Ablation Process Of Collinear Double-pulse LIBS And Its Effect On Spectral Intensity

Laser-induced breakdown spectroscopy(LIBS)is an element quantitative detection technology based on atomic emission spectrometry,which has been widely studied and applied in industry,technology and environmental detection in recent years.However,the defects of high detection limit,low signal intensity and high signal-to-noise ratio limit the further development of LIBS.In order to improve the detection performance of LIBS,in recent years,many researchers have studied the mechanism of signal enhancement in ns Double-Pulse Laser Induced Breakdown Spectroscopy(DP-LIBS).But there are still many contradicting explanations can be found in the literature.Especially in the collinear dual-pulse LIBS technology,it is only mentioned that the dual-pulse laser can improve the emission signal by increasing removal mass and reheating plasma in general.The detailed mechanisms responsible of the ablation and plasma heating processes affect the spectral enhancement are not completely clear and an in-depth study is needed to conduct in this issue.In order to further explore the mechanisms of enhancing spectral signal in collinear dual-pulse LIBS technology,the ablation sampling method of focus-offset collinear dual-pulse is designed.Both in focus-offset collinear dual-pulse LIBS and traditional con-focal collinear dual-pulse LIBS,the spectral intensity and surface ablation morphology obtained by preheating and reheating mode are studied respectively.The different modes of excitation,pulse delay,laser energy and the influence of ablation amount on the enhancement of spectral signal are discussed,further clarifies the effects of sample ablation and plasma heating process in spectral signal enhancement in dual-pulse LIBS.The main research contents include:1.Firstly,the spectral intensity and surface ablation morphology of focus-offset collinear dual-pulse and traditional con-focal dual-pulse under preheating and reheating plasma excitation modes are studied.The results show that:(1)Focus-offset preheating collinear dual-pulse produced deep ablation in the target surface,and the increase of laser energy can significantly change the morphology and increase ablation mass only when the preheated laser energy is small.The maximum ablation depth occurs approximately at 1?s pulse delay.Focus-offset reheating dual-pulse only produced surface ablation and the ablation amount varies little with the heating laser energy.In traditional collinear dual-pulse,both excitation modes produced deep ablation.At the same delay time,the ablation morphology obtained by changing the energy of heating pulse is not much different,and the maximum depth appears near 500 ns pulse delay.(2)In the two modes of focus-offset collinear dual-pulse,the optimal pulse delay of spectral intensity increases with the heating pulse energy,while the optimal pulse delay of traditional collinear dual-pulse is independent of laser energy.In the focus-offset preheating mode with small preheating pulse energy,the maximum spectral intensity appears near 1?s pulse delay,which is consistent with the delay of maximum depth ablation.It shows that the enhancement of double-pulse LIBS spectral is mainly determined by the amount of ablation under the small preheating pulse energy,while the effect of secondary plasma heating on the spectral intensity is more significant under the other three modes.(3)The enhancement factor of ion and atomic spectra by focus-offset preheating double pulses are similar and correlated with pulse energy,it shows that the same enhancement mechanism of the two is the increase of ablation.In focus-offset reheating double pulses,the enhancement factor of ion spectra increase greatly when the energy is large,while the atomic spectra change little,it shows that the secondary heating of plasma is more conducive to the enhancement of ion spectra.In the conventional con-focal double-pulse modes,the enhancement of ion lines is higher than that of atomic lines,but the factor of ion lines is close to that of atomic lines in preheating mode,while the factor of ion lines in reheating mode is much larger than that of atomic lines.2.By using time-resolved spectroscopy,the ablation morphology of the sample and its variation with the pulse delay are obtained in the focus-offset dual-pulse LIBS with a time resolution of 1?s,and the ablation by heating laser and its influence on the spectral intensity are further studied.The results show that the optimal delay of ablation depth and spectral intensity occurs at 1.8?s in the preheating focus-offset collinear dual-pulse LIBS,and the change of depth and intensity has a high consistency.It shows that the enhancement of spectral signal is mainly due to the increase of ablated samples.With the same total energy,the focus-offset reheating double pulses only produced slight surface ablation.The spectral intensity of the focus-offset reheating double pulse increases within 100 ns pulse delay,and depends on the combined action of the increase of ablation and the secondary excitation of plasma.