基于自吸收量化的激光诱导等离子体表征方法

Laser-induced plasma characterization using self-absorption quantification method

为了表征激光诱导等离子体的定量特征参数,提出了一种谱线自吸收量化的方法,通过获得分析元素谱线的半高全宽来量化谱线自吸收程度,进而得到等离子体的特征参数,包括电子温度、元素含量比以及辐射物质的绝对数密度.与传统激光诱导击穿光谱定量分析方法相比,新方法由于计算过程与谱线强度弱相关,所以分析结果基本不受自吸收效应的影响,同时也无需额外的光谱效率校准.基于铝锂合金的实验结果表明,该方法能够实现精确的相对定量分析和等离子体的特性诊断.

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 e V and 0.97 e V, respectively. The weight ratio w Mg/w Al 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 × 10^17 cm^-3, which is comparable to the electron density calculated from the Hαline broadening（1.72 × 10^17 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.