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基于共振激发的激光探针成分分析技术基础研究
中文摘要

激光探针,又称激光诱导击穿光谱(Laser-induced breakdown spectroscopy,简称LIBS)是一种基于激光烧蚀的发射光谱分析技术,它利用激光在待测样品表面激发出等离子体,通过分析等离子体发射光谱,获得待测样品的元素信息。虽然激光探针拥有几乎无需样品制备、快速响应、原位和远程探测等优点,但谱线干扰、自吸收效应,以及检测灵敏度低等因素仍制约着激光探针技术进一步发展。基于此,本文研究了基于共振激发的激光探针技术(LIBS assisted with laser-induced fluoroescence,简称LIBSLIF),利用波长可调谐的激光对等离子体进行共振激发来改善激光探针分析检测性能,通过研究空间选择性激发、非聚焦激光受激吸收和分子自由基激发三种新型共振激发方法,进一步提高了激光探针技术的检测灵敏度和准确度。具体研究内容如下: 搭建了基于共振激发的激光探针实验平台。自主设计了一体化光路系统,该系统集成激光控制、激光监测、光谱采集、照明和成像五个功能,在对待测点进行光谱测量的同时,可以实时获取表面形貌信息,保证所见即所得;为方便光谱数据处理,自主开发了一体化光谱处理软件,集成了查看光谱、光谱预处理、背景去除、光谱归一化、谱线寻峰和定量分析等功能。 研究了目标原子能态选择的方法。通过讨论LIBS-LIF中荧光强度与共振激发目标原子能态的关系,发现以钢铁中痕量钴元素为例证明了选择数量占优的原子能态可以优化LIBS-LIF的荧光强度,提高共振激发效率,灵敏度提高了约l5倍,定标模型线性度R²从0.985优化至0.996,分析误差降低了约40%;并将该方法应用于钢铁中硅、铜、锰三种元素和石英玻璃中镱、铝、磷三种元素的检测,研究了不同元素目标激发原子能态选择的具体方法。 研究了空间选择性共振激发的方法。针对LIBS-LIF中基体元素激发线与待测元素激发线较近带来的激发干扰问题,研究了钢铁等离子体分布的非均性,发现了对钢铁等离子体中心进行激发时,基体铁原子荧光光谱强度较强,而微量铬、镍原子荧光光谱强度较弱;而对钢铁等离子体外层进行激发时,则实验结果相反。因此通过精确控制共振激光聚焦点位置可以减小基体铁元素的激发干扰,提高微量铬和镍原子的共振激发效率,定量分析的灵敏度分别提高了约10倍和20倍。 研究了激光受激吸收效应对等离子体自吸收效应抑制的方法。针对激光等离子体外层基态冷原子会导致自吸收效应的问题,利用波长可调谐的非聚焦激光辐照整个等离子体,大幅减小了等离子体外层冷原子的数量,抑制了等离子体中的自吸收效应。通过主量金属元素光谱自蚀现象的变化,验证了该方法的可行性;通过计算钢铁等离子体中微量铜和铬元素光谱的自吸收系数,定量评估自吸收效应的变化,证明了该方法可显著减少定量分析中的自吸收效应,将定标模型线性度R²从0.90以下提高至0.98以上,铜和铬的分析误差分别降低至1/3和1/8,大幅提高定量分析的线性度和准确度。 研究了等离子体中分子自由基共振激发的方法。针对部分难激发元素的激发波长位于真空紫外区域、无法实现空气中共振激发的难题,通过待分析元素与其他物质的化学反应来构造瞬态分子自由基,从而通过分析该分子自由基的光谱来标定待测元素,并对该分子自由基进行共振激发,将其光谱强度增强至少2个数量级。该方法将激发区和观察区转移至可见-近紫外区,实现了空气中难激发元素的灵敏检测。以钢铁中碳元素的检测为例,详细阐述了分子自由基共振激发的机理和实验过程,并将分析灵敏度提高约4.6倍,分析模型线性度从0.858上升至0.991,分析误差降低至1/5。以C-C、Al-O、Si-O和B-O自由基的共振激发为例,证明了该方法的普适性和广阔的应用前景。 综上所述,本文对基于共振激发的激光探针技术进行了系统的研究,利用共振激发目标原子能态选择、空间选择性激发、激光受激吸收和分子自由基共振激发的方法有效地减小了光谱干扰和自吸收效应,显著地提高了检测灵敏度和准确度,为推进激光诱导击穿光谱技术的发展和应用奠定了理论和实验基础。 关键词:激光探针 激光诱导击穿光谱 激光诱导荧光 等离子体 共振激发

英文摘要

Laser-induced breakdown spectroscopy (LIBS) is a laser-ablation-based spectroscopy method, in which elemental information is deduced by analyzing spectra emitted from laser-induced plasmas. Though LIBS has the advantages of minimal sample preparation, fast response, in-situ and remote detection, its drawbacks of spectral interference, self-absorption effect, and poor sensitivity significantly hinder its further development. Therefore, LIBS assisted with laser-induced fluorescence (LIBS-LIF) was investigated to improve analytical performance of LIBS in this thesis work, and three novel methods for resonant excitation in LIBS-LIF were proposed, which further improved analytical sensitivity and accuracy. The detailed contents are as follows: An experimental setup for LIBS-LIF analysis was built, and an optical system was designed, integrating laser control, laser monitoring, spectral collecting, sample surface lighting and imaging. For convenient data analyses, a software was developed for spectral processing, integrating exploring, pre-processing, background removal, normalizing, peak searching, and quantitative analysis, etc. A method for selecting resonance-excited target atoms was studied. The relation between fluroscent intensity and number of target atoms was discussed and demontrated by detecting trace cobalt element in steels with stimulating cobalt atoms in different states. The result showed that the atoms with a higher population offered higher fluroscent intensity and analytical sensitivity. Silicon, copper, and manganese in steels, as well as ytterbium, aluminum, and phosphorus in silica glass were used to validate this method of selecting target atoms in different element. Spatially selective excitation for reducing excitation interference was proposed. Inhomogeneity of the laser-induced plasmas was investigated with resonant excitation. Taking chromium and nickel elements in steels as examples, it was discovered that optimal excitation locations were the center of the plasmas for the matrix iron, but the periphery for Cr and Ni. By focusing an excitation laser at the optimal locations, both the excitation efficiency and the analytical sensitivity were improved. A novel method using laser-stimulated absorption was proposed to reduce the self-absorption effect. Namely, a non-focused wavelength-tunable laser was used to radiate the whole plasma, and the cold atoms in plasma periphery which cause the self-absorption were dramatically reduced by the excitation. The self-absorption effect was reduced. The feasibility of this method was proven by observing the extent of self-reversal disappearing in spectral lines of matrix elements. The improvement of analytical linearity and accuracy was demonstrated by analyzing low-concentration elements in steels using this method. For solving the problem that some hard-to-excite elements cannot be resonantly excited in open air, a method of forming and resonant exciting molecular radicals was proposed, by which, the transient radicals in plasmas could be obtained by chemical reactions in open air. The laser-induced fluorescences from these radicals can be observed and deduced to provide elemental information. The excitation and observation windows are moved to VIS-NUV (200-800 nm), which make it feasible to detect hard-to-excite elements in open air. The physical mechanism and experimental process were discussed by taking carbon-nitrogen radicals as an example. High enhancement factor, analytical sensitiviy and accuracy in quantitative analyses were demonstrated by detecting low-concentration carbon in steels. Universality of this method was proven by resonant exciting dicarbon, aluminum monoxide, silica monoxide, and boron monoxide radicals. In summary, the LIBS-LIF technique was systemically investigated in this thesis. Methods of selecting target atoms, spatially selective excitation, laser-stimulated absorption, and molecular radical excitation were proposed to reduce spectral interference and self-absorption effect, by which analytical sensitivity and accuracy were significantly improved. This thesis provides a guide of theory and experiment for further development of LIBS applications. Key words: Laser probe, Laser-induced breakdown spectroscopy, Laser-induced fluorescence, Plasma, Resonant excitation

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