A spectroscopic analysis study of graphite using laser technique

Results of single and double pulses (SP- and DP) Q-switch Nd:YAG laser induced breakdown spectroscopy (LIBS) at atmospheric pressure on the structure of graphite target are reported. Various parameters, such as laser energy, placement of the focus with respect to the surface of illumination, and time delay between laser pulse and ICCD camera pulse were used as variables. This study contributes to a better understanding of the LIBS plasma dynamics of the SP- and DP effect by observing the temporal evolution of emission line of graphite. The gate width and the delay time were adjusted to achieve the best possible signal to noise ratio. Electron densities were found from Stark broadening. The temporal behavior of the last one was also estimated. It was found that the electron density for DP of 2 × 20 mJ is higher than that for SP of 40 mJ of the same total energy. The results show a faster decay of continuum, spectral lines, and a shorter plasma life.     

Titanium plasma spectroscopy studies under double pulse laser excitation

Laser-induced breakdown spectroscopy (LIBS) was applied for parametric studies of titanium (Ti) plasma using single and double pulsed laser excitation scheme. Here a pulsed Nd:YAG laser was employed for generation of laser produced plasma from solid Ti target at ambient pressure. Several ionized titanium lines were recorded in the 312–334 nm UV region. The temporal evolution of plasma parameters such as excitation temperature and electron number density was evaluated. The effect of incident laser irradiance, position of the laser beam focal point with respect to the surface of illumination, single and double laser pulse effect on plasma parameters were also investigated. This study contributes to a better understanding of the LIBS plasma dynamics of the double laser pulse effect on the temporal evolution of various Ti emission lines, the detection sensitivity and the optimal dynamics of plasma for ionized states of Ti. The results demonstrate a faster decay of the continuum and spectral lines and a shorter plasma life time for the double pulse excitation scheme as compared with single laser pulse excitation. For double pulse excitation technique, the emissions of Ti lines intensities are enhanced by a factor of five which could help in the improvement of analytical performance of LIBS technique. In addition, this study proved that to avoid inhomogeneous effects in the laser produced plasma under high laser intensities, short delay times between the incident laser pulse and ICCD gate are required.     

非平坦样品激光诱导等离子体光谱特性研究

激光诱导击穿光谱（LIBS）作为一种快速、实时的元素分析技术,由于其在痕量元素探测、地质环境监测等领域有着广阔的应用前景,而受到人们极大的关注。在实际应用中,样品表面是影响等离子体产生及其特性的关键环境因素之一。在大气环境下,利用脉宽为8 ns、波长为1 064 nm的纳秒脉冲激光产生等离子体,对比研究了天然岩石样品在非平坦和平坦表面条件下等离子体的发射光谱。基于激光辅助辐射波模型,阐释了非平坦样品表面对其光谱特性的影响。通过对比等离子体时间积分光谱,发现非平坦样品的谱线强度相比于平坦样品的谱线强度减弱了近70%,该结果说明非平坦样品表面对LIBS真实测量数据的负面影响不可忽视。针对褐铁矿样品中的谱线Fe Ⅰ 404.58 nm和Fe Ⅰ 438.35 nm,研究了在平坦和非平坦样品表面下的峰值强度以及其衰减因子随激光能量的变化规律,结果表明非平坦样品表面条件下采集的光谱强度始终低于平坦样品表面的光谱强度。光谱强度的衰减因子先随激光能量增大而逐渐降低,并在激光能量33 mJ达到最小值,后随激光能量的进一步增大而增大。实验结果进一步表明在非平坦样品表面条件下产生了密度较低的等离子体,并且非平坦与平坦样品的电子密度的比值在激光能量33 mJ时达到最小,此结果与光谱强度的衰减因子随激光能量的变化趋势一致,这是源于非平坦样品表面会形成较大激光入射角度,使得激光等离子体能量吸收区厚度变薄,产生等离子体屏蔽效应所对应的激光能量阈值升高。此外,样品表面状态和激光能量对等离子体温度的影响甚微。阐述了非正入射时等离子体特征参数与正入射时等离子体特征参数的联系和差异,揭示了非平坦样品激光等离子体特征参量变化的内在物理机制,为室外LIBS探测技术在元素定性和定量分析中光谱强度的校正提供参考。     

铅的单脉冲LIBS定量光谱检测与比较

单脉冲激光诱导击穿光谱技术(single-shot,LIBS)是一种在国际上被广泛使用的物质及元素检测技术,具有快速、准确、无需样品制备等诸多优点.为了满足单脉冲LIBS的实验要求,实验用激光器的选取是非常重要的,它直接关系到诱导激光的强度以及脉冲宽度的大小,而这些又对实验结果产生深刻的影响.因此,选取正确的激光器,是单脉冲LIBS成功的关键.正是基于这个目的,比较了不同波长、不同激光能量的激光脉冲作用下,纯铅发射光谱的定量变化,对于单脉冲LIBS中激光器的选取有着很好的借鉴意义.     

激光脉冲能量对激光诱导Al等离子体辐射特征的影响

使用Nd：YAG激光器烧蚀金属Al靶获得等离子体,利用光谱时-空分辨技 术,在52 mJ～145 mL/pulse激光脉冲能量范围内,关于激光脉冲能量对激光诱导等离子体 辐射特征的影响进行了研究。使用的气体是Ar气,压强为10 kPa。结果发现,激光脉冲能量 升高,引起特征谱线强度增加,连续谱强度也增加;但能量过高,会击穿周围气体,产生气 体微等离子体。此时,特征谱和连续谱几乎不再增强;最大特征辐射强度在145 mJ、10 kPa 、靶前0.1 mm处、延时180 ns获得;同一条件下获得最强背景连续谱,而信号-背景差是在 145 mJ、10 kPa、靶前1.0 mm处、延时450 ns达到最大值。基于Al等离子体不同激光脉冲能 量下的时间-空间分辨谱,对结果进行了简单的讨论。并分别确定了获得最大特征辐射和信 号-背景差的条件。     

铀元素的激光诱导击穿光谱测量分析

为促进LIBS技术在微量重金属元素检测以及核污染检测领域的应用,提高检测灵敏度和准确性,采用了激光双脉冲LIBS技术和光电双脉冲LIBS技术,分别对土壤和二氧化硅中的铀元素进行分析。首先,对激光脉冲能量、电压和采集延时等参数进行优化,提高铀元素特征谱线的强度和信噪比;然后在优化实验参数条件下,对含不同浓度铀元素的土壤样品和二氧化硅样品进行激发;选取UII 367.01 nm、 UII 454.36 nm两条铀元素的特征谱线作为分析线,通过铀元素浓度与特征谱线强度的线性关系,建立定标曲线。双脉冲激光激发条件为：激光脉冲1作为预脉冲,主要参数为1 064 nm, 90 mJ, 9.2 ns,激光脉冲2作为再加热脉冲,主要参数为355 nm, 50 mJ, 8 ns,两个脉冲的时间间隔800 ns,光谱采集相对第二个脉冲延时1μs,得到铀元素在土壤和二氧化硅两种样品中的浓度检测下限分别为572和110 mg·kg^-1,拟合优度值R²分别为0.958和0.999。在光电双脉冲激发条件下,激光脉冲作为预脉冲,主要参数为355 nm, 50 mJ, 8 ...     

Comparative Study of Two Methods of Orthogonal Double-Pulse Laser-Induced Breakdown Spectroscopy of Aluminum

Double-pulse laser induced breakdown spectroscopy (DP-LIBS) of aluminum sample is studied experimentally in orthogonal configuration in air. In this configuration, two schemes of reheating and pre-ablation are examined and the results are compared with single pulse one. The effect of delay time between two laser pulses on emission line intensities of plasma is investigated. Some of the parameters that have been involved in different mechanism of signal enhancement such as plasma temperature, sample heating effects, atmospheric effects, and modification of the ablation dynamics are more discussed. Investigation of the effect of laser pulse energy on emission line intensities in single pulse LIBS experiment demonstrate that because of saturation effects the intensities will not increase necessarily by increasing the laser pulse energy. Moreover, the results show that the electron temperature and rate of mass removal in orthogonal configuration of DP-LIBS is higher than that of single pulse with the same total energy. It is suggested that for correct comparison between single and double pulse results, the optimum pulse energy in single pulse should be considered. Overall, our results demonstrate that under optimized conditions the signal enhancement is much more in pre-ablation configuration than re-heating configuration.     

钾元素单脉冲和双脉冲激光诱导击穿光谱研究

基于自行研制的新型液体射流的激光诱导击穿光谱(LIBS)实验装置,研究了实验条件(如积分延时、脉冲间隔、激光能量等)对K元素单脉冲LIBS和双脉冲LIBS等离子发射的影响.实验得知相对单脉冲激光激发,双脉冲激光激发可以显著提高等离子体发射谱线强度,增加谱线强度的衰减时间,提高LIBS数据的稳定性.通过最佳实验条件下K766.49nm谱线强度随溶液浓度的分析,得到该实验系统中,双脉冲激光激发时K元素的检测灵敏度和检测限约是单脉冲激光激发时的37倍.实验结果为双脉冲LIBS技术应用于水体金属的检测提供了一定依据.     

钾元素单脉冲和双脉冲激光诱导击穿光谱研究

基于自行研制的新型液体射流的激光诱导击穿光谱(LIBS)实验装置,研究了实验条件(如积分延时、脉冲间隔、激光能量等)对K元素单脉冲LIBS和双脉冲LIBS等离子发射的影响.实验得知相对单脉冲激光激发,双脉冲激光激发可以显著提高等离子体发射谱线强度,增加谱线强度的衰减时间,提高LIBS数据的稳定性.通过最佳实验条件下K766.49nm谱线强度随溶液浓度的分析,得到该实验系统中,双脉冲激光激发时K元素的检测灵敏度和检测限约是单脉冲激光激发时的37倍.实验结果为双脉冲LIBS技术应用于水体金属的检测提供了一定依据.     

Double-pulse laser-induced breakdown spectroscopy for trace element analysis in sintered iron oxide ceramics

Double-pulse laser-induced breakdown spectroscopy (LIBS) is an emerging technique for accurate compositional analysis of many different materials. We present a systematic study of collinear double-pulse LIBS for analysis of the trace and side elements boron, manganese, copper, aluminum, titanium, silicon, chromium, nickel, potassium, and calcium in sintered iron oxide targets. The samples were ablated in air by single-pulse and double-pulse Nd:YAG laser radiation (6 ns pulse duration, laser wavelength of 532 nm) and spectra were recorded with an Echelle spectrometer equipped with an ICCD camera. We investigated the evolution of atomic and ionic line emission intensities for different interpulse delay times between the laser pulses (from 100 ns to 50 μs) and gate delays after the second laser pulse. We also varied the energy partition between the first and second laser pulse and the size of the irradiated spot at the sample surface. For the trace and side elements, we observed double-pulse LIBS signals that were enhanced as compared to single-pulse measurements depending on the interpulse delay time, the energy partition between the pulses, and the spot size. For the elements boron, copper, aluminum, titanium, chromium, potassium, and calcium limits of detection below 10 ppm were achieved.     

Diagnostics of Tin Plasma Produced by Visible and IR Nanosecond Laser Ablation

We present the optical emission spectroscopic studies of the Tin (Sn) plasma, produced by the fundamental (1064 nm) and second (532 nm) harmonics of a Q switched Nd: YAG pulsed laser having pulse duration of 5 ns and 10 Hz repetition rate which is capable of delivering 400 mJ at 1064 nm, and 200 mJ at 532 nm using Laser Induced Breakdown Spectroscopy (LIBS). The laser beam was focused on target material by placing it in air at atmospheric pressure. The experimentally observed line profiles of four neutral tin (Sn I) lines at 231.72, 248.34, 257.15 and 266.12 nm were used to extract the electron temperature (T_e) using the Boltzmann plot method and determined its value 6360 and 5970 K respectively for fundamental and second harmonics of the laser. Whereas, the electron number density (N_e) has been determined from the Stark broadening profile of neutral tin (Sn I) line at 286.33 nm and determined its value 5.85 x 10¹⁶ and 6.80 x 10¹⁶cm^–3 for fundamental and second harmonics of the laser respectively. Both plasma parameters (T_e and N_e) have also been calculated by varying distance from the target surface along the line of propagation of plasma plume and also by varying the laser irradiance. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

激光感生击穿光谱技术（LIBS）的原理及影响因素

激光感生击穿光谱技术（LIBS）具有不需制备样品,快速分析,可进行实时控制的特点显示了巨大实际应用价值,文中介绍了激光感生击穿光谱技术（LIBS）的发展历史、研究现状及其原理。同时通过重复频率为20 Hz,最高单脉冲能量为50　mJ的纳秒激光（10 ns,1　064 nm）研究了在不同激发条件下Cr和Co薄膜产生的光谱信号。得出对于Cr膜当激发能量小于10 mJ时,信号差异不明显。但是当激发能量超过10　mJ时将产生明显的变化。分析了激光特性、延迟时间、实验装置、环境气体的种类及压力、单/双脉冲、以及采用的理论分析方法对LIBS探测结果的影响。     

利用Nd: YAG激光诱导击穿光谱法检测橡胶共混物中锌的均匀性分布

激光诱导击穿光谱是一种基于激光的定性、定量分析化学元素的方法,可用于分析混合室中的橡胶共混物。本文采用波长为1 064 nm、脉冲能量为100 mJ的Nd: YAG激光诱导击穿光谱法检测橡胶共混物中锌的分散情况。实验表明,正确混料决定了橡胶产品的质量,较短的混合时间将会导致混料的不均匀分散;焦距等参数设置对光谱线强度影响较大。激发标定样品可产生标定曲线。通过行扫描可分析锌在橡胶共混物中的分散情况。通过估算沿着行扫描方向的强度分布,可检测由于混合时间较短和旋转速度不同而引起的锌均匀性变化。     

液相基质中重金属元素激光诱导击穿光谱实验参数的优化

针对混合溶液中重金属元素的激光诱导击穿光谱(LIBS)测量系统,为提高测量系统的检测灵敏度,以提高混合溶液中Ca和Cr金属元素LIBS光谱线强度的信噪比为目标,对LIBS测量系统中的激光脉冲能量、液相样品流速、ICCD门宽、延时等实验参数进行了优化,得到最优化参数激光脉冲能量、样品流速、ICCD门宽、延时分别为35 m J、30 ml/min、1400 ns和2400 ns,为降低LIBS技术应用于混合溶液中痕量重金属元素的检出限提供了实验参数支撑.     

不同能量水下激光诱导等离子体的轴向辐射分布特性研究

激光诱导击穿光谱(LIBS)技术作为一项新兴的水下原位探测技术,备受海洋探测技术领域的关注。将这项技术推向实用化的关键之一是改善LIBS的远程探测能力,因此需要采用超击穿阈值的高能量探测激光。为观察超击穿阈值情况下的等离子体辐射和动态击穿特性,采用图像与光谱相结合的方法,以KCl溶液为样品进行了系列实验研究。通过对1～20 mJ不同能量激发下的等离子体图像分析,获得了不同激发条件下总辐射的轴向跨度和最亮点位置信息。随激光脉冲能量增大,等离子体长度增加,从1 mJ时的0.49 mm增加到20 mJ时的1.83 mm,同时辐射最亮点位置向激光入射方向移动了0.79 mm。结合光谱探测分析,得出等离子体特征辐射的轴向空间分布也对激光能量有明显的依赖性。虽然不同能量下谱线强度呈相似的轴向空间分布,但钾原子辐射最强处的位置和相应强度均随能量变化,在5 mJ激发下获得最佳辐射强度。实验结果表明,为满足远程LIBS应用需求,提高激光能量时应考虑其对原子辐射的影响。还对不同能量下的谱线的半高宽和信背比进行了观测分析。     

激光诱导击穿光谱技术对土壤中重金属元素Cr的定量分析

为促进LIBS技术在土壤微量重金属元素检测中的应用,提高特征谱线的光谱强度和信背比,对实验参数进行优化,并对Cr元素进行分析。首先对激光器激发能量、样品距透镜距离和光谱仪采集延时等实验参数进行优化。对比激光器能量从60 mJ到110 mJ的谱线强度和信背比,当选用90 mJ的激发能量时可以得到最佳实验结果。其次,选择不同样品到透镜的距离,对比从焦前5 mm到焦后5 mm得到的实验结果,得出样品与透镜距离为焦后1 mm（即聚焦位置121 mm）时,Cr元素的特征谱线和信背比达到最佳。最后,分析对比光谱仪采集延时对谱线强度和信背比的影响,结果显示,与能量对等离子辐射强度的影响趋势大致相同,当采集延时为1 000 ns时,实验结果最佳。在最佳实验条件下（即激光器能量90 mJ、聚焦位置121 mm、采集延时1 000 ns）,对12种含有重金属Cr元素的土壤样品进行了光谱检测,为减弱外界环境的干扰,对同一样品的10个激光烧蚀位置得到的光谱做平均值预处理,选择Cr(Ⅰ)357.86 nm,Cr(Ⅰ)425.44 nm,Cr(Ⅰ)427.49 nm为特征谱线,通过建立样品掺杂浓度和光谱强度的定标曲线,得到了三条谱线的检测限LOD分别为74.62,64.07和67.49 mg·kg-1,拟合优度值R2分别为0.98,0.97和0.99,均方根误差值RMSE分别为0.41,0.33和0.35。同时,引入偏最小二乘法及支持向量机算法进一步提高了定标模型精度。研究表明,通过对实验参数进行优化及改善LIBS技术对微量元素的定量探测参数,得到了最优的光谱强度和信背比,并通过对Cr元素进行定量分析,计算定标曲线的Lorenz拟合得到检测限、拟合优度和均方根误差等实验参数,提高了LIBS对土壤中重金属元素的检测精度,这对于利用LIBS技术检测微量重金属元素具有重要的参考意义。     

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。     

激光诱导击穿光谱诊断合金铸件表面/界面质量

熔模精密铸造技术广泛应用于高温合金叶片生产。高温合金单晶叶片铸件尺寸精度高,内部和表面不得有缺陷,但在浇铸时易受浇铸温度场、熔体流场及与模壳发生界面反应的影响而导致表面结瘤、凹陷、缩孔等缺陷。激光诱导击穿光谱（LIBS）是一种非常有效的表面及界面分析技术,对于诊断分析铸件表面/界面质量具有明显的优势,对样品损伤小,可对不平整表面进行分析。研究优化了LIBS深度分析的条件,采用2 mm孔径光阑分光,500 mJ,1 064 nm脉冲高斯激光在焦外激发单晶高温合金获得了良好的空间分辨力,降低了能量密度较低的二级以上衍射环能量辐射造成表层烧蚀引起的深度分析信号失真。体积烧蚀速率与激光输出能量线性正相关,与光阑直径正相关,与激发频率无关。采用LIBS技术对DD407高温合金单晶叶片表面和近表面进行了深度分析。发现SiO₂-Al₂O₃莫来石结构的型壳在铸造过程中会引起DD407单晶叶片表面和近表面的Al,Ti,Ni,Cr,Co贫化,其中Al和Ti贫化显著且深度达50 μm。铸件尖锐部位较厚大部位的合金元素贫化明显。叶片经过脱芯脱壳后表面沉积钙镁盐类、钠盐物质。通过沸水清洗和超声清洗后,钠盐完全去除,钙镁物质均大幅减少,仍有3~5 μm厚。LIBS可方便的进行合金铸件表面和界面的成分分布分析,给出表面/界面质量的清晰判据,展现了较好的应用前景。     

用激光诱导击穿光谱技术比较土豆和百合中的微量元素

激光诱导击穿光谱(LIBS)已成功地运用于固体、液体、气体样品中微量元素的测量以及微生物、细菌等的鉴别分析上,而在植物样品上面的应用则是一个较新的课题。该实验用纳秒Nd∶YAG激光器击穿真空冻干的土豆、百合样品,用光纤光谱仪测量了其LIBS光谱。通过鉴别、分析LIBS光谱,得到了这两种样品中微量元素的成分。并且对土豆干和百合干中Ca,Na,K,Fe,Al,Mg六种金属元素典型谱线的强度进行了统计分析,并由此得到了样品中这六种微量元素含量的对比情况。实验结果表明土豆干中的Ca和Na等含量都明显高于百合干,而百合干中Mg的含量较高。实验结果还表明用LIBS技术检测、对比冻干植物样品中微量元素含量是一种快速、有效的分析方法。     

铁合金中微量钛元素的激光诱导击穿光谱研究

采用激光诱导击穿光谱对铁(Fe)合金中的钛元素(Ti)的含量进行测量。实验中激光器在最大能量输出(50 mJ),延时为1 μs时光谱信号的强度值最大。在此条件下,分别使用传统定标法和Fe Ⅰ 438.35 nm及Fe Ⅰ 427.12 nm两条谱线的内标法对铁合金中的Ti进行定量分析。内标法得到的拟合相关系数(r)分别为0.997 8和0.993 9,优于传统法得到的r(0.956 3)。提出了一种双谱线平均内标法,拟合得出r为0.998 4。同时,在浓度为0.063%～1.9%的范围内传统定标法测量的相对误差为23.7%,内标法的相对误差为6.0%,采用平均内标法后相对误差降为3.9%。最后,通过测量的Ti光谱计算了激光能量为50 mJ时所产生的等离子体温度为6 654.3 K,电子密度为1.072×1022 cm-3,并讨论了激光能量与烧蚀产生等离子体温度之间的关系。