responsible for the observed variation in intensity is suggested, in which laser-produced bubbles play important roles. Bubble formation is essential to ignite plasma in the liquid and more feasible at the higher liquid temperature. However, the abundant bubbles at the higher temperature can scatter the incident laser beam more effectively to decrease the energy delivered for the laser-induced plasma. Thus, these two roles have effects on the optical emission intensities in opposite ways. The validity of the suggested mechanism is discussed based on the plasma temperature, temperature dependence of the refractive index of water, plasma electron density, scattered light intensity, and plasma ignition threshold energy. Our result indicates that the temperature of the liquid is also an important parameter to be considered in the laser-induced breakdown spectroscopy analysis of bulk liquid samples and its application in deep-sea exploration.