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Sep.  2019
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Study on B2Σ+~ X2Σ+ spectra and temperature of CN radicals based on LIBS

  • Corresponding author: LI Chuanliang, clli@tyust.edu.cn
  • Received Date: 2018-11-22
    Accepted Date: 2018-12-10
  • In order to study the B2Σ+~X2Σ+ spectra of CN radical and temperature under different conditions, laser-induced breakdown spectroscopy was used to break high purity graphite in air environment and produce CN free radicals. Emission spectra of B2Σ+~X2Σ+ were measured by high resolution spectrometer. The spectra of CN radicals under different conditions were studied by changing laser energy and laser focus position. The results show that, when laser energy is tuned from 30mJ to 50mJ and the increasing step size is 5mJ, spectral intensity increases with the increase of laser energy. When single pulse energy is 50mJ, spectral intensity reaches its maximum. In addition, when the distance between the upper surface of graphite and laser focal point is 8mm, signal-to-noise ratio reaches the maximum. LIFBASE software is used to fit the spectral data. The vibration temperature of CN radical is about 104K. The rotating temperature is about 4000K. Vibration temperature of CN radicals decreases and rotational temperature increases with the increase of distance. These results play an important role in studying cosmic stars and exploring high temperature chemical reaction.
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Study on B2Σ+~ X2Σ+ spectra and temperature of CN radicals based on LIBS

    Corresponding author: LI Chuanliang, clli@tyust.edu.cn
  • 1. College of Applied Science, Taiyuan University of Science and Technology, Taiyuan 030024, China
  • 2. State Key Laboratory for Electronic Test Technology, North University of China, Taiyuan 030051, China

Abstract: In order to study the B2Σ+~X2Σ+ spectra of CN radical and temperature under different conditions, laser-induced breakdown spectroscopy was used to break high purity graphite in air environment and produce CN free radicals. Emission spectra of B2Σ+~X2Σ+ were measured by high resolution spectrometer. The spectra of CN radicals under different conditions were studied by changing laser energy and laser focus position. The results show that, when laser energy is tuned from 30mJ to 50mJ and the increasing step size is 5mJ, spectral intensity increases with the increase of laser energy. When single pulse energy is 50mJ, spectral intensity reaches its maximum. In addition, when the distance between the upper surface of graphite and laser focal point is 8mm, signal-to-noise ratio reaches the maximum. LIFBASE software is used to fit the spectral data. The vibration temperature of CN radical is about 104K. The rotating temperature is about 4000K. Vibration temperature of CN radicals decreases and rotational temperature increases with the increase of distance. These results play an important role in studying cosmic stars and exploring high temperature chemical reaction.

引言
  • CN自由基是一种广泛分布的瞬态自由基[1], 它是宇宙星体的主要组成部分,对研究恒星、行星、彗星、星云、尘埃,测量宇宙背景温度等天体物理学领域有重要的作用[2-3]。此外,CN自由基还在高温化学反应中扮演着重要角色,它经常出现在爆炸、燃烧、气体化学、聚合反应等各种化学反应中[4-5]。为了准确预测反应过程中的辐射情况,需要准确测量反应时的温度。在高温化学反应中,已经不能只对宏观温度进行测量,需要对振动温度和转动温度等各自由度进行测量。1971年,SCHAEFER等人最早对CN的59个分子态进行了从头计算[6]。1974年,DAS等人采用多组态自洽场方法(multi-configurational self-consistent field,MCSCF)研究了CN的基态和一些低激发态电子态,获得了X2Σ+, A2Π, B2Σ+, D2Π, E2Σ+和H2Π电子态的势能曲线和光谱常数[7]。RAM等人用傅里叶变换光谱仪、微波和红外测量技术对CN自由基进行了研究,得出B2Σ+和X2Σ+电子态的光谱常数[8]。激光诱导荧光光谱和火焰光谱等光谱技术也用于研究CN自由基B2Σ+~X2Σ+和A2П~X2Σ+的跃迁,获取其振-转常数[9-12]

    目前,激光诱导击穿光谱技术(laser-induced breakdown spectroscopy,LIBS)也推广应用于自由基光谱测量中。GUO等人利用LIBS技术对AlO进行测量,得到了电子态B2Σ+~X2Σ+的光谱数据,并对实验过程中激光能量和延时时间对AlO自由基光谱信号的影响进行了探究[13]。LIU等人通过对火焰进行诱导击穿,得到了CH自由基的光谱数据[14]。最近,RAO等人采用LIBS技术测得了CN自由基B2Σ+~X2Σ+电子带系中上下态的振动能级之差Δν=0, Δν=+1和Δν=-1的3个振动带系的发射光谱[15]。MOUSAVI等人利用LIBS技术在不同的气体环境下对CN自由基进行了测量,得到了B2Σ+~X2Σ+电子带系中Δν=0的5条光谱线[16]

    此前的研究重点大都集中在CN自由基的振-转常数上,而对燃烧和高温化学反应动力学有重要意义的振动温度和转动温度研究不多。

    本文中采用了LIBS技术烧蚀高纯石墨产生CN自由基分子,通过测量CN自由基的B2Σ+~X2Σ+跃迁谱线,获得其振动温度和转动温度,并研究了温度与激光脉冲能量和样品与焦点距离之间的关系。

1.   实验装置
  • 图 1为实验系统装置图。包括激光器(Quantel Nd:YAG)、光谱仪(SR-500i)、CCD(Andor DV401A-BVF)、延时发生器、计算机等仪器。脉冲激光从激光器发出,经由反射镜反射、透镜聚焦后垂直向下照射到石墨样品表面。激光脉冲解离了空气中的N2和石墨样品,形成N原子、C原子等物质组成的等离子体团,结合形成CN自由基。CN自由基的辐射跃迁光,经过收集透镜后进入光谱仪分光,最后由CCD采集传入计算机进行处理。

    Figure 1.  Measurement setup for B2Σ+~X2Σ+ spectra of CN radicals

    在LIBS实验系统中,纳秒脉冲激光器的主要作用是产生高功率的激光来击打样品使其发生熔化、蒸发、电离成等离子体。激光器发出高能量密度的脉冲激光通过光阑照射到反射镜上,在经过凸透镜汇聚到样品表面。不同的样品其击穿阈值也不同,对激光器功率要求也不一样。气体状态的样品要想击穿成为等离子体,所需的激光功率密度大概为1011W/cm2;固体样品激光功率的击穿阈值大约109W/cm2[17]

2.   结果与分析
  • 为了得到CN自由基的光谱,本实验中采用石墨碳单质样品。石墨样品在高功率密度激光的击穿下,容易在表面形成坑洞,从而影响到光谱测量的质量[18]。为此,实验中使用了转盘,避免激光持续击打在样品表面同一点。此外,光谱信号与光谱采集时间和激光脉冲的延时有关。为了获得高质量光谱信号,作者测量了一系列延时情况下的光谱数据,发现延时时间在1μs时信噪比(signal-to-noise ratio,SNR)最高。同时在不影响光路的前提下,改变光纤探头的位置,结果发现放置光纤探头在激光靶点斜上方5cm处得到的光谱信号信噪比最高。

  • 实验测量CN自由光谱信号如图 2所示。实验中测量的光谱范围为380nm~400nm,观测到的信号为B2Σ+~X2Σ+的0~0, 1~1, 2~2, 3~3和4~4带振转跃迁光谱。激光能量的大小会对实验测得的光谱数据产生影响。激光能量值范围有限,为30mJ~50mJ,为了选取最佳的激光能量值,保持延时时间1μs,透镜到样品表面的距离50mm不变,激光能量值30mJ~50mJ,每次增加5mJ,获得的结果如图 3所示。图 4为光谱信噪比随能量变化的关系图。从图 3图 4中可以看出, 随着激光能量的增大,光谱强度也随着增大,到50mJ时达到最大值。而信噪比在激光能量为40mJ时有所下降,但变化幅度不大,信噪比最大值出现在50mJ处。因此,最佳的激光能量值为50mJ。

    Figure 2.  Vibration-rotation transition lines of B2Σ+~X2Σ+ spectra of CN radical

    Figure 3.  Line intensity vs. laser energy

    Figure 4.  Signal-to-noise ratio vs. laser energy

    在脉冲时间、激光照射在样品表面的光斑大小等其它条件一定时,激光的能量越大,功率密度就越大。样品受热熔化最后变成气态,进而产生含有大量原子、离子和自由电子的等离子体。它们分布在聚焦点上方,形成了一个“保护罩”,会阻碍激光继续烧蚀样品,影响样品的烧蚀质量,还会直接吸收激光能量,使等离子体内部更加不稳定,从而降低光谱信号的信噪比。样品的烧蚀质量越大,测到的光谱就越强[19]

  • 当聚焦点与样品表面的距离改变时,激光照射到样品表面的光斑大小也发生了改变,其功率密度就会发生改变。激光照射到样品表面的功率密度影响着样品的烧蚀质量,从而影响到了的光谱信号强度[20]。实验中将激光焦点与样品表面重合处设为原点,聚焦平面在样品表面下1mm处记为1mm。以此类推,得出CN B2Σ+~X2Σ+ 5条谱线在不同聚焦位置的光谱强度数据如表 1所示。

    distance/mm 0~0/count 1~1/count 2~2/count 3~3/count 4~4/count
    0 4806.52 4217.63 3427.39 3373.29 3388.32
    1 12791.80 8873.51 5671.82 5261.27 4746.70
    2 16033.50 12521.30 9363.87 9186.32 8875.62
    4 22375.30 15277.10 10233.50 9092.39 8882.33
    6 22062.50 16716.50 12062.00 11233.20 10989.60
    8 28865.50 21877.10 15109.70 13604.60 12604.40
    10 19631.40 18489.30 15047.70 14882.80 14242.20
    12 19503.00 15945.50 11487.50 10766.00 10015.80
    14 12614.60 11115.70 8288.62 7797.58 7224.41
    16 9988.06 7833.82 5423.86 4923.01 4407.88
    18 10347.50 7233.77 4692.11 4015.33 3585.56
    20 7397.54 5115.66 3347.43 2900.11 2607.99

    Table 1.  Laser focus position and spectral intensity

    根据表 1中不同聚焦位置的光谱强度数据,选取389nm附近的一段背景作为噪声,计算得到信噪比随着不同聚焦位置的变化趋势图, 如图 5所示。0~0和1~1带的信噪比在聚焦位置0mm~8mm范围内整体趋势逐渐增大,在8mm~20mm范围内逐渐减小,在8mm处达到最大值。2~2, 3~3和4~4带信噪比在0mm~8mm范围内整体趋势逐渐增大,在8mm和10mm持平,在10mm~20mm逐渐减小,在8mm和10mm处达到最大值。因此,最佳的聚焦位置为聚焦平面在样品表面下8mm处。

    Figure 5.  Signal-to-noise ratio vs. focus position

  • 分子振转光谱测温法是通过理论计算分子振-转谱线的强度分布与实验的谱线强度分布进行拟合得到分子的振动温度和转动温度的。辐射谱线强度公式[5]

    式中, νJ′表示高振动-转动能级; νJ″表示低振动-转动能级;Nν′J′为高能级粒子布局数;Aν″J″ν′J′为Einstein自发辐射跃迁概率;h为Planck常数;c为光速;νν″J″ν′J′为跃迁波数。Nν′J′的表达式为[5]

    式中, N0为总的粒子数;k为Boltzmann常数;ge为电子态简并度;Te, TvTr分别为电子态、振动态和转动态的温度;Ee, EvEr分别为电子态、振动态和转动态的能量;Qe, QvQr分别为电子态、振动态和转动态的配分函数[21]。通过(1)式和(2)式可以得到任意振动温度Tv、转动温度Tr所对应的谱线强度。

    将不同聚焦位置测得的光谱数据导入到LIFBASE软件[22],得到CN自由基B2Σ+~X2Σ+谱带的振动温度和转动温度随着聚焦位置变化的关系图,如图 6所示。由于实验中所得光谱受到多种展宽机制的影响,谱线会有一定的展宽。谱线的展宽可以通过对该谱线的非线性拟合得到。在LIFBASE软件中,不断调整振动温度、转动温度和谱线展宽的初值,当模拟计算得到的光谱与实验测到的光谱最为吻合时,即均方差达到最小值时,记下此时的振动温度和转动温度。

    Figure 6.  Vibration temperature and rotation temperature vs. distance

    图 6中可以观察到, CN自由基的振动温度随着距离的增加整体呈现下降的趋势,而转动温度呈现上升的趋势。激光聚焦点与样品表面的距离从0mm~10mm,振动温度初始时从12000K增加到13000K,然后逐渐下降到7900K;激光聚焦点与样品表面的距离从10mm~16mm时,振动温度趋于平稳,保持在7800K左右;16mm~20mm振动温度又有下降,到激光聚焦点与样品表面的距离为20mm时,振动温度为6100K。转动温度与振动温度的变化趋势不同,激光聚焦点与样品表面的距离从0mm~10mm,转动温度上升幅度越来越大,当激光聚焦点与样品表面的距离达到10mm时,转动温度为4800K;激光聚焦点与样品表面的距离从10mm~18mm,转动温度先下降到4100K,然后上升到6500K;激光聚焦点与样品表面的距离为20mm时,转动温度下降到了5000K。

    此外,作者拟合最佳实验条件(延时时间为1μs,激光能量为50mJ和激光聚焦点与样品表面的距离为8mm)下的光谱数据,得到其振动温度Tv=7500K,转动温度Tr=4000K。拟合的卡方值为96.7231,峰值相关性为0.979316,这表明了作者所得温度拟合的精度较高。同时,作者的结果与参考文献[4]和参考文献[15]中给出的结果一致,说明该系统具有较高的可靠性。

3.   结论
  • 本文中用LIBS技术测出了CN自由基B2Σ+~X2Σ+的5条谱线。详细探究了激光能量、聚焦位置对光谱数据的影响。设置了激光能量为30mJ~50mJ,步长为5mJ。光谱强度逐渐增大,激光能量增长为50mJ时达到最大值。5条谱线聚焦位置从0mm~8mm整体趋势逐渐增大,8mm~20mm逐渐减小,在8mm处达到最大值。将不同聚焦位置测得的光谱数据导入LIFBASE软件,研究了振动温度和转动温度随聚焦位置的变化规律,拟合出激光能量为50mJ和激光聚焦点与样品表面的距离为8mm的测量条件下,CN自由基B2Σ+~X2Σ+的振动温度为7500K、而转动温度为4000K。这些结果对研究燃烧过程、高温化学反应和天体物理学等方面有着重要的作用。

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