Ni原子双飞秒脉冲激光诱导击穿光谱的信号增强研究

在大气环境下采用波长为800nm,脉宽为30fs的飞秒激光研究了Ni的双脉冲激光诱导击穿光谱,与单飞秒脉冲激光诱导击穿光谱相比,双飞秒脉冲在最优的双脉冲相对延时下,其信号强度增强接近10倍,实验研究了双脉冲相对延时在0-1300ps范围内不同延时对激光诱导击穿光谱信号强度增强因子的影响。整个相对延时区域可以分为三个阶段：在0-50ps区域内信号增强因子是一个持续增大的过程,在50ps左右,达到一个最大值;在50-300ps区域内,信号增强因子呈现出一个先下降后上升的过程;在300-1300ps,信号增强因子基本保持不变。     

Ni原子双飞秒脉冲激光诱导击穿光谱的信号增强研究

在大气环境下采用波长为800nm,脉宽为30fs的飞秒激光研究了Ni的双脉冲激光诱导击穿光谱,与单飞秒脉冲激光诱导击穿光谱相比,双飞秒脉冲在最优的双脉冲相对延时下,其信号强度增强接近10倍,实验研究了双脉冲相对延时在0-1300ps范围内不同延时对激光诱导击穿光谱信号强度增强因子的影响。整个相对延时区域可以分为三个阶段：在0-50ps区域内信号增强因子是一个持续增大的过程,在50ps左右,达到一个最大值;在50-300ps区域内,信号增强因子呈现出一个先下降后上升的过程;在300-1300ps,信号增强因子基本保持不变。     

不同实验条件对激光诱导等离子体的影响机理研究

常温常压下,采用波长532 nm的Nd:YAG纳秒激光器激发诱导空气中的铝合金,由高分辨率的光谱仪和ICCD对等离子体发射光谱采集和实现光电转换。研究激光能量、ICCD门延迟和聚焦透镜到样品表面的距离(lens-to-sample distance,LTSD)对谱线信号强度和等离子体电子温度的影响,并分析了产生影响的物理机制。结果表明,固定ICCD门延迟和LTSD,随着激光能量的增大,谱线强度和电子温度均增大;计算结果表明,当激光能量从20 mJ增加到160 mJ时,原子谱线Al I 396.15 nm,Mg I 518.36 nm,离子谱线Mg II 279.54 nm谱线强度相较于20 mJ分别提高了12.83,6.45,10.56倍。固定激光能量和LTSD,ICCD门延迟在100~4000 ns范围内变化时,随着延迟的增加,谱线强度和等离子体电子温度均呈指数形式衰减。固定ICCD门延迟和激光能量,采用焦距为75 mm的聚焦透镜,研究了LTSD对等离子体参数的影响机理。结果表明,聚焦透镜到样品的距离对等离子体的谱线强度和电子温度有较大的影响。等离子体的特征谱线强度和等离子体的电子温度的变化规律基本一致,分别在聚焦透镜到样品表面的距离为73 mm和79 mm处取得峰值,并在73 mm处对应最大值。     

单脉冲和再加热正交双脉冲激光诱导击穿光谱对比研究

为了提高激光诱导击穿光谱技术(LIBS)的检测灵敏度和辐射光谱特性,采用再加热正交双脉冲结构对样品中的4种元素Fe,Pb,Ca和Mg以及含有不同浓度重金属元素Cr的土壤样品进行分析。研究了4条特征谱线FeⅠ：404.581 nm,PbⅠ：405.78 nm,CaⅠ：422.67 nm和MgⅠ：518.361 nm的光谱强度和信背比随两激光脉冲之间时间间隔的变化关系,获得了两激光脉冲之间最佳的时间间隔为1.0 μs。在单脉冲和双脉冲条件下,得到了4条特征谱线FeⅠ：404.581 nm,PbⅠ：405.78 nm,CaⅠ：422.67 nm和MgⅠ：518.361 nm光谱强度的增强倍数分别为2.23,2.31,2.42和2.10;分析了特征谱线FeⅠ：404.581 nm和CaⅠ：422.67 nm谱线强度随时间的演化特性以及4条特征谱线信背比随光谱采集延时的变化关系,双脉冲能有效延长光谱强度的衰减时间以及提高特征谱线的信背比;比较分析了等离子体温度和电子密度随时间的演化特性,在双脉冲条件下,等离子体温度最大升高了730 K,电子密度最大增加了1.8×1016 cm-3。单脉冲和双脉冲条件下获得重金属元素Cr的检测限分别为38和20 μg·g-1,再加热正交双脉冲技术使元素检测限下降近2倍。以上结果表明：再加热正交双脉冲能有效地提升LIBS技术的检测灵敏度和光谱特性,为进一步降低元素的检测限提供了有效的方法。     

样品温度对纳秒激光诱导Cu等离子体特征参数的影响

为了研究样品温度对激光诱导击穿Cu等离子体特征参数的影响,以黄铜为研究对象,在优化的实验条件下采用波长为532 nm的Nd∶YAG纳秒脉冲激光诱导激发不同温度下的块状黄铜,测量了Cu等离子体的特征谱线强度和信噪比;同时在局部热平衡条件下利用Boltzmann斜线法和Stark展宽法分析计算了不同的样品温度条件下等离子体电子温度和电子密度。实验结果表明,在激光功率为60 mW时,随着样品温度的升高,Cu的特征谱线强度和信噪比逐渐增加,样品温度为130 ℃时达到最大值,然后趋于饱和。计算表明,黄铜样品中Cu元素Cu Ⅰ 329.05 nm,Cu Ⅰ 427.51 nm,Cu Ⅰ 458.71 nm,Cu Ⅰ 510.55 nm,Cu Ⅰ 515.32 nm,Cu Ⅰ 521.82 nm, Cu Ⅰ 529.25 nm,Cu Ⅰ 578.21 nm八条谱线在130℃的相对强度相较于室温（18 ℃）下分别提高了11.55倍、4.53倍、4.72倍,3.31倍、4.47倍、4.60倍、4.25倍、4.55倍,光谱信噪比分别增大了1.35倍,2.29倍、1.76倍、2.50倍、2.45倍、2.28倍、2.50倍,2.53倍。分析认为,升高样品温度会增大样品的烧蚀质量,相对于温度较低状态增加了等离子体中样品粒子浓度,进而提高等离子体发射光谱强度。所以,适当升高样品温度能够提高谱线强度和信噪比,从而增强LIBS技术检测分析光谱微弱信号的测量精度,改善痕量元素的检测灵敏度。同时研究了改变样品温度时等离子体电子温度和电子密度的变化趋势。计算表明,当样品温度从室温上升到130 ℃的过程中,等离子体的电子温度由4 723 K上升到7 121 K时基本不再变化。这种变化规律与发射谱线强度和信噪比变化趋势一致。分析认为,这主要是由于在升高样品温度的初始阶段,激光烧蚀量增大,等离子体内能增大,从而导致等离子体电子温度升高。当激光烧蚀样品的量达到一定值后不再变化,激光能量被激发溅射出来的样品蒸发物以及尘粒的吸收、散射和反射,导致激光能量密度降低,电子温度趋于饱和,达到某种动态平衡。选用一条Cu原子谱线（324.75 nm）的Stark展宽系数计算激光等离子体的电子密度,同时研究改变样品温度时等离子电子密度的变化趋势,计算表明在样品温度为130 ℃时,Cu Ⅰ 324.75 nm对应的等离子电子密度相较于室温（18 ℃）条件下增大了1.74×1017 cm-3。该变化趋势与电子温度的变化趋势一致。适当升高样品温度使得电子密度增大,从而提高电子和原子的碰撞几率,激发更多的原子,这是增强光谱谱线强度的原因之一。由此可见,升高样品温度是一种便捷的提高LIBS检测灵敏度的有效手段。     

金纳米与磁场作用下LIBS检测灵敏度改善研究

针对激光诱导击穿光谱技术（LIBS）中等离子体的发射光谱增强问题,提出一种磁场增强LIBS与纳米颗粒增强LIBS（NELIBS）相结合的方法。采用热蒸发法在样品表面沉积一层直径20 nm的金纳米颗粒。利用波长为1 064 nm,最大能量为200 mJ的Nd∶YAG脉冲激光器在室温,一个标准大气压下对纯铜和黄铜进行诱导击穿。调整激光能量为30~110 mJ,分别使用传统LIBS、磁场增强LIBS、NELIBS以及两种方法结合对纯铜进行激光诱导击穿,得到特征谱线(Cu Ⅰ 521.8 nm)的强度增强因子和信噪比,并对其增强机理进行分析。在相同环境下使用四种方式对黄铜和纯铜进行诱导击穿以探测样品中的微量元素。当在样品表面沉淀金纳米颗粒或者将沉淀有金纳米颗粒的样品放在磁场中进行诱导击穿时,发现纯铜样品的光谱中存在Mg元素的特征谱线Mg Ⅱ 279.569 nm,黄铜样品的光谱中存在Si元素的特征谱线 Si Ⅰ 251.611 nm。实验结果表明：单独施加磁场约束或增加纳米金颗粒均可以有效增强等离子体光谱强度,但增强效果弱于两种方法结合,磁场约束对光谱的增强效果弱于NELIBS的增强效果。当结合NELIBS与磁场约束LIBS时,谱线增强因子最高可达14.3(Cu Ⅰ 521.8 nm),相比于磁场增强LIBS和NELIBS,最大增强因子分别提高了28%和59%。四种情况中当激光脉冲能量逐渐增大时,等离子体向外膨胀的强度增大,磁场产生的洛伦兹力束缚等离子的能力相对减弱,同时纳米金颗粒对等离子体发射光谱的增强作用被削弱,谱线强度降低,等离子体的增强因子逐渐减小后趋于稳定。通过NELIBS与磁场约束LIBS结合方式,不仅可以有效提高等离子体的发射谱线强度,改善光谱信号信噪比,而且传统LIBS方法中由于谱线强度低、背景噪声大而无法探测的微量元素可以被探测到,LIBS技术对微量元素的探测能力得到显著提高,微量元素的探测下限变得更低。NELIBS与磁场约束LIBS结合的方法具有更高的灵敏度和准确度,为激光诱导击穿光谱技术的谱线增强方法提供了新的思路,在该领域具有广阔的应用前景。     

Spectral evolution of nano-second laser interaction with Ti target in nitrogen ambient gas

The present work aimed to study the variation in the plasma parameters (temperature and density) of the Ti plasma generated by 1,064 and 532 nm lasers at different ambient N₂ pressures for different delay times. The characterization of the plasma-assisted pulsed laser ablation of the titanium target is discussed. The emission spectra of the titanium plasma produced in the present study have been carefully investigated over the whole UV–NIR (200–1,000 nm) spectral range. Boltzmann plots of suitable spectral lines have been employed to derive the excitation temperature, and the electron density is derived from the Stark widths of the Ti II spectral line at 350.49 nm.     

The role of microwaves in the enhancement of laser-induced plasma emission

We studied experimentally the effect of microwaves (MWs) on the enhancement of plasma emission achieved by laser-induced breakdown spectroscopy (LIBS). A laser plasma was generated on a calcium oxide pellet by a Nd:YAG laser (5 mJ, 532 nm, 8 ns) in reduced-pressure argon surrounding gas. A MW radiation (400 W) was injected into the laser plasma via a loop antenna placed immediately above the laser plasma to enhance the plasma emission. The results confirmed that when the electromagnetic field was introduced into the laser plasma region by the MWs, the lifetime of the plasma was extended from 50 to 500 µs, similar to the MW duration. Furthermore, the plasma temperature and electron density increased to approximately 10900 K and 1.5×10¹⁸ cm^-3, respectively and the size of the plasma emission was extended to 15 mm in diameter. As a result, the emission intensity of Ca lines obtained using LIBS with MWs was enhanced by approximately 200 times compared to the case of LIBS without MWs.     

再加热双脉冲激光诱导等离子体的时空演变特性研究

为了研究再加热双脉冲激光诱导击穿光谱(LIBS)对信号的增强机制,分别采用单脉冲LIBS和再加热双脉冲LIBS两种方式烧蚀合金钢样品产生等离子体,利用高分辨率的中阶梯光栅光谱仪采集等离子体发射光谱信号,同时用快速成像ICCD相机观测等离子体形态的变化,研究了两种烧蚀方式下等离子体的时空演变特性。通过比较两种烧蚀方式下等离子体产生初期光谱信号和图像的时间演变规律,发现再加热双脉冲LIBS提高了等离子体温度,且当信号采集延时等于再加热双脉冲的脉冲间隔时,等离子体温度的衰减速率发生变化;再加热双脉冲LIBS使等离子体图像强度增加,等离子体的中心区域高度和宽度分别增大了23.5%和15.1%。空间分布的研究结果表明,与单脉冲LIBS相比,当到样品表面的距离大于0.6 mm时,等离子体中的Fe Ⅱ和N Ⅰ谱线强度有较明显的增强,而Fe Ⅰ谱线在空间不同位置处的增强程度都较小,局部区域有减小的现象;再加热双脉冲LIBS使等离子体温度增加了约2 000 K,等离子体中产生了一个较大的高温区域。综合时空演变的实验结果说明再加热双脉冲对光谱信号增强的机制主要是由于第二束激光对第一束激光烧蚀样品产生的等离子体再次激发,使等离子体温度增加,进而引起等离子体辐射强度增加。     

Evaluation of plasma produced by first and second harmonic nano-second laser for enhancing the capability of laser induced breakdown spectroscopy technique

Evaluation of plasmas produced and optimized for improving the capability of convenential laser induced breakdown spectroscopy (LIBS) for analytical purposes of solid samples is the main goal of the present work. The plasma produced in the present study was generated by focusing a single nano-second Nd:YAG laser at the fundamental wavelength of 1064 nm and at the second harmonic wavelength of 532 nm on an Al target in air at atmospheric pressure. The emission spectrum was recorded time resolved over the whole UV-NIR (200–1000 nm) spectral range. This work describes an extension of previously reported studies and focuses now on the determination of the plasma parameters at the optimum condition – highest signal-to-noise ratio (SNR) and minimum limit of detection (LOD) — of the LIBS technique, which is now widely applied to the elemental analysis of materials in atmospheric air. Parameters of the produced plasma in the time interval from 0 to 10 μs are determined for to further understanding the LIBS plasma dynamics. O I and Mn I spectral lines are used in the present work as thermometric lines for the determination of the plasma temperature based on Boltzmann plots. Stark broadening of lines yields the electron density. The widths of the H _α -line at 656.27 nm, of the O I line at 844.65 nm, of Al II lines at 281.65 nm and 466.30 nm and of the Si I line at 288.15 nm has been utilized for that. The plasma temperature ranged from 0.73 eV to around 1 eV for the different laser energies with both laser wavelengths for the optimized plasma used for LIBS analysis. This temperature is very close to that well known for the other spectrochemical analytical techniques or in excitation sources such as inductively coupled plasma-optical emission spectrometry (ICP-OES).     

Comparison between single- and double-pulse LIBS at different air pressures on silicon target

A comparative study between single- and double-pulse laser-induced breakdown spectroscopy (LIBS) was performed on an n-type silicon(111) target. A new mobile double-pulse instrument for LIBS analysis was used for the measurements. The experiment was carried out at different air pressures of 0.7, 470 and 1000 hPa. It has been found that, in the case of double-pulse LIBS, the emission intensities of atomic and ionic lines are strongly enhanced at higher pressures. Using Stark broadening of the atomic lines of silicon, it was found that the electron number densities for single and double pulses are approximately the same (N_e∼10¹⁷ cm^-3). Plasma excitation and ionization temperatures were determined from a Boltzmann plot. The double-pulse laser-induced plasma was studied at different interpulse delay times of 1, 2, 5, 10, 15, 25 and 50 μs. The results indicated that the interaction between the laser, plasma and target gives higher atomic and ionic intensities at shorter interpulse delay times. PACS 52.38.Mf; 79.20.Ds; 52.50.Jm     

激光诱导Ni等离子体特性的研究

以Nd·YAG激光器的二倍频输出作为激发源,获得了激光诱导Ni等离子体的发射光谱,基于发射光谱,对等离子体电子激发温度和电子密度进行了测量,其典型值分别为3 714 K,4.67×1016 cm-3。测量了等离子体电子激发温度和电子密度的空间分布,发现沿垂直于激光传播方向的径向,随到中心点距离的增加,等离子体辐射的强度减小,但线型和线宽不变,表明等离子体电子激发温度和电子密度沿径向均匀分布。沿激光传播方向,随到样品表面距离的增加,等离子体辐射强度、电子激发温度和电子密度先增加后降低,在距样品表面1.5 mm处,达到最大值。采用激光诱导击穿光谱技术进行相关探测时,收集距离样品表面1.5 mm处的发射谱,有利于提高探测灵敏度。     

Spectral evolution of nano-second laser interaction with Ti target in Air

Time-resolved optical emission spectroscopy has been successfully employed to investigate the evolution of plasma produced by the interaction of IR- and visible-pulsed laser beams with a titanium target in ambient air at atmospheric pressure. The characterization of the plasma-assisted pulsed laser ablation of the titanium target is discussed in this study. The emission spectrum produced by the titanium plasma in the wavelength range 200–1,000 nm has been carefully investigated for different experimental conditions. Boltzmann plots have been used in the calculation of the excitation temperature employing Ti II spectral lines at 286.23, 321.71, 325.29, 348.36, and 351.08 nm; this set of lines was tested and proved to be suitable for the measurement of the plasma temperature. The obtained temperature is in good agreement with the one obtained from Ti II spectral lines previously suggested by Hermann et al. [J. Appl. Phys. 77, 2928–2936, 1995, 22]. Moreover, the Stark broadening method has been employed for electron density measurements. In this study, the Stark width of the Ti II spectral line at 350.49 nm was used.     

Evolution Characteristics of the Laser-Plasma Electron Temperature of Heavy Metal Cr in Liquid Matrix

Using the nanosecond single-pulse laser-induced breakdown-spectroscopy (LIBS) technique, we determine the LIBS spectrum of a mixed solution of soluble compounds as a sample. Based on the LIBS measurement system established and the best experimental parameters, we measure the plasma emission spectrum of some lines of the Cr element. When the ICCD gate delay is measured from 500 to 2500 ns every 500 ns, and the liquid sample flow rate is set in the range of 35 ml · min^?1?55 ml · min^?1 with an interval of 5 ml · min^?1, the partial emission line of the Cr element in the laser plasma is measured, and the Boltzmann oblique line is drawn after the integrated intensity obtained. We calculate the evolution characteristics of the laser-plasma electron temperature with experimental parameters. The results show that the electron temperature values obtained from the intensity of the spectral lines of different elements are consistent with each other, indicating the reliability of the experimental measurement data. The range of electron temperature is obtained from the intensity of the Cr element line in the plasma range from 4071.0 K to 5628.4 K. As the flow rate continues to increase, the plasma electron temperature also increases but gradually becomes gentle. We select Cr 357.87 nm as the analytical line in the experiment and measure the particle density ratios of the Cr element in the ground state at different ICCD gate delays and sample flow rates. It provides experimental parameter support for further LIBS quantitative analysis of trace heavy metal elements in a liquid matrix.

     

Double-pulse neodymium YAG/Carbon dioxide laser-induced breakdown spectroscopy for excitation of bulk and trace analytes

The mechanisms involved in signal enhancement and persistence of the plasma in double-pulse laser-induced breakdown spectroscopy are investigated, and their implications to improving figures of merit for bulk and trace analytes in sample are discussed. For double-pulse laser-induced breakdown spectroscopy, 1064 nm neodymium YAG laser is used for ablation and 10.6 µm transversely excited atmospheric carbon dioxide laser in near-collinear geometry is used for reheating. Significant improvement in signal detection and sensitivity of both bulk and trace analytes using double-pulse laser-induced breakdown spectroscopy as compared to conventional single-pulse laser-induced breakdown spectroscopy are observed. Using double-pulse laser-induced breakdown spectroscopy in near-collinear geometry, Cu and Fe as bulk and trace analytes, respectively, in brass sample, showed 5 and 6 times improvement in persistence of the spectral emission. Temporal and time-integrated studies show that ionic lines are significantly enhanced compared to neutral lines. Plasma characterization employing spectroscopic methods showed significant enhancement in plasma temperature resulting in higher signal as well as increased plasma persistence of the species studied.     

激光诱导击穿光谱增强机制及技术研究进展

激光诱导击穿光谱技术是一种新的材料识别及定量分析技术。但是光谱的重复性低限制其由定性分析向定量分析的发展。因此提高激光诱导等离子光谱信号信噪比及等离子体的空间稳定性对于提高光谱信号的可重复性、降低基体效应等不利因素影响有着积极的作用。同时光谱信号信噪比的增强可降低对激光器输出能量的要求,有效降低了激光诱导击穿光谱集成系统的成本,有利于此技术向更多领域拓展。本文对实验中采用的双脉冲或多脉冲增强,放电脉冲再激发,空间限域,磁场束缚和微波辅助增强四大类方法加以总结及概括。在此基础上深入探讨光谱增强的物理机制,从而为进一步提高光谱信号稳定性及定量化分析的精确度提供有力的理论支持。     

Double pulse UV laser induced breakdown spectroscopy of stainless steel

Results of experimental investigations of 304 austenitic stainless steel (ASS) ultraviolet spectral range by single and double pulse laser induced breakdown spectroscopy (LIBS) at atmospheric pressure are reported. Various parameters, such as laser energy, placement of the laser beam focus with respect to the surface of illumi-nation, and collinear double laser pulse delay were used as variables. This study contributes to a better under-standing of the LIBS plasma dynamics by observing the temporal evolution of various emission lines. Temperature measurements were made by the Boltzmann diagram method using singly ionized Fe lines, and electron densities were found from Stark broadening. The temporal behaviors of these parameters were also estimated. It was found that the electron temperature for double pulses is higher than that for single pulse of the same total energy. For double pulse LIBS, the iron line emission intensities are enhanced and the analytical performance is improved. For instance, the intensity of iron line Fe I 275.01 nm was a factor of about 300 times higher if a double pulse of 2 × 20 mJ was used instead of a single pulse of 40 mJ when focusing the beam 4.7 mm behind the target surface. Published in Zhurnal Prikladnoi Spektroskopii, Vol. 79, No. 5, pp. 654–660, 2006.     

飞秒激光成丝诱导Cu等离子体的温度和电子密度

研究了飞秒激光成丝诱导铜击穿光谱,利用光发射光谱对产生的铜等离子体光谱强度沿着丝长度进行了测量,获得了在不同样品与聚焦透镜间距离的Cu(I)的强度分布.结果显示,由于强度钳箍效应成丝诱导的光谱在较大的透镜样品间距离范围内有较强的辐射强度.另外,利用玻尔兹曼图和斯塔克展宽计算了整个成丝繁衍距离中Cu等离子体温度和电子密度.     

基于自吸收量化的激光诱导等离子体表征方法

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.     

Femtosecond laser-induced Cu plasma spectra at different laser polarizations and sample temperatures

Laser-induced breakdown spectroscopy (LIBS) is a good technique for detecting and analyzing material elements due to the plasma emission produced by the high-power laser pulse. Currently, a significant topic of LIBS research is improving the emission intensity of LIBS. This study investigated the effect of laser-polarization on femtosecond laser-ablated Cu plasma spectra at different sample temperatures. The measured lines under circularly polarized lasers were higher than those under linearly and elliptically polarized lasers. The enhancement effect was evident at higher Cu temperatures when comparing the plasma spectra that have circular and linear polarizations for different target temperatures. To understand the influence of laser-polarization and sample temperature on signal intensity, we calculated the plasma temperature (PT) and electron density (ED) . The change in PT and ED was consistent with the change in the atomic lines as the laser polarization was being adjusted. When raising the Cu temperature, the PT increased while the ED decreased. Raising the Cu temperature whilst adjusting the laser-polarization is effective for improving the signal of femtosecond LIBS compared to raising the initial sample temperature alone or only changing the laser polarization.