Characteristic Of Laser Induced Plasma Of Nickel Target In Air And Water

In science, the key to understanding any situation is careful observations and measurements. The key to observing and measuring, however, is being there—in the moment—and that has always proved challenging for oceanographers. Analysis techniques based on spectra have the advantage of in-situ, real time, non-contact, multi-element detection for solid, gas, liquid samples, and take more activities in oceanic application. Laser induced breakdown spectroscopy (LIBS) is a powerful analytical technique that can be used for the detection and characterization of materials and holds the distinct advantage over many other techniques since little or no sample preparation is needed before analysis, and it can be suit for long distance, rapid real-time, multi-elements analysis in field operations, particularly in extreme environments. However, plasma induced in bulk water holds the character of very weak emission, and also short life time duration, leading to the emission spectrum is hard to detection. There is much work to do when applied LIBS to under water analysis. As laboratory investigation of LIBS in-situ under water application, the author's work is focus on the core problem facing, self-absorption and effects of laser parameters on LIBS, when applying LIBS in bulk water and expect to provide useful experiment data for LIBS oceanic application.As the beginning of the thesis, a brief background introduction of the author'work is presented, followed by the description of fundamental mechanical of LIBS, the current situation and develop tendency, character of LIBS spectra. The principle, parameters and performance of instruments used in this work are described in detail. The author'work is mainly focus on the followings: (1) the temporal evaluation of continuum spectrum and discrete nickel lines were analyzed, the self-absorption phenomenon appeared was explained; (2) the characters of different emission spectrum under different ablation methods such as single pulse, dual pulse, long duration single pulse; (3) the influence of Na+, K+ which are rich in sea water on the emitted spectra was also investigated.Laser induced breakdown spectroscopy has been shown to be a promising technique for element analysis. However, self-absorption effect deeply influences the LIBS measurements. Four atomic lines Ni I 341.476/351.034/351.505/352.454 nm which belong to the same electronic configuration (3d9(2D)4p - 3d9(2D)4s) of Ni have been chosen for self-absorption investigation. Self-absorption of Ni I 351.034 nm corresponding to the highest energy level 3D1 of 3d9(2D)4s was not observed in the plasma emission investigated. While for the other three lines, a strong self-absorption appeared at the prophase of the plasma and tended to weaker as the temporal evaluation of Ni plasma with different durations. The self-absorption at Ni I 352.454 nm was the most serious and still visible at the delay of 1,100 ns, compared with the lines of Ni I 341.476/351.505 nm whose self-absorption duration is 900 ns and 500 ns respectively. It was also found that the self-absorption effect had power dependence and decreased with the increasing of laser pulse energy. The obtained results suggest that the self-absorption effect could be alleviated by suitable atomic line selection, operating at higher pulse energy and detecting with a longer delay.To evaluate the potential using of LIBS for in-situ under water application, extensive experimental investigations of LIBS have been carried out with a nickel sample submerged in water under both single pulse and dual-pulse laser ablation. It was found that, for short duration laser pulse ablation, the continuum emission lifetime was nearly equal to the duration of laser pulse, with nickel atomic emission hardly appeared. As the laser pulse duration going up to 50 ns, several nickel atomic lines were turned up on the continuum background. With dual-pulse laser ablation, the intensity of theses nickel atomic lines were enhanced significantly, which demonstrated that dual-pulse had better LIBS performance. The LIBS signals of nickel target within the wavelength range of 330 nm-390 nm were obtained under dual-pulse laser ablation. And severe self-absorption phenomenon was observed among those nickel atomic emission lines. The possible reason for self-absorption has been discussed and was attributed to the relevant transmission energy levels. To verify the effect of cations in water, LIBS investigation was carried out with the nickel target submerged in Na+, K+ solution or deep-sea water. It was found that both intensity and signal to background ratio had been improved with the cations in water. All those obtained results suggested that LIBS had great potential to be used in in-situ under water application.As the last part of this thesis, a general discussion of the work was presented. Meanwhile the feasibility of applying LIBS in metal in-situ analysis was discussed. The thesis ended with the tentative idea for the work in the future.