Influence of the position relationship between gas–liquid interface and laser focus on plasma evolution characteristics in jet LIBS technology

In order to understand the characteristics of breakdown process, plasma evolution and spectral emission in liquid jets laser-induced breakdown spectroscopy methods under the influence of the position variation between laser focus and gas–liquid interface, this work takes the plasma generated by laser-induced liquid jets as the object of study and discusses the changes in the spatial and temporal evolution characteristics and spectral radiation of the plasma when the position parameters between the laser focal point and the gas–liquid interface are different. The initial breakdown position is always between the front interface and the focus when the laser focus moves along the vertical direction of the interface, forming a phenomenon like 'interface effect'. The relationship between laser pulse energy and breakdown probability exhibits a law similar to a 'hysteresis curve' in the study of 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 0.2 mm away from the front interface. Simultaneously, the spectral signal intensity and signal-to-back ratio of the characteristic peaks of target elements in water reach maximum values, and the spectral signal becomes more stable (relative standard deviation value reaches 2%). The Ca element's ion radiation at 393.366 nm and atomic radiation at 422.673 nm are studied using narrow-band filtering imaging and time-space resolution spectroscopy. The findings demonstrate that the laws of ion and atomic radiation are not perfectly consistent in space and time.