Laser-induced plasma characterization using self-absorption quantification method

Abstract

Laser-induced breakdown spectroscopy (LIBS) is a well-known analytical technique based on the atomic emission spectroscopy.The elemental composition and relative abundance information can be obtained by analyzing the plasma radiation generated by focusing high-energy pulsed laser on the sample.It has a wide range of applications due to its many advantages,such as minimal-to-no sample preparation,broad applicability,and in-situ capability.But in LIBS,the self-absorption effect of the emitted line can reduce the spectral line intensity,and then affect the precision and accuracy of LIBS quantitative analysis.So there are many methods and researches to reduce or eliminate the adverse effects of selfabsorption on spectral lines.In this paper,a self-absorption quantification analysis method is proposed to characterize the laser-induced plasma quantitative parameters.This self-absorption quantification analysis method,which utilizes the intensity independent information in the self-absorbed spectral lines,is proposed to characterize the induced plasma and perform quantitative measurements.The plasma characteristics including electron temperature,elemental concentration ratio,and absolute species number density can be derived directly through quantifying the self-absorption degree of the analytical spectral lines.Compared with the traditional laser-induced breakdown spectroscopy,the new method is weakly related to the spectral intensity:neither the analysis results are affected by the self-absorption effects,nor the additional spectral efficiency calibration is required.The LIBS spectrum of an aluminum-lithium alloy (nominal weight compositions are Al 94.6%,Mg 1.8%,Li 0.8%,Cu 2.59%,and Mn 0.21%) is used to calculate the spatiallyaveraged electron temperature and the concentration ratio between Mg and Al,and the species number densities is obtained by using the proposed self-absorption quantification method.The results of experiment on aluminum-lithium alloy show that the mean electron temperatures obtained by the modified Saha-Boltzmann plots determined by Mg and Al are 0.96 eV and 0.97 eV,respectively.The weight ratio wMg/wAl in the plasma is calculated to be 0.0171,which is approximately coincident with the nominal value of 0.0169.The absolute singly ionized number density of matrix element Al is 1:65×1017 cm-3,which is comparable to the electron density calculated from the Hα line broadening (1:72×1017 cm-3).Evidently,the free electrons present in the plasma are mainly contributed by the singly ionized matrix element Al.These experimental results of aluminum-lithium alloy validate that the proposed method is qualified to realize accurate absolute quantitative measurements and fast diagnose the plasma characteristics,which verifies the practicability,advantages,and precision of this method.This self-absorption quantification method is of great significance for quantitative LIBS analysis,especially the CF-LIBS analysis.