光谱学与光谱分析, 2019, 39 (1): 250, 网络出版: 2019-03-17  

基于Abel逆变换的激光诱导等离子体辐射特性研究

Investigation on the Characteristic of Laser Induced Plasma by Abel Inversion
作者单位
河南科技大学物理工程学院, 河南 洛阳 471023
摘要
激光诱导击穿光谱(LIBS)作为一种新型的物质成分测量方法已经在越来越多的领域得到广泛应用, 但是与传统的分析方法相比, LIBS技术的分析性能还需进一步提高。 LIBS技术的理论基础是激光诱导等离子体, 从物理机理上研究等离子体特性, 对LIBS系统实验参数的优化具有指导作用, 也为提高LIBS技术的检测能力奠定理论基础。 激光诱导等离子体是一个与空间相关的非稳态辐射源, 空间分辨光谱测量是探究等离子体物理特性的重要手段之一。 为研究激光诱导等离子体的辐射特性, 采用1 064 nm的Nd∶YAG调Q固体激光器烧蚀合金钢样品产生等离子体, 利用空间分辨装置测量二维空间的等离子体辐射光谱信号, 通过分析可知实验采集的光谱信号是信号探测器测量路径上的积分光谱强度, 由此计算得到的等离子体参数也是观测路径上的平均值。 为了深入研究等离子体由内层到外层的辐射规律, 首先测量得到等离子体路径积分光谱强度的横向空间分布, 然后以等离子体为光学薄和圆柱对称的前提条件, 采用三次样条函数算法对路径积分光谱强度进行Abel逆变换, 反演得到等离子体由内层到外层谱线辐射率的径向空间分布。 选取等离子体辐射光谱中的原子谱线Fe Ⅰ: 374.55 nm和Mn Ⅰ: 403.08 nm为研究对象, 分析等离子体辐射光谱的空间分布特征, 研究结果表明, 等离子体辐射路径积分光谱强度的横向分布呈现出中心位置强度大边缘位置强度小的特征, 这是由于等离子体膨胀扩张的结果引起的; 通过Abel逆变换得到等离子体光谱辐射率的径向分布, 结果表明等离子体从内层到外层谱线的辐射率经过了先增加后减小的变化规律, 等离子体中心处出现辐射率的极小值, 造成这种现象的主要原因是由于等离子体辐射源中心区域具有较低的电子密度; 选取等离子体辐射光谱中Fe元素的11条原子谱线, 采用Boltzman法分别由谱线相应的积分光谱强度和辐射率计算等离子体温度, 得到等离子体温度的横向空间和径向空间的二维分布, 两者具有类似的变化规律; 由等离子体温度的横向空间分布可以看出, 随着离样品表面距离的增加, 等离子体温度呈现单调减小的趋势, 等离子体中心到边缘区域等离子体温度逐渐降低, 这是由等离子体膨胀扩张以及与环境气体相互作用共同的结果; 由等离子体温度的径向空间分布可以看出等离子体由内层到外层等离子体温度逐渐降低, 这是由于等离子体膨胀扩张冷却引起的。 由此可见, 采用Abel逆变换能够实现等离子体由内层到外层的辐射特性分析, 为深入理解等离子体产生和演变的物理机理提供实验依据, 从而为提高激光诱导击穿光谱技术的分析性能奠定理论基础。
Abstract
Laser-Induced Breakdown Spectroscopy (LIBS) has been widely used in more and more fields as a new measurement method of material composition. However, compared with the traditional analysis methods, the analytical performance of LIBS needs to be further improved. The basis of LIBS is the laser-induced plasma. It is helpful to optimize the experimental parameters of LIBS system and lays the theoretical foundation for improving the detection capability of LIBS. The laser induced plasma is a non-steady radiation source associated with the space. Spatial-resolved spectroscopy is one of the most important ways to explore the physical properties of plasma. In order to study the characteristics of laser induced plasma, a Q-switched Nd∶YAG laser operating at the wavelength of 1 064 nm was used to ablate the alloy steel and the plasma was generated. The two-dimensional distribution of plasma emission was measured by the spatial resolution device. It is analyzed that the spectral signal collected in the experiment is the integrated intensity of the spectrum along the line of sight of the signal detector. So the plasma parameter calculated by the integrated intensity is the average of the observed path. In order to investigate the emission characteristics from the inner layer to the outer layer of the plasma, we measured the transverse spatial distribution of integrated intensity firstly. Then, assuming that the plasma is optically thin and cylindrically symmetrical, a method of Abel inversion based on cubic spline functions was performed on the integrated intensity. And the radial spatial distribution of the emissivity of the plasma from the inner layer to the outer layer was obtained. The atomic emission lines of Fe Ⅰ: 374.55 nm and Mn Ⅰ: 403.08 nm were selected to analyze the spatial distribution characteristics of the plasma emission. It has shown that the distribution of the integrated intensity presents a greater intensity value in the central location and smaller intensity at the edge of the plasma. This is due to the expansion of the plasma. The radial distribution of the spectral emissivity of the plasma was obtained by Abel inverse transformation. It has shown that the emissivity increased and then decreased from the inner to the outer of the plasma. A minimum value of emissivity appears at the center of the plasma as a result of the lower electron density in the central region of the plasma source. Eleven atomic lines of Fe in the plasma emission spectra were selected to calculate the plasma temperature by Boltzman method. The corresponding integral spectral intensities and emissivity were used respectively. The two-dimensional distributions of the transverse and radial spatial distributions of the plasma temperature were obtained. They have the similar variation rule. It can be seen that transverse spatial distributions of the plasma temperature decreases monotonously with the increase of the distance from the sample surface. And the plasma temperature gradually decreases from the center of the plasma to the edge which is the result of the expansion of the plasma and the interaction with the ambient gas. From the radial spatial distribution of the plasma temperature, it can be seen that the temperature of the plasma gradually decreases from the inner layer to the outer layer due to the expansion and cooling of plasma. Therefore, the radiation characteristics of the plasma can be obtained by using the Abel inverse. It provides an experimental basis for further understanding of the physical mechanism of the laser induced plasma. It also lays a theoretical foundation for improving the analytical performance of laser-induced breakdown spectroscopy.

王静鸽, 李贺贺, 李新忠, 张利平, 李小龙. 基于Abel逆变换的激光诱导等离子体辐射特性研究[J]. 光谱学与光谱分析, 2019, 39(1): 250. WANG Jing-ge, LI He-he, LI Xin-zhong, ZHANG Li-ping, LI Xiao-long. Investigation on the Characteristic of Laser Induced Plasma by Abel Inversion[J]. Spectroscopy and Spectral Analysis, 2019, 39(1): 250.

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