Effects of the combination of sample temperature and spatial confinement on laser-induced breakdown spectroscopy
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摘要: 升高样品温度和采用空间约束能提高激光诱导击穿光谱的信号强度，两种技术的结合可以进一步提高激光诱导击穿光谱的光谱强度。本文在空气环境中研究了升高样品温度和空间约束效应两种方法相结合对激光诱导击穿光谱的影响，测量了激光诱导铝等离子体的时间分辨光谱。实验结果表明：升高样品温度能增加激光诱导击穿光谱的信号强度，高温样品能耦合更多的激光能量；当圆柱形腔被用于约束等离子体时，信号强度得到了进一步提高。两个实验条件的结合对于激光诱导击穿光谱信号增强的效果明显强于单独升高样品温度或者单独采用空间约束的增强效果。单一200 °C高温下样品的Al(I) 396.2 nm线强度增加了1.4倍；单一空间约束条件下的Al(I) 396.2 nm线强度增加了1.3倍；而在200 °C和空间约束的组合条件下，Al(I) 396.2 nm线强度增加了2.1倍。这个结合效应增强效果产生主要由于激光照射高温样品产生更强的冲击波，从而能更有效地压缩高温下产生的更大尺寸的等离子体羽，进一步提高了激光诱导击穿光谱的强度。Abstract: The signal intensity of Laser-Induced Breakdown Spectroscopy (LIBS) can be improved by increasing sample temperature and confining space confinement. The combination of the two techniques can further improve the spectral intensity of LIBS. In this paper, the effects of increasing a sample’s temperature and spatial confinement on LIBS are studied in air, and the time-resolved spectra of laser-induced aluminum plasma are measured. The experimental results show that increasing the sample’s temperature can increase the signal intensity of LIBS since a sample with a higher temperature can absorb more laser energy; when the cylindrical cavity is used to confine the plasma, the spectral emission is further improved. The effect of the combination of the two experimental conditions is that the signal intensity of LIBS is significantly stronger than that of either condition alone. The intensity of Al (I) 396.2 nm increases to 1.4 times at 200 °C with higher temperature conditions alone, 1.3 times when spatial confinement is applied alone, and 2.1 times at 200 °C with spatial confinement. The emission intensity with the combined effects is higher than the sum of that under the two individual conditions. The effect of the combination is mainly based on the fact that laser irradiation of the sample under a higher temperature generates stronger shock waves that can more effectively compress a larger-sized plasma plume, thereby further improving the spectral intensity of LIBS.
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图 1 实验装置示意图（M为反射镜；I为光阑；Pd为光电二极管；DM为双色镜；L为透镜）
Figure 1. Schematic diagram of experimental setup (M is the mirror; I is the iris; Pd is the photodiode; DM is the dichroic mirror; L is the lens)
图 2 不同样品温度下铝的LIBS时间积分光谱，延迟时间为6 μs，积分时间为20 μs，激光能量为40 mJ
Figure 2. Time-integrated spectra of aluminum plasma at different sample temperatures when the delay time is 6 μs, the integrated time is 20 μs, and the laser energy is 40 mJ
图 3 不同样品温度下Al(I) 396.2 nm峰强度随着延迟时间的变化，门宽为0.5 μs，激光能量为40 mJ
Figure 3. Evolution of peak intensity of Al (I) 396.2 nm at different sample temperatures when gate width is 0.5 μs, and the laser energy is 40 mJ
图 4 有无空间约束下不同样品温度的Al(I) 396.2 nm峰强度随着延迟时间的变化情况，门宽为0.5 μs，激光能量为40 mJ
Figure 4. Evolution of peak intensity of Al (I) 396.2 nm with and without space confinement as a function of delay time at different sample temperatures when gate width is 0.5 μs, and laser energy is 40 mJ
图 5 有无空间约束下不同样品温度的Al(I) 396.2 nm信背比随着延迟时间的变化，门宽为0.5 μs，激光能量为40 mJ
Figure 5. Evolution of SBR of Al (I) 396.2 nm with and without space confinement as a function of delay time at different sample temperatures when gate width is 0.5 μs, and the laser energy is 40 mJ
图 6 无（a），有（b）空间约束下不同样品温度的Al等离子体光谱对比，延迟时间为12.5 μs，门宽为0.5 μs，激光能量为40 mJ
Figure 6. Comparison of spectra of Al plasmas without (a) and with (b) spatial confinement at different sample temperatures, when the delay time is 12.5 μs, the gate width is 0.5 μs, and the laser energy is 40 mJ
图 7 有无空间约束下不同样品温度Al(I) 396.2 nm峰强度对比，延迟时间为12.5 μs，门宽为0.5 μs，激光能量为40 mJ
Figure 7. Comparison of peak intensity of Al (I) 396.2 nm at different sample temperatures with and without space confinement when the delay time is 12.5 μs, the gate width is 0.5 μs, and the laser energy is 40 mJ
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