空气中激光等离子体通道的荧光探测和声学诊断两种方法的比较实验研究

使用荧光探测和声学诊断两种方法，对超强飞秒激光脉冲在空气中传输形成的等离子体通道同时进行了测量.两种方法都很好地反映了等离子体通道的演化过程.对两种方法的比较研究发现，声学诊断相对于荧光探测方法来说，具有较高的灵敏度和空间分辨能力，实验装置也更简单，更适宜于等离子体通道的常规测量. 关键词： 强飞秒激光等离子体通道 荧光探测 声学诊断     

空气中激光等离子体通道导电性能的研究

超强飞秒激光在大气中传输时，可以形成很长的等离子体通道.对通道电阻率和测量电极与等离子体通道间的接触电阻进行了测量，并对影响等离子体通道电阻率的主要因素进行了研究分析.提出减小等离子体通道电阻的方案. 关键词： 等离子体通道 电阻率 接触电阻     

激光大气等离子体的空间分布特性

利用Nd:YAG激光器产生的1.06 μm激光束（脉冲能量为500 mJ,脉冲宽度为10 ns, 重复频率为30 Hz）聚焦形成长约8 cm、直径5 cm的激光大气等离子体柱,分别沿激光束方向和垂直于激光束方向探测了该等离子体柱的空间分辨光谱,并由此反演得出电子密度和电子温度空间分布特性。实验结果表明：激光大气等离子体中各种离子和电子呈泪滴型分布,即沿激光束方向不对称,而垂直激光束方向对称分布,最大电子密度1018/cm3,最高电子温度3 000 K。     

等离子体通道的声学诊断方法

超短脉冲强激光在空气中传输可以形成很长的等离子体通道，目前对这种等离子体通道有许多诊断方法，文章介绍了一种十分简易但很有效的方法——声学测量方法．该方法可以根据通道不同位置的声音信号，测量通道的长度和通道内电子密度等一些参数，具有其他传统诊断方法无可比拟的优越性．     

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.     

激光等离子体对反射波频移影响的实验研究

研究了微波被激光等离子体反射时反射波的频率变化。实验中微波发生器产生的微波入射到激光等离子体并被等离子体反射,反射波的频率由频谱分析仪测量,发现反射波的频率与入射波的频率明显不同,分析了在激光等离子体膨胀和熄灭两种情况下激光功率密度和入射波频率对反射波频移影响的原因。结果表明：在等离子体膨胀和熄灭过程中,反射波频移的最大值随激光功率密度的增加而增加;随入射微波频率的增加而减小。     

Combination of spark discharge and nanoparticle-enhanced laser-induced plasma spectroscopy

A combination of spark discharge and nanoparticle-enhanced laser-induced plasma spectroscopy is investigated. Depositing Au nanoparticles at the surface of a brass target can enhance the coupling of the target and the laser. More atoms in the brass sample are excited. As a secondary excitation source, spark discharge reheats the generated plasma, which further amplifies the enhancement results of nanoparticles. The spectral intensity with the spark discharge increases more obviously with nanoparticle concentration increasing than without the spark discharge. Also, plasma temperature and electron density are calculated by the Boltzmann plot and Stark broadening. The changes in the plasma temperature and electron density are consistent with the spectral emission changes.     

On the improvement of signal repeatability in laser-induced air plasmas

The relatively low repeatability of laser-induced breakdown spectroscopy (LIBS) severely hinders its wide commercialization. In the present work, we investigate the optimization of LIBS system for repeatability improvement for both signal generation (plasma evolution) and signal collection. Timeintegrated spectra and images were obtained under different laser energies and focal lengths to investigate the optimum configuration for stable plasmas and repeatable signals. Using our experimental setup, the optimum conditions were found to be a laser energy of 250 mJ and a focus length of 100 mm. A stable and homogeneous plasma with the largest hot core area in the optimum condition yielded the most stable LIBS signal. Time-resolved images showed that the rebounding processes through the air plasma evolution caused the relative standard deviation (RSD) to increase with laser energies of > 250 mJ. In addition, the emission collection was improved by using a concave spherical mirror. The line intensities doubled as their RSDs decreased by approximately 25%. When the signal generation and collection were optimized simultaneously, the pulse-to-pulse RSDs were reduced to approximately 3% for O(I), N(I), and H(I) lines, which are better than the RSDs reported for solid samples and showed great potential for LIBS quantitative analysis by gasifying the solid or liquid samples.     

Conditions for laser-induced plasma to effectively remove nano-particles on silicon surfaces

Particles can be removed from a silicon surface by means of irradiation and a laser plasma shock wave.The particles and silicon are heated by the irradiation and they will expand differently due to their different expansion coefficients,making the particles easier to be removed.Laser plasma can ionize and even vaporize particles more significantly than an incident laser and,therefore,it can remove the particles more efficiently.The laser plasma shock wave plays a dominant role in removing particles,which is attributed to its strong burst force.The pressure of the laser plasma shock wave is determined by the laser pulse energy and the gap between the focus of laser and substrate surface.In order to obtain the working conditions for particle removal,the removal mechanism,as well as the temporal and spatial characteristics of velocity,propagation distance and pressure of shock wave have been researched.On the basis of our results,the conditions for nano-particle removal are achieved.     

Effect of parameters on Si plasma emission in collinear double-pulse laser-induced breakdown spectroscopy

Collinear dual-pulse laser-induced breakdown spectroscopy was carried out on Si crystal by using a pair of nanosecond Nd:YAG laser sources emitting at 1064 nm. The spectral intensities and signalto-noise ratios of selected Si atomic and ionic lines were used to evaluate the optical emission. The optical emission intensity was recorded while varying the interpulse delay time and energy ratio of the two pulsed lasers. The effects of the data acquisition delay time on the line intensity and signal-to-noise ratio have been investigated as well. Based on the results, the optimal interpulse delay time, energy ratio of the two pulsed lasers, and data acquisition delay time for achieving the maximum atomic and ionic line intensities were found for generation of Si plasma with the collinear dual-pulse laser approach. The dominant mechanism for the observed line intensity variation was also discussed. In addition, the plasma temperature and electron number density at different gate delay times and different interpulse delay times were derived. A significant influence of plasma shielding on the electron temperature and electron number density at shorter interpulse delay times was observed.     

NaCl对激光诱导土壤等离子体辐射的增强效应

为了改善激光诱导击穿光谱质量,采用Nd:YAG激光器输出的纳秒脉冲激光激发土壤样品,由光栅光谱仪和光电检测系统记录等离子体发射光谱,研究了激光输出能量（100~500 mJ）和NaCl样品添加剂（质量分数0%~20%）对等离子体辐射强度的影响。实验结果表明,在激光能量为200 mJ的优化条件下,可以提高光谱强度和信噪比,而更高激光能量诱导的等离子体中光谱线自吸严重。当土壤样品中加入适量NaCl添加剂以后,能够进一步改善激光光谱质量。NaCl加入量为10%时,等离子体中元素Fe,Ba,Al和Ti的谱线强度比无添加剂时分别提高了37%,32%, 32% 和70%,光谱信噪比分别提高了33%,26%,16% 和38%。而样品烧蚀量、电子温度和电子密度比无添加剂时分别提高了14%,9.6%和26%。     

NaCl对激光诱导土壤等离子体辐射的增强效应

为了改善激光诱导击穿光谱质量,采用Nd:YAG激光器输出的纳秒脉冲激光激发土壤样品,由光栅光谱仪和光电检测系统记录等离子体发射光谱,研究了激光输出能量（100~500 mJ）和NaCl样品添加剂（质量分数0%~20%）对等离子体辐射强度的影响。实验结果表明,在激光能量为200 mJ的优化条件下,可以提高光谱强度和信噪比,而更高激光能量诱导的等离子体中光谱线自吸严重。当土壤样品中加入适量NaCl添加剂以后,能够进一步改善激光光谱质量。NaCl加入量为10%时,等离子体中元素Fe,Ba,Al和Ti的谱线强度比无添加剂时分别提高了37%,32%, 32% 和70%,光谱信噪比分别提高了33%,26%,16% 和38%。而样品烧蚀量、电子温度和电子密度比无添加剂时分别提高了14%,9.6%和26%。     

Mechanisms of broadening of emission lines of a laser-induced plasma generated on the surface of rigid targets

This article reports the results of an experimental investigation of the basic mechanisms that cause the broadening of emission lines of a laser-induced plasma generated on the surface of rigid targets in a gaseous atmosphere. The contribution of Stark and resonance mechanisms of emission-line broadening for the resonance doublet Al I 3s²3p²P_3/2,1/2-3s²4s²S_1/2 and triplet Mg I 3s3p³P _0,1,2 ⁰ -3s4s³S₁ is discussed. The concentrations of neutral Al atoms in the laser-induced plasma are evaluated by the magnitude of resonance line broadening. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 234–237, March–April, 2000.     

Micro-pinch formation and extreme ultraviolet emission of laser-induced discharge plasma

Extreme ultraviolet(EUV) source produced by laser-induced discharge plasma(LDP) is a potential technical means in inspection and metrology. A pulsed Nd:YAG laser is focused on a tin plate to produce an initial plasma thereby triggering a discharge between high-voltage electrodes in a vacuum system. The process of micro-pinch formation during the current rising is recorded by a time-resolved intensified charge couple device camera. The evolution of electron temperature and density of LDP are obtained by optical emission spectrometry. An extreme ultraviolet spectrometer is built up to investigate the EUV spectrum of Sn LDP at 13.5 nm. The laser and discharge parameters such as laser energy, voltage, gap distance,and anode shape can influence the EUV emission.     

激光诱导Ca等离子体温度的实验研究

等离子体温度是激光诱导击穿光谱测量中一个重要的因素。采用Nd:YAG脉冲激光器作为光源击穿样品形成等离子体,其发射光谱由中阶梯光栅光谱仪和ICCD进行分光和光电转换。通过实验得出了300~450 nm波段的光谱图,定性分析出了Ca Ⅱ 315.9, 317.9, 393.4, 396.9 nm和Ca Ⅰ 422.7 nm等发射谱线。根据激光诱导击穿光谱定量公式,等离子体温度的变化对谱线强度有影响。先假设实验中等离子体处于局部热平衡状态,选用Ca的4条一价离子谱线,根据Boltzmann斜线法计算出了等离子体温度,并得到了等离子体温度与Ca质量分数的关系。随着Ca质量分数的增加,等离子体温度也相应增加。但当Ca质量分数小于0.50%时等离子体温度增加的幅度较小,而质量分数大于0.50%时等离子体温度的上升幅度相对较大。最后经过验证,实验中等离子体处于局部热平衡状态的假设成立。     

Study on quantitative analysis of slag based on spectral normalization of laser-induced plasma image

To reduce the influence of laser-induced breakdown spectroscopy （LIBS） experimental parameter fluctuations to quantitative analysis of slag components, a normalization method using integral intensity of plasma image was proposed and a series of experiments with slag samples were performed. Mg II 279.55 nm, Ca II 396.85 and Ca I 422.67 nm were selected as analytical lines, and analytical curves of reference mass fractions versus spectral line intensities were established. With the incre- ment of set threshold for edge extraction of plasma image, the determination coefficients and relative standard deviations of analytical curves were improved gradually and reached the optimmn values when the threshold was equal to 10 000. Comparing with the results without normalization and normalized by whole spectrum area, the relativity between spectral line intensity and mass fraction can be enhanced etfieiently after normalized by integral intensity of plasma image. The verification experiments with Ti alloy samples further confirmed the conclusions mentioned above.     

Radio-frequency compressed electron pulse-width characterization by cross-correlation between electron bunches and laser-induced plasma

A method is proposed to determine the temporal width of high-brightness radio-frequency compressed electron pulses based on cross-correlation technique involving electron bunches and laser-induced plasma. The temporal evolution of 2-dimensional transverse profile of ultrafast electron bunches repelled by the formed transient electric field of laser-induced plasma on a silver needle is investigated, and the pulse-width can be obtained by analyzing these time-dependent images.This approach can characterize radio-frequency compressed ultrafast electron bunches with picosecond or sub-picosecond timescale and up to 105 electron numbers.     

磁约束技术对激光诱导等离子体辐射特性的影响

为了提高激光诱导击穿光谱质量,利用Nd：YAG激光器烧蚀土壤样品,研究了磁场作用下的激光诱导等离子体辐射特性。实验结果表明,在相同激光输出能量条件下,随着磁场强度的增大,等离子体的辐射强度逐渐增强。计算可知,当采用的磁场强度为0.5 T时,样品元素Al,Fe,Ba和Ti的光谱线强度比无磁场作用时的分别增强了52.35%,46.64%,64.01%和51.73%,光谱信噪比分别提高了45.44%,69.64%,40.26%和41.33%;而等离子体的电子温度和电子密度分别提高了1 355.01 K和0.531016 cm-3。可见,利用磁场约束等离子体的技术是提高激光光谱质量的一种有效方法。     

激光诱导击穿光谱机理及爆炸物研究

激光诱导击穿光谱(LIBS)技术是基于激光诱导等离子体发光探测物质成分的方法。激光诱导等离子体的发射谱线由两部分组成:一部分是电子-离子复合及自由电子相互作用辐射的宽带连续谱;另一部分是原子和离子发射的特征谱线。对这些谱线进行识别并测量其强度,给出物质成分的定性和定量信息。分别讨论了样品所附着的基质为金属和电介质的两种情况,并对ns和fs级激光脉宽的影响作了介绍。     

Characteristics of indirect laser-induced plasma from a thin film of oil on a metallic substrate

Optical emissions from the major and trace elements embodied in a transparent gel prepared from cooking oil were detected after the gel was spread in a thin film on a metallic substrate. Such emissions are due to the indirect breakdown of the coating layer. The generated plasma, a mixture of substances from the substrate, the layer, and the ambient gas, was characterized using emission spectroscopy. The characteristics of the plasma formed on the metal with and without the coating layer were investigated. The results showed that Al emission induced from the aluminum substrates coated with oil films extends away from the target surface to ablate the oil film. This finally formed a bifurcating circulation of aluminum vapor against a spherical confinement wall in the front of the plume, which differed from the evolution of the plasma induced from the uncoated aluminum target. The strongest emissions of elements from the oil films can be observed at 2 mm above the target after a detection delay of 1.0 μs. A high temperature zone has been observed in the plasma after the delay of 1.0 μs for the plasma induced from the coated metal. This higher temperature determined in the plasma allows the consideration of the sensitive detection of trace elements in liquids, gels, biological samples, or thin films.