激光诱导击穿空气等离子体时间分辨特性的光谱研究

空气等离子体的时间行为对空气环境下激光诱导等离子体形成过程的研究有重要意义.本文将纳秒Nd:YAG脉冲激光(1064 nm)聚焦于一个大气压的空气中,诱导其产生等离子体.利用具有纳秒时间分辨功能的PI-MAX-II型ICCD,采用时间分辨光谱方法,研究了大气环境下激光诱导等离子体的时间行为.大气环境下的激光诱导等离子体光谱广泛分布于300—900 nm范围内,并且是由带状光谱和线状光谱叠加而成的.根据美国国家标准与技术研究院原子发射谱线数据库,对等离子体光谱中的氧、氮、氢等元素的特征谱线进行了识别和归属.给出了激光诱导击穿大气等离子体光谱随时间演化的直观图像,根据空气等离子体发射谱线计算了等离子体电子温度和等离子体电子密度.这些结果对于提高在大气环境下进行的在线测量结果的准确性和精确性具有重要的科学意义.     

气体压强对纳秒激光诱导空气等离子体特性的影响

环境气体的压强对激光诱导等离子体特性有重要影响.基于发射光谱法开展了气体压强对纳秒激光诱导空气等离子体特性影响的研究,探讨了气体压强对空气等离子体发射光谱强度、电子温度和电子密度的影响.实验结果表明,在10-100 kPa空气压强条件下,空气等离子体发射光谱中的线状光谱和连续光谱依赖于气体压强变化,且原子谱线和离子谱线强度随气体压强的变化有明显差别.随着空气压强增大,激光击穿作用区域的空气密度增加,造成激光诱导击穿空气几率升高,从而等离子体辐射光谱强度增大.空气等离子体膨胀区域空气的约束作用,增加了等离子体内粒子间的碰撞几率以及能量交换几率,并且使离子-电子-原子的三体复合几率增加,因此造成原子谱线OⅠ777.2 nm与NⅠ821.6 nm谱线强度随着气体压强增大而增大,在80 kPa时谱线强度最高,随后谱线强度缓慢降低.而离子谱线N Ⅱ 500.5 nm谱线强度在40 kPa时达到最大值,气体压强大于40 kPa后,谱线强度随压强增加而逐渐降低.空气等离子体电子密度均随压强升高而增大,在80 kPa后增长速度变缓.等离子体电子温度在30 kPa时达到最大值,气体压强大于30 kPa后,等离子体电子温度逐渐降低.研究结果可为不同海拔高度的激光诱导空气等离子体特性的研究提供重要实验基础,为今后激光大气传输、大气组成分析提供重要的技术支持.     

激光能量对粉煤灰未燃碳测量的影响

搭建了基于激光诱导击穿光谱技术的成分分析应用研究台架,应用于粉煤灰未燃碳的检测,考察不同能量的脉冲激光烧蚀粉煤灰样品时的等离子体特性。使用多通道光纤光谱仪和CCD探测器对激光烧蚀形成的等离子体发射信号进行分光和探测。分析碳谱线强度、等离子体温度和电子密度随激光能量变化的趋势,掌握激光能量对粉煤灰未燃碳测量的影响规律。研究结果显示,随着激光能量的增大,碳谱线强度、等离子体温度和电子密度均先增大后减小,空气击穿明显增强。随后碳谱线强度的变化趋于平缓并开始下降。合适的激光能量可以增强等离子体发射信号,并避免强烈空气击穿的不利影响,有助于提高测量精度。     

二维偏振光谱技术对燃烧场的诊断

利用单脉冲激光诱导偏振光谱技术测量了甲烷/空气预混火焰、酒精灯火焰和固体燃剂燃烧场中OH的二维分布。简述了激光诱导偏振光谱技术的基本原理和二维测量的实验方法;通过测量火焰中OH自由基A~2∑~+-X~2∏(0,0)跃迁带中Q1(8)吸收线的强度,获得了燃烧场中OH的二维分布。实验结果对了解火焰构造,研究燃烧机理等有一定的参考价值。     

激光诱导煤粉等离子体的特性研究

本文研究了煤粉形态对于激光诱导煤粉等离子体特性的影响,以指导应用激光感生击穿光谱进行煤质测量时最佳样品形态的选择.建立了一套激光诱导击穿光谱的实验台架,对同一煤种的4个不同粒径范围的粉状样品进行激光激发与光谱分析,利用钙原子不同跃迁能级发射谱线的强度分布计算了0.3～0.5μs区间内的等离子体温度,并依据谱线Stark展宽与电子密度的关系得到了等离子体的电子密度.再对激发不同粒径煤粉样品产生的等离子体温度与电子密度进行了对比.实验证明,煤粉粒径越小,等离子体温度越高且电子密度越大,也即样品的等离子化程度越高,越有利于煤中元素的定量分析.     

纳秒激光诱导空气等离子体的红外辐射特性研究

纳秒激光诱导空气等离子体存在从紫外、可见、近红外乃至射频微波的宽谱段辐射,但目前的研究大多关注紫外到可见波段的光谱辐射。激光等离子体作为一种新型的红外辐射源具有很多优势,相比于红外干扰弹以及红外干扰手段而言,空气等离子体红外辐射源可以灵活布置,成本低廉,因此研究空气等离子体的红外辐射特性就很有必要。针对目前脉冲激光诱导空气等离子体的红外干扰研究需要,对激光波长为532 nm的纳秒脉冲激光诱导空气等离子体的红外辐射特性进行实验研究,探讨激光能量对空气等离子体红外辐射强度的影响规律,以及空气等离子体红外辐射的角度分布特性,分析了等离子体红外辐射的可能产生机制。实验结果表明,激光诱导空气等离子体在950～1 700 nm范围内的红外光谱为线状谱和连续谱的叠加。其中线状谱主要是氮和氧的中性原子谱线,并且氮原子红外辐射占主导。随着激光能量的增加,由于空气击穿产生的氧和氮原子数量增加,导致空气等离子体红外辐射的谱线强度逐渐增大。随着红外探测角度的变化,在探测角度为75°时,OⅠ 1 128.63 nm和NⅠ1 246.96和1 362.42 nm谱线强度达到最大,在探测角度为120°时,NⅠ 1 011.46和1 053.96 nm谱线强度达到最大,这是因为空气等离子体红外辐射强度随探测角度变化呈现空间非对称性,表明空气等离子体内不同粒子的空间分布呈现非对称性。     

自由定标激光诱导击穿光谱技术在炉渣成分定量分析中的应用

采用自由定标激光诱导光谱技术(CF-LIBS)对炉渣中几种主要成分(CaO,SiO2,Al2O3,MgO)进行了定量分析.利用Nd:YAG激光脉冲在空气中烧蚀炉渣样品产生等离子体,等离子体光谱由中阶梯光栅光谱仪记录.通过几种主要元素的原子谱线和离子谱线的玻尔兹曼图,计算了等离子体的温度.利用Ca的一条谱线Stark展宽...     

空气及水汽的激光诱导击穿光谱特性实验研究

以大气颗粒物测量为目标,实验研究了作为背景的空气分子的激光诱导击穿光谱(laser induced breakdown spectrometry, LIBS),以NIST原子发射谱线数据库为参考,对其中的O,N,H等主要元素的特征谱线进行了标识。研究了CCD光谱仪实验参数对空气等离子体发射谱线的影响,得出积分延时大于7 μs时可以较好地减小空气分子所产生的干扰谱线,积分时间宽度大于仪器最小值(1. 1 ms)时对谱线信号强度影响较小等结论。还研究了水汽的激光诱导击穿光谱,分析了O,N,H等元素的发射谱线信号强度的变化,发现H的发射谱线信号强度与水汽含量之间具有很好的线性关系。这些结果对于在大气环境条件下,对大气颗粒物的识别具有重要的指导意义。     

采用激光诱导击穿光谱技术探测等离子体温度

阐述了激光诱导击穿技术的基本原理，研究了激光诱导击穿光谱技术在探测等离子体温度方面的应用，并进行了实验研究。在等离子体达到局部热平衡时，通过探测Cu的等离子体特征谱线相对强度的方法，达到用激光诱导击穿光谱技术探测等离子体温度的目的。实验结果表明，该方法方便、快捷，具有一定的实际应用价值。     

液相和固相钢铁的激光诱导击穿光谱特性

为深入了解激光诱导击穿光谱技术应用于钢液成分的检测机制,对45#钢样品分别为高温熔融液态和冷却凝固后的固态样品的激光诱导击穿光谱特性进行了对比分析.实验表明,相比固体样品,在相同实验条件下对钢液进行直接测量的稳定性相对较差.各波段的光谱强度液态钢的要明显强于固态钢,两者的等离子体特性存在较大差别.采用Fe的五条原子谱线...     

Laser-induced plasma in methane and dimethyl ether for flame ignition and combustion diagnostics

In this paper we report the investigation of the laser-induced breakdown and ignition behaviour of methane/air and dimethyl ether (DME)/air mixtures. Moreover, the optical emission from the induced plasma is utilized for determining the mixture composition quantitatively by means of laser-induced breakdown spectroscopy (LIBS). To the best of the authors’ knowledge, LIBS and laser ignition of DME have not been reported in literature before. The technique under investigation is finally employed for combustion diagnostics in laminar as well as turbulent flames. In the laminar premixed and non-premixed flames the LIBS spectra allow spatially resolved measurements of the equivalence ratio and enable studying the mixing of gases provided through the burner with the surrounding room air. In addition, the breakdown threshold of the applied laser pulse energy yields an estimate for the local temperature. In the turbulent cases single-shot LIBS spectra are recorded at fixed position allowing the derivation of local statistical fluctuations of the equivalence ratio in partially premixed jet flames. The results show that laser-induced breakdowns have a strong potential for flame diagnostics and, under suitable conditions, for the ignition of combustible mixtures.     

激光烧蚀金属靶时气体电离分析

我们使用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,关于环境气体分解及其对等离子体辐射特征的影响进行了研究。使用的气体是Ａｒ,Ａｉｒ,Ｎ２。结果发现,在等离子体形成初期,等离子体时－空分辨谱中有大量的环境气体离子谱线。基于Ａｌ等离子体不同激光能量下的时间－空间分辨谱。对激光烧蚀金属靶时气体电离现象进行了分析,对气体分解造成的影响进行了简单的讨论。     

1维非稳腔空间增强探测CARS技术对稳态燃烧场温度的测量

 建立了1维非稳腔空间增强探测CARS实验系统，该系统由光源（YAG激光器、染料激光器）、实验光路和信号采集系统组成。分别测量了空气和化学平衡比为1，甲烷流量为0.7 L/min的甲烷-空气预混火焰中的氮气Q支的CARS实验谱。给出了火焰不同高度处小范围内的温度分布结果，并对实验结果进行了分析，结果表明：预混火焰温度随高度的增加呈下降趋势，测量结果的不确定度优于7%。该技术可用于稳态燃烧场温度的测量。     

Excitation Mechanisms of Copper Ionic and Atomic Lines Emitted from a Low‐Pressure Argon Laser‐Induced Plasma

Abstract

Low‐pressure laser‐induced plasmas generated with a pulsed Nd∶YAG laser have complicated structures both temporally and spatially. The emission characteristics of the plasma are investigated for optimizing the experimental parameters in atomic emission spectrometry. The emission intensities of copper emission lines, measured in a time‐resolved as well as a time‐integrated mode, are strongly dependent on the kind of copper lines, ionic or atomic line, and the excitation energy. Also, the pressure of argon gas is the most important parameter for determining the behavior of these emission lines, including argon lines. Generally, copper ionic lines are dominantly emitted from the initial breakdown zone, because the copper ions are produced mainly in the hot breakdown zone. However, the Cu II 229.44‐nm line is emitted also from the expansion zone of the plasma. It results from an additional excitation process through the charge‐transfer collision particularly effective for the corresponding excited level. In this work, the excitation mechanisms for Cu I, Ar I, and Ar II lines are also discussed. The excitations occurring in the laser‐induced plasma can be well understood by taking the temporal and spatial variations in their intensities into consideration.     

火焰原子吸收对K元素激光诱导击穿光谱测量的影响

当使用激光诱导击穿光谱技术（LIBS）测量火焰场内碱金属元素时,等离子体内碱金属原子所发出的LIBS信号,会受到等离子体外火焰中基态碱金属原子的吸收,影响测量精度。基于Beer Lambert定律和CH4 空气火焰场内气态含K物质的热力学平衡原理,建立了火焰场内K元素LIBS信号的原子吸收模型,并分析了实际生物质颗粒燃烧K元素释放浓度范围内,火焰气氛、K元素浓度分布以及总K浓度对火焰原子吸收效率的影响。研究发现,随着O2/CH4的摩尔比值的增加,火焰中热力学平衡状态下K原子占总K的比例从约25%逐步降低,火焰原子吸收效率也从86.8%逐步降低。当O2/CH4的摩尔比值大于2时,火焰尾气中会存在剩余O2,此时火焰内K原子的吸收效率均低于13%。同时,火焰中K元素浓度分布以及总K浓度的合理调整亦对火焰原子吸收效率具有降低作用。在此基础上,提出了创造氧化性气氛、调整K浓度分布来降低火焰原子吸收效率和提高LIBS测量精度的解决途径。     

Near-infrared cavity enhanced absorption spectroscopy of hot water and OH in an oven and in flames

A compact diode laser operating around 1.5 μm was used to measure cavity enhanced absorption spectra of hot water molecules and OH radicals in radiative environments under atmospheric conditions. Spectra of air were measured in an oven at temperatures ranging from 300 K to 1500 K. These spectra contained rovibrational lines from water and OH. The water spectra were compared to simulations from the HITRAN and HITEMP databases. Furthermore, spectra were recorded in the flame of a flat methane/air burner and in an oxyacetylene flame produced by a welding torch. The results show that cavity enhanced absorption spectroscopy provides a sensitive method for rapid monitoring of species in radiative environments. Received: 22 February 2001 / Revised version: 23 April 2001 / Published online: 7 June 2001     

Calibration-free laser-induced breakdown spectroscopy for quantitative elemental analysis of materials

The application of calibration-free laser-induced breakdown spectroscopy (CF-LIBS) for quantitative analysis of materials, illustrated by CF-LIBS applied to a brass sample of known composition, is presented in this paper. The LIBS plasma is produced by a 355?nm pulsed Nd:YAG laser with a pulse duration of 6?ns focussed onto a brass sample in air at atmospheric pressure. The time-resolved atomic and ionic emission lines of Cu and Zn from the LIBS spectra recorded by an Echelle spectrograph coupled with a gated intensified charge coupled detector are used for the plasma characterization and the quantitative analysis of the sample. The time delay where the plasma is optically thin and is also in local thermodynamic equilibrium (LTE), necessary for the elemental analysis of samples from the LIBS spectra, is deduced. An algorithm relating the experimentally measured spectral intensity values with the basic physics of the plasma is developed. Using the algorithm, the Zn and Cu concentrations in the brass sample are determined. The analytical results obtained from the CF-LIBS technique agree well with the certified values of the elements in the sample, with an accuracy error <1%.     

Emission spectroscopy of flame fronts in aluminum suspensions

Spatially resolved emission spectra from Bunsen-type flames stabilized in aluminum suspensions in air and oxygen–argon/helium mixtures were obtained using a mechanical-optical scanning system. A low resolution (1.5 nm) spectrometer was used to acquire the broad spectra over the 350–1000 nm range, and a high-resolution (0.04 nm) instrument was used for observation of AlO molecular bands and non-ionized atomic aluminum. The temperature of condensed phase emitters in the flame was derived using polychromatic fitting of the continuum spectra to Planck’s law. AlO temperature was found by fitting of the theoretically calculated shape of the band to experimental data. Peak temperatures of the condensed emitters were found to be approximately 3250 K in aluminum-air flames and approximately 3350 K for oxygen–argon/helium flames. Temperatures derived from AlO spectra coincide with the temperature of the condensed emitters with measurement accuracy and are only 100–200 °C lower than the computed equilibrium flame temperatures. The radial distribution of the temperature profile of the continuous emitters was found via Abel deconvolution and recovered the double-front structure of the Bunsen flame cone, with the outer flame being attributed to a diffusion flame of the fuel-rich products with ambient air. The observation of atomic aluminum lines seen in emission from the outer flame edge and partial self-absorption from the inner flame confirms the structure associated with the double-front structure. The implications of these results for the regime of particle combustion in a dust flame are discussed.     

Emission properties of laser ablation of SnO2: Sb transparent conducting film and KTiOPO4 crystal

Optical emission spectra of Nd:YAG laser ablation of KTiOPO₄ (KTP) crystal and SnO₂:Sb transparent conducting thin film were recorded and analyzed in vacuum and in air. The integral intensities of spectral lines from laser-ablated KTP crystal were obtained as functions of distance from the target surface and laser power density in vacuum and in air. The ambient gas effects on pulsed laser ablation of target were discussed. We also performed laser ablation of SnO₂:Sb transparent conducting thin film in air and the electron temperature and full-width at half-maximum (FWHM) of atomic and ionic spectral lines in the plasma were quantified using Boltzmann plot method and Lorentzian fit, respectively. Integral intensities of atomic and ionic Sn spectral lines were also obtained as functions of distance from the target surface and laser irradiance. The intensity ratio of ionic and atomic Sn spectral lines as a function of laser power density was got which gives some information about the variation of ionization ratio with laser irradiance in the plasma produced by high-power laser.