Characteristics of laser induced plasma near a flat gas-liquid interface and its effect on the performance of Laser induced breakdown spectroscopy (LIBS) detection.

Moving the laser focus to the vicinity of the gas-liquid interface is the key point for many new enhanced and new methods to improve the quality of spectral signals in water LIBS detection. Understanding the generation and evolution characteristics of the plasma induced by pulsed laser near the gas-liquid interface is of great significance for the establishment of evolution models and improvement of these new LIBS methods. In this paper, a set of slow horizontal flow auxiliary system is established to provide an ideal flat gas-liquid two-phase interface experimental condition. Experimental research on vertical incidence plane system was conducted using techniques such as time-resolved imaging, plasma characterization diagnosis, and spectral analysis. And the detection capabilities of the system were also tested. The characteristics and mechanisms of LIBS near the gas-liquid two-phase interface were investigated with the laser incident on the sample along the vertical direction. Simulation of the laser beam focusing process and observation of laser beam spot images show that the shift of plasma generation position relative to the focal point results from the refraction of the laser beam entering the solution from the air and the ‘interface effect’ of propagation on the vertical direction. Moreover, the plasma forms only the optical power density surpasses the breakdown threshold. In this work, plasma with smaller size, rounder shape, stronger radiation, higher temperature, and higher density can be produced when the focus position is in the liquid column 0.3 mm away from the upper interface. Simultaneously, for example, the Mg ion line at 285.213 nm, the obtained spectral intensity to signal-to-background ratio reaches the maximum value, and a better spectral signal can be obtained, which is 2-4 times of other positions, and the detection limits of the elements Na, Mg, and Ca also reach the lowest level, with 1.6-2.4 times of the detection limit of other focusing positions for Mg and 1.4-1.7 times for Ca, respectively.