飞秒激发纳秒共振增强的双脉冲激光诱导击穿光谱方法研究

Femtosecond laser-induced breakdown spectroscopy (LIBS) analysis method has many advantages in the analytical methods. The interaction of femtosecond laser and material provides some special features to the ablation process as a lower damage threshold and a higher efficiency, which is different for the nanosecond laser. The other advantages include the small heat affected zone, better depth resolution, and faster broadband background decay. By comparing single-shot LIBS with femtosecond and nanosecond laser pulses, femtosecond spectrum is well-resolved and presented a very low background emission, allowing signal accumulation for a large number of pulses. And, plasma emission is found to vary much more rapidly with time than in the case of the nanosecond laser produced plasma. Unfortunately, due to the absence of laser-plume interaction in the case of femtosecond LIBS, the intensity of the overall signal is much lower than that in the case of nanosecond LIBS. In this project, femtosecond excited nanosecond resonant enhanced double-pulse LIBS is formed by combining femtosecond LIBS with nanosecond resonant laser spectroscopy. The plasma spectroscopy is produced by femtsecond laser. The subsequent nanosecond laser is a tunable dye laser. The plasma generated by femtosecond is irradiated by the second pulse with the selected wavelength. The photoresonant will take place between the selective excitation of species in the plume and the second pulse. This study can enhance the emission intensity of femtosecond LIBS, and decrease the limit of detection. The sensitivity of femtsocond LIBS is improved in the analytical methods.

飞秒(fs)激光诱导击穿光谱(LIBS)在分析方法中具有较低的烧蚀阈值、高的烧蚀效率、较小的热损伤区域，还可以通过成丝效应进行远程的LIBS。fs产生的等离子体有快的衰减时间，相比于纳秒(ns)激光有较低连续辐射谱，对这方面的研究有利于提高光谱线状谱与连续谱的比值，从而使光谱信噪比进一步提高。由于fs持续时间短，几乎不存在光与等离子体的相互作用，使得fsLIBS的信号远小于nsLIBS的信号强度。本申请将fs激发与ns共振光谱技术相互结合，利用fs的优点去激发样品产生等离子体，随后可调谐的ns激光的输出波长与fs产生等离子体中待分析元素的共振吸收波长相对应。目的是利用fs在LIBS中的优点，再通过可调谐的ns激光有选择性地对特定的元素进行共振激发来增强谱线强度,提高光谱发射效率，克服fs在LIBS中的谱线强度较低的缺点，提高fsLIBS的探测极限，改善fsLIBS用于元素分析的探测灵敏度。

飞秒激光诱导击穿光谱(LIBS)在分析方法中具有较低的烧蚀阈值、高的烧蚀效率、较小的热损伤区域，还可以通过成丝效应进行远程的LIBS。由于飞秒持续时间短，几乎不存在光与等离子体的相互作用，使得飞秒LIBS的信号远小于纳秒LIBS的信号强度。本项目将用飞秒烧蚀待测样品产生等离子体，随后用纳秒脉冲对等离子体再激发，借此来提高飞秒LIBS光谱谱线强度的目的。主要研究成果如下：1）采用共轴的飞秒和纳秒双脉冲激光诱导方案来研究等离子体光谱，提出了激光脉冲能量对信号强度增强因子的影响，随着纳秒激光能量的增加，增强的倍数也会增加，在飞秒纳秒LIBS中获得的光谱强度是在飞秒LIBS中获得的光谱强度的19倍；2）研究了飞秒纳秒脉冲间延迟时间对光谱强度的影响，光谱谱强度首先随脉冲间延迟而增加，然后开始下降，在纳秒+飞秒中，光谱的增强归因于激光和液体层的耦合,在飞秒+纳秒中，光谱的增强则归因于等离子体再激发效应；3）讨论了飞秒纳秒LIBS对等离子体中分子光谱的影响，飞秒+纳秒LIBS方案可以获得更高的信背比，飞秒+纳秒LIBS方案下的信背比的最大值是纳秒+飞秒DP-LIBS方案下信背比最大值的2倍左右；4）讨论了飞秒纳秒双脉冲LIBS的时间分辨光谱，利用Boltzmann图和Stark展宽法得到了时间分辨等离子体温度和电子密度，发现飞秒+纳秒激发的时间分辨等离子体温度和电子密度高于纳秒+飞秒激发的时间分辨等离子体温度和电子密度，还观察了两种实验条件下烧蚀坑的形貌，发现飞秒+纳秒激发产生了更强的烧蚀，对应于更强的等离子体发射。项目增强了飞秒激光诱导击穿光谱谱线的信号强度，克服飞秒激光在LIBS中的谱线强度较低的缺点，改善飞秒激光诱导击穿光谱用于元素分析的探测灵敏度,为飞秒LIBS用于元素分析提供了重要的支持。