平行板约束对激光诱导PMMA等离子体中CN分子光谱的影响 
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1 引言
激光诱导击穿光谱(LIBS),又称为激光诱导等离子体光谱(LIPS)或激光火花光谱(LSS),作为原子发射光谱(AES),因具有操作简便、成本低廉、检测范围广、多组分元素能同时检测等优点,在物质组分分析方面有着独特的优势[1-3]。LIBS的基本原理相对简单,即一束具有高能量密度的激光脉冲照射样品,在激光照射样品区的上方形成高温、高密度等离子体,产生等离子体辐射的光经透镜会聚到光谱探测器中,再通过探测器得到光谱波长和强度信息。LIBS技术已经被应用到环境监测、空间探测、生物医学及一些**应用等领域[4-9]。当前,提高LIBS技术的探测灵敏度是人们关注的热点之一,很多实验技术已经被用来提高LIBS信号的强度,代表性的方法有双脉冲LIBS[10-11]、火花放电辅助LIBS[12-13]、预加热LIBS[14-15]、磁约束LIBS[16-19]及空间约束LIBS[20-25]等。
在上述增强光谱技术中,空间约束LIBS凭借操作简单、效率高、成本低等优点被广泛研究。空间约束LIBS加强效应的主要机理是反射冲击波与等离子体的相互作用[26]。在采用空间约束LIBS的实验中,当激光脉冲会聚到样品表面时,在样品表面产生等离子体,同时伴随冲击波的产生,冲击波的传播速度高于等离子体羽膨胀的速度;当冲击波在其膨胀过程中遇到约束腔的内壁时,冲击波会被反弹并开始反向传播,随后将和膨胀的等离子体羽相遇,能将等离子体羽压缩至较小尺寸[27],使得粒子间的碰撞几率增加,从而产生更多的高激发态粒子,这些粒子跃迁至低的能级时能使辐射更强。Gao等[20]研究了空间约束下纳秒激光诱导Cu等离子体,在无空间约束情况下,冲击波以半球形不断向外膨胀,而在空间约束的情况下,冲击波被反弹并将等离子体羽压缩,使等离子体辐射出更强的光。Wang等[22]讨论了不同形状约束腔对LIBS的影响,结果表明,空间约束可以提高等离子体的光谱强度和电子温度,且在圆柱形约束腔下,等离子体的光谱强度和电子温度最高。Shen等[23]研究了圆柱形约束腔下激光诱导Al等离子体光谱,在空间约束下,波长为394.40 nm和396.15 nm处的Al(I)的谱线强度显著增强,增强因子约为7。Wang等[28]研究了空间约束下激光诱导Cu等离子体中Cu(I)光谱的持续时间,结果发现,空间约束可以在一定延迟时间范围内增强光谱强度,且这个延迟时间的范围取决于激光的能量和约束腔的直径。
尽管许多国内外学者在这个领域都进行了大量的工作,但这些工作主要针对LIBS中的原子和离子光谱,而关于空间约束下激光诱导等离子体中的分子光谱的研究相对较少。在LIBS中,分子光谱能提供样品中非常有价值的化学信息,可以用来分析爆炸物、细菌、生物材料、卤素元素、同位素及各种有机化合物等[29-32],所以有必要研究空间约束对激光诱导等离子体中分子光谱的影响。另外,聚甲基丙烯酸甲酯(PMMA),又称亚克力,是最为常见的聚合物材料。PMMA价格低、韧性强、易于机械加工,具有良好的光学和绝缘特性。同时,PMMA表面光滑平整,在聚焦激光脉冲实验的过程中,平整的表面有利于光谱信号的稳定。因此,在本实验中采用PMMA作为样品,研究了空间约束对激光诱导等离子体中分子光谱的影响,测量了CN分子的时间分辨光谱,并利用CN分子光谱计算了CN分子的振动温度。
2 实验装置
空间约束下激光诱导PMMA等离子体的实验装置如
图 1. 纳秒激光诱导PMMA等离子体光谱实验装置示意图
Fig. 1. Schematic of experimental setup for nanosecond laser-induced PMMA plasma spectroscopy
3 结果与讨论
LIBS的变化过程主要分为三个阶段:待测样品被脉冲激光激发;产生激光诱导的等离子体;等离子体的快速膨胀和冷却[33-34]。其中,最有趣的过程就是等离子体的动态衰减过程,这个过程中的等离子体光谱随着延迟时间不断变化[35]。为了了解等离子体光谱的时间演化,测量了在有、无空间约束下的时间分辨光谱。无、有平行板约束下CN分子的时间分辨光谱如
图 2. CN分子(Δν=0)的时间分辨光谱。(a)无空间约束腔;(b)有空间约束腔
Fig. 2. Time-resolved spectra of CN molecule (Δν=0). (a) Without spatial confinement cavity; (b) with spatial confinement cavity
图 3. CN(0-0)光谱峰强度随延迟时间和波长的变化。(a)延迟时间;(b)波长
Fig. 3. Evolution of CN (0-0) spectral peak intensity with delay time and wavelength. (a) Delay time; (b) wavelength
光谱的信号强度是一个重要的指标,增加激光能量能直接地增强光谱的强度。
图 4. CN(0-0)光谱峰强度随延迟时间的变化。(a)无空间约束腔;(b)有空间约束腔
Fig. 4. Evolution of CN(0-0) spectral peak intensity with delay time. (a) Without spatial confinement cavity; (b) with spatial confinement cavity
图 5. 延迟时间为5 μs时CN(0-0)光谱峰强度随激光能量的变化
Fig. 5. Evolution of CN(0-0) spectral peak intensity with laser energy at delay time of 5 μs
温度信息是等离子体重要的参数之一,通过温度信息,能更有助于理解空间约束下CN分子光谱的增强效应。
图 6. CN(Δν=0)光谱对比。(a)无空间约束腔;(b)有空间约束腔
Fig. 6. Comparison of CN(Δν=0) spectra. (a) Without spatial confinement cavity; (b) with spatial confinement cavity
图 7. 不同激光能量下,CN分子的振动温度随延迟时间的变化。(a) 70 mJ;(b) 80 mJ
Fig. 7. Evolution of vibration temperatureof CN molecule with delay time under different laser energies. (a) 70 mJ; (b) 80 mJ
4 结论
研究平行板约束下的激光诱导PMMA等离子体,测量PMMA等离子体中CN(Δν=0)的时间分辨光谱。当平行板被用于约束PMMA等离子体时,CN(Δν=0)光谱峰强度得到明显增加,并且,激光能量越强,CN(Δν=0)光谱峰强度增加得越明显。另外,通过拟合CN(Δν=0)光谱,获得了CN分子的振动温度,发现采用空间约束的振动温度高于无空间约束下振动温度的情况。PMMA等离子体中CN光谱的增强是基于冲击波与等离子体羽的相互作用的,平行板反射冲击波,等离子体羽得到压缩,等离子体羽中的各种粒子数密度增加,进而增加了粒子之间的碰撞几率和等离子体温度,导致PMMA等离子体中CN分子辐射出幅值更大的光谱。
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杨雪, 陈安民, 李苏宇, 姜远飞, 金明星. 平行板约束对激光诱导PMMA等离子体中CN分子光谱的影响[J]. 中国激光, 2020, 47(8): 0811002. Yang Xue, Chen Anmin, Li Suyu, Jiang Yuanfei, Jin Mingxing. Effect of Parallel Plate Constraint on CN Molecular Spectra in Laser-Induced PMMA Plasma[J]. Chinese Journal of Lasers, 2020, 47(8): 0811002.
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