再加热双脉冲激光诱导等离子体的时空演变特性研究

为了研究再加热双脉冲激光诱导击穿光谱(LIBS)对信号的增强机制，分别采用单脉冲LIBS和再加热双脉冲LIBS两种方式烧蚀合金钢样品产生等离子体，利用高分辨率的中阶梯光栅光谱仪采集等离子体发射光谱信号，同时用快速成像ICCD相机观测等离子体形态的变化，研究了两种烧蚀方式下等离子体的时空演变特性。通过比较两种烧蚀方式下等离子体产生初期光谱信号和图像的时间演变规律，发现再加热双脉冲LIBS提高了等离子体温度，且当信号采集延时等于再加热双脉冲的脉冲间隔时，等离子体温度的衰减速率发生变化;再加热双脉冲LIBS使等离子体图像强度增加，等离子体的中心区域高度和宽度分别增大了23.5%和15.1%。空间分布的研究结果表明，与单脉冲LIBS相比，当到样品表面的距离大于0.6 mm时，等离子体中的Fe Ⅱ和N Ⅰ谱线强度有较明显的增强，而Fe Ⅰ谱线在空间不同位置处的增强程度都较小，局部区域有减小的现象;再加热双脉冲LIBS使等离子体温度增加了约2 000 K，等离子体中产生了一个较大的高温区域。综合时空演变的实验结果说明再加热双脉冲对光谱信号增强的机制主要是由于第二束激光对第一束激光烧蚀样品产生的等离子体再次激发，使等离子体温度增加，进而引起等离子体辐射强度增加。

Research on Temporal and Spatial Evolution of Reheating Double-Pulse Laser-Induced Plasma

In order to investigate the emission enhancement mechanisms of reheating Double Pulse Laser-Induced Breakdown Spectroscopy (DP-LIBS), single pulse LIBS (SP-LIBS) and reheating DP-LIBS were carried out on an alloy steel sample respectively. The plasma emission was collected by an Echelle spectrometer with high resolution, while the plasma structure was monitored via fast-photography. The temporal and spatial evolutio ns of the plasma generated by SP-LIBS and reheating DP-LIBS were being studied. It is found that the plasma temperature in reheating DP-LIBS was higher than that of SP-LIBS, and there was a turning point for the decay rate of plasma temperature in reheating DP-LIBS when the delay time was equal to the inter-pulse time of DP-LIBS. Moreover, the inte nsity of the plasma image was increased by reheating DP-LIBS, and the height and width of the central region of the plasma were increased about 23.5% and 15.1% respectively. The results of spatial distribution showed that the inte nsity of Fe Ⅱ and N I lines in the plasma were obviously enhanced by reheating DP-LIBS when the distance from the sample surface was larger than 0.6 mm. While the inte nsity enhancement for Fe Ⅰ lines were little, even in some positio ns the inte nsity of Fe Ⅰ lines decreased. The plasma temperature of double-pulse configuration was about 2 000 K higher than that of SP-LIBS, and a larger hot region in the plasma was generated. It is evidenced that the emission enhancement mechanisms in reheating DP-LIBS is that the second laser pulse re-excited the plasma induced by the first laser pulse, and the higher plasma temperature resulted from the re-exciting process.